// SPDX-License-Identifier: GPL-2.0+
/*
 * NILFS segment constructor.
 *
 * Copyright (C) 2005-2008 Nippon Telegraph and Telephone Corporation.
 *
 * Written by Ryusuke Konishi.
 *
 */

#include <linux/pagemap.h>
#include <linux/buffer_head.h>
#include <linux/writeback.h>
#include <linux/bitops.h>
#include <linux/bio.h>
#include <linux/completion.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/freezer.h>
#include <linux/kthread.h>
#include <linux/crc32.h>
#include <linux/pagevec.h>
#include <linux/slab.h>
#include <linux/sched/signal.h>

#include "nilfs.h"
#include "btnode.h"
#include "page.h"
#include "segment.h"
#include "sufile.h"
#include "cpfile.h"
#include "ifile.h"
#include "segbuf.h"


/*
 * Segment constructor
 */
#define SC_N_INODEVEC   16   /* Size of locally allocated inode vector */

#define SC_MAX_SEGDELTA 64   /*
                              * Upper limit of the number of segments
                              * appended in collection retry loop
                              */

/* Construction mode */
enum {
        SC_LSEG_SR = 1, /* Make a logical segment having a super root */
        SC_LSEG_DSYNC,  /*
                         * Flush data blocks of a given file and make
                         * a logical segment without a super root.
                         */
        SC_FLUSH_FILE,  /*
                         * Flush data files, leads to segment writes without
                         * creating a checkpoint.
                         */
        SC_FLUSH_DAT,   /*
                         * Flush DAT file.  This also creates segments
                         * without a checkpoint.
                         */
};

/* Stage numbers of dirty block collection */
enum {
        NILFS_ST_INIT = 0,
        NILFS_ST_GC,            /* Collecting dirty blocks for GC */
        NILFS_ST_FILE,
        NILFS_ST_IFILE,
        NILFS_ST_CPFILE,
        NILFS_ST_SUFILE,
        NILFS_ST_DAT,
        NILFS_ST_SR,            /* Super root */
        NILFS_ST_DSYNC,         /* Data sync blocks */
        NILFS_ST_DONE,
};

#define CREATE_TRACE_POINTS
#include <trace/events/nilfs2.h>

/*
 * nilfs_sc_cstage_inc(), nilfs_sc_cstage_set(), nilfs_sc_cstage_get() are
 * wrapper functions of stage count (nilfs_sc_info->sc_stage.scnt). Users of
 * the variable must use them because transition of stage count must involve
 * trace events (trace_nilfs2_collection_stage_transition).
 *
 * nilfs_sc_cstage_get() isn't required for the above purpose because it doesn't
 * produce tracepoint events. It is provided just for making the intention
 * clear.
 */
static inline void nilfs_sc_cstage_inc(struct nilfs_sc_info *sci)
{
        sci->sc_stage.scnt++;
        trace_nilfs2_collection_stage_transition(sci);
}

static inline void nilfs_sc_cstage_set(struct nilfs_sc_info *sci, int next_scnt)
{
        sci->sc_stage.scnt = next_scnt;
        trace_nilfs2_collection_stage_transition(sci);
}

static inline int nilfs_sc_cstage_get(struct nilfs_sc_info *sci)
{
        return sci->sc_stage.scnt;
}

/* State flags of collection */
#define NILFS_CF_NODE           0x0001  /* Collecting node blocks */
#define NILFS_CF_IFILE_STARTED  0x0002  /* IFILE stage has started */
#define NILFS_CF_SUFREED        0x0004  /* segment usages has been freed */
#define NILFS_CF_HISTORY_MASK   (NILFS_CF_IFILE_STARTED | NILFS_CF_SUFREED)

/* Operations depending on the construction mode and file type */
struct nilfs_sc_operations {
        int (*collect_data)(struct nilfs_sc_info *, struct buffer_head *,
                            struct inode *);
        int (*collect_node)(struct nilfs_sc_info *, struct buffer_head *,
                            struct inode *);
        int (*collect_bmap)(struct nilfs_sc_info *, struct buffer_head *,
                            struct inode *);
        void (*write_data_binfo)(struct nilfs_sc_info *,
                                 struct nilfs_segsum_pointer *,
                                 union nilfs_binfo *);
        void (*write_node_binfo)(struct nilfs_sc_info *,
                                 struct nilfs_segsum_pointer *,
                                 union nilfs_binfo *);
};

/*
 * Other definitions
 */
static void nilfs_segctor_start_timer(struct nilfs_sc_info *);
static void nilfs_segctor_do_flush(struct nilfs_sc_info *, int);
static void nilfs_segctor_do_immediate_flush(struct nilfs_sc_info *);
static void nilfs_dispose_list(struct the_nilfs *, struct list_head *, int);

#define nilfs_cnt32_ge(a, b)   \
        (typecheck(__u32, a) && typecheck(__u32, b) && \
         ((__s32)((a) - (b)) >= 0))

static int nilfs_prepare_segment_lock(struct super_block *sb,
                                      struct nilfs_transaction_info *ti)
{
        struct nilfs_transaction_info *cur_ti = current->journal_info;
        void *save = NULL;

        if (cur_ti) {
                if (cur_ti->ti_magic == NILFS_TI_MAGIC)
                        return ++cur_ti->ti_count;

                /*
                 * If journal_info field is occupied by other FS,
                 * it is saved and will be restored on
                 * nilfs_transaction_commit().
                 */
                nilfs_warn(sb, "journal info from a different FS");
                save = current->journal_info;
        }
        if (!ti) {
                ti = kmem_cache_alloc(nilfs_transaction_cachep, GFP_NOFS);
                if (!ti)
                        return -ENOMEM;
                ti->ti_flags = NILFS_TI_DYNAMIC_ALLOC;
        } else {
                ti->ti_flags = 0;
        }
        ti->ti_count = 0;
        ti->ti_save = save;
        ti->ti_magic = NILFS_TI_MAGIC;
        current->journal_info = ti;
        return 0;
}

/**
 * nilfs_transaction_begin - start indivisible file operations.
 * @sb: super block
 * @ti: nilfs_transaction_info
 * @vacancy_check: flags for vacancy rate checks
 *
 * nilfs_transaction_begin() acquires a reader/writer semaphore, called
 * the segment semaphore, to make a segment construction and write tasks
 * exclusive.  The function is used with nilfs_transaction_commit() in pairs.
 * The region enclosed by these two functions can be nested.  To avoid a
 * deadlock, the semaphore is only acquired or released in the outermost call.
 *
 * This function allocates a nilfs_transaction_info struct to keep context
 * information on it.  It is initialized and hooked onto the current task in
 * the outermost call.  If a pre-allocated struct is given to @ti, it is used
 * instead; otherwise a new struct is assigned from a slab.
 *
 * When @vacancy_check flag is set, this function will check the amount of
 * free space, and will wait for the GC to reclaim disk space if low capacity.
 *
 * Return: 0 on success, or one of the following negative error codes on
 * failure:
 * * %-ENOMEM   - Insufficient memory available.
 * * %-ENOSPC   - No space left on device (if checking free space).
 */
int nilfs_transaction_begin(struct super_block *sb,
                            struct nilfs_transaction_info *ti,
                            int vacancy_check)
{
        struct the_nilfs *nilfs;
        int ret = nilfs_prepare_segment_lock(sb, ti);
        struct nilfs_transaction_info *trace_ti;

        if (unlikely(ret < 0))
                return ret;
        if (ret > 0) {
                trace_ti = current->journal_info;

                trace_nilfs2_transaction_transition(sb, trace_ti,
                                    trace_ti->ti_count, trace_ti->ti_flags,
                                    TRACE_NILFS2_TRANSACTION_BEGIN);
                return 0;
        }

        sb_start_intwrite(sb);

        nilfs = sb->s_fs_info;
        down_read(&nilfs->ns_segctor_sem);
        if (vacancy_check && nilfs_near_disk_full(nilfs)) {
                up_read(&nilfs->ns_segctor_sem);
                ret = -ENOSPC;
                goto failed;
        }

        trace_ti = current->journal_info;
        trace_nilfs2_transaction_transition(sb, trace_ti, trace_ti->ti_count,
                                            trace_ti->ti_flags,
                                            TRACE_NILFS2_TRANSACTION_BEGIN);
        return 0;

 failed:
        ti = current->journal_info;
        current->journal_info = ti->ti_save;
        if (ti->ti_flags & NILFS_TI_DYNAMIC_ALLOC)
                kmem_cache_free(nilfs_transaction_cachep, ti);
        sb_end_intwrite(sb);
        return ret;
}

/**
 * nilfs_transaction_commit - commit indivisible file operations.
 * @sb: super block
 *
 * nilfs_transaction_commit() releases the read semaphore which is
 * acquired by nilfs_transaction_begin(). This is only performed
 * in outermost call of this function.  If a commit flag is set,
 * nilfs_transaction_commit() sets a timer to start the segment
 * constructor.  If a sync flag is set, it starts construction
 * directly.
 *
 * Return: 0 on success, or a negative error code on failure.
 */
int nilfs_transaction_commit(struct super_block *sb)
{
        struct nilfs_transaction_info *ti = current->journal_info;
        struct the_nilfs *nilfs = sb->s_fs_info;
        int err = 0;

        BUG_ON(ti == NULL || ti->ti_magic != NILFS_TI_MAGIC);
        ti->ti_flags |= NILFS_TI_COMMIT;
        if (ti->ti_count > 0) {
                ti->ti_count--;
                trace_nilfs2_transaction_transition(sb, ti, ti->ti_count,
                            ti->ti_flags, TRACE_NILFS2_TRANSACTION_COMMIT);
                return 0;
        }
        if (nilfs->ns_writer) {
                struct nilfs_sc_info *sci = nilfs->ns_writer;

                if (ti->ti_flags & NILFS_TI_COMMIT)
                        nilfs_segctor_start_timer(sci);
                if (atomic_read(&nilfs->ns_ndirtyblks) > sci->sc_watermark)
                        nilfs_segctor_do_flush(sci, 0);
        }
        up_read(&nilfs->ns_segctor_sem);
        trace_nilfs2_transaction_transition(sb, ti, ti->ti_count,
                            ti->ti_flags, TRACE_NILFS2_TRANSACTION_COMMIT);

        current->journal_info = ti->ti_save;

        if (ti->ti_flags & NILFS_TI_SYNC)
                err = nilfs_construct_segment(sb);
        if (ti->ti_flags & NILFS_TI_DYNAMIC_ALLOC)
                kmem_cache_free(nilfs_transaction_cachep, ti);
        sb_end_intwrite(sb);
        return err;
}

void nilfs_transaction_abort(struct super_block *sb)
{
        struct nilfs_transaction_info *ti = current->journal_info;
        struct the_nilfs *nilfs = sb->s_fs_info;

        BUG_ON(ti == NULL || ti->ti_magic != NILFS_TI_MAGIC);
        if (ti->ti_count > 0) {
                ti->ti_count--;
                trace_nilfs2_transaction_transition(sb, ti, ti->ti_count,
                            ti->ti_flags, TRACE_NILFS2_TRANSACTION_ABORT);
                return;
        }
        up_read(&nilfs->ns_segctor_sem);

        trace_nilfs2_transaction_transition(sb, ti, ti->ti_count,
                    ti->ti_flags, TRACE_NILFS2_TRANSACTION_ABORT);

        current->journal_info = ti->ti_save;
        if (ti->ti_flags & NILFS_TI_DYNAMIC_ALLOC)
                kmem_cache_free(nilfs_transaction_cachep, ti);
        sb_end_intwrite(sb);
}

void nilfs_relax_pressure_in_lock(struct super_block *sb)
{
        struct the_nilfs *nilfs = sb->s_fs_info;
        struct nilfs_sc_info *sci = nilfs->ns_writer;

        if (sb_rdonly(sb) || unlikely(!sci) || !sci->sc_flush_request)
                return;

        set_bit(NILFS_SC_PRIOR_FLUSH, &sci->sc_flags);
        up_read(&nilfs->ns_segctor_sem);

        down_write(&nilfs->ns_segctor_sem);
        if (sci->sc_flush_request &&
            test_bit(NILFS_SC_PRIOR_FLUSH, &sci->sc_flags)) {
                struct nilfs_transaction_info *ti = current->journal_info;

                ti->ti_flags |= NILFS_TI_WRITER;
                nilfs_segctor_do_immediate_flush(sci);
                ti->ti_flags &= ~NILFS_TI_WRITER;
        }
        downgrade_write(&nilfs->ns_segctor_sem);
}

static void nilfs_transaction_lock(struct super_block *sb,
                                   struct nilfs_transaction_info *ti,
                                   int gcflag)
{
        struct nilfs_transaction_info *cur_ti = current->journal_info;
        struct the_nilfs *nilfs = sb->s_fs_info;
        struct nilfs_sc_info *sci = nilfs->ns_writer;

        WARN_ON(cur_ti);
        ti->ti_flags = NILFS_TI_WRITER;
        ti->ti_count = 0;
        ti->ti_save = cur_ti;
        ti->ti_magic = NILFS_TI_MAGIC;
        current->journal_info = ti;

        for (;;) {
                trace_nilfs2_transaction_transition(sb, ti, ti->ti_count,
                            ti->ti_flags, TRACE_NILFS2_TRANSACTION_TRYLOCK);

                down_write(&nilfs->ns_segctor_sem);
                if (!test_bit(NILFS_SC_PRIOR_FLUSH, &sci->sc_flags))
                        break;

                nilfs_segctor_do_immediate_flush(sci);

                up_write(&nilfs->ns_segctor_sem);
                cond_resched();
        }
        if (gcflag)
                ti->ti_flags |= NILFS_TI_GC;

        trace_nilfs2_transaction_transition(sb, ti, ti->ti_count,
                            ti->ti_flags, TRACE_NILFS2_TRANSACTION_LOCK);
}

static void nilfs_transaction_unlock(struct super_block *sb)
{
        struct nilfs_transaction_info *ti = current->journal_info;
        struct the_nilfs *nilfs = sb->s_fs_info;

        BUG_ON(ti == NULL || ti->ti_magic != NILFS_TI_MAGIC);
        BUG_ON(ti->ti_count > 0);

        up_write(&nilfs->ns_segctor_sem);
        current->journal_info = ti->ti_save;

        trace_nilfs2_transaction_transition(sb, ti, ti->ti_count,
                            ti->ti_flags, TRACE_NILFS2_TRANSACTION_UNLOCK);
}

static void *nilfs_segctor_map_segsum_entry(struct nilfs_sc_info *sci,
                                            struct nilfs_segsum_pointer *ssp,
                                            unsigned int bytes)
{
        struct nilfs_segment_buffer *segbuf = sci->sc_curseg;
        unsigned int blocksize = sci->sc_super->s_blocksize;
        void *p;

        if (unlikely(ssp->offset + bytes > blocksize)) {
                ssp->offset = 0;
                BUG_ON(NILFS_SEGBUF_BH_IS_LAST(ssp->bh,
                                               &segbuf->sb_segsum_buffers));
                ssp->bh = NILFS_SEGBUF_NEXT_BH(ssp->bh);
        }
        p = ssp->bh->b_data + ssp->offset;
        ssp->offset += bytes;
        return p;
}

/**
 * nilfs_segctor_reset_segment_buffer - reset the current segment buffer
 * @sci: nilfs_sc_info
 *
 * Return: 0 on success, or a negative error code on failure.
 */
static int nilfs_segctor_reset_segment_buffer(struct nilfs_sc_info *sci)
{
        struct nilfs_segment_buffer *segbuf = sci->sc_curseg;
        struct buffer_head *sumbh;
        unsigned int sumbytes;
        unsigned int flags = 0;
        int err;

        if (nilfs_doing_gc())
                flags = NILFS_SS_GC;
        err = nilfs_segbuf_reset(segbuf, flags, sci->sc_seg_ctime, sci->sc_cno);
        if (unlikely(err))
                return err;

        sumbh = NILFS_SEGBUF_FIRST_BH(&segbuf->sb_segsum_buffers);
        sumbytes = segbuf->sb_sum.sumbytes;
        sci->sc_finfo_ptr.bh = sumbh;  sci->sc_finfo_ptr.offset = sumbytes;
        sci->sc_binfo_ptr.bh = sumbh;  sci->sc_binfo_ptr.offset = sumbytes;
        sci->sc_blk_cnt = sci->sc_datablk_cnt = 0;
        return 0;
}

/**
 * nilfs_segctor_zeropad_segsum - zero pad the rest of the segment summary area
 * @sci: segment constructor object
 *
 * nilfs_segctor_zeropad_segsum() zero-fills unallocated space at the end of
 * the current segment summary block.
 */
static void nilfs_segctor_zeropad_segsum(struct nilfs_sc_info *sci)
{
        struct nilfs_segsum_pointer *ssp;

        ssp = sci->sc_blk_cnt > 0 ? &sci->sc_binfo_ptr : &sci->sc_finfo_ptr;
        if (ssp->offset < ssp->bh->b_size)
                memset(ssp->bh->b_data + ssp->offset, 0,
                       ssp->bh->b_size - ssp->offset);
}

static int nilfs_segctor_feed_segment(struct nilfs_sc_info *sci)
{
        sci->sc_nblk_this_inc += sci->sc_curseg->sb_sum.nblocks;
        if (NILFS_SEGBUF_IS_LAST(sci->sc_curseg, &sci->sc_segbufs))
                return -E2BIG; /*
                                * The current segment is filled up
                                * (internal code)
                                */
        nilfs_segctor_zeropad_segsum(sci);
        sci->sc_curseg = NILFS_NEXT_SEGBUF(sci->sc_curseg);
        return nilfs_segctor_reset_segment_buffer(sci);
}

static int nilfs_segctor_add_super_root(struct nilfs_sc_info *sci)
{
        struct nilfs_segment_buffer *segbuf = sci->sc_curseg;
        int err;

        if (segbuf->sb_sum.nblocks >= segbuf->sb_rest_blocks) {
                err = nilfs_segctor_feed_segment(sci);
                if (err)
                        return err;
                segbuf = sci->sc_curseg;
        }
        err = nilfs_segbuf_extend_payload(segbuf, &segbuf->sb_super_root);
        if (likely(!err))
                segbuf->sb_sum.flags |= NILFS_SS_SR;
        return err;
}

/*
 * Functions for making segment summary and payloads
 */
static int nilfs_segctor_segsum_block_required(
        struct nilfs_sc_info *sci, const struct nilfs_segsum_pointer *ssp,
        unsigned int binfo_size)
{
        unsigned int blocksize = sci->sc_super->s_blocksize;
        /* Size of finfo and binfo is enough small against blocksize */

        return ssp->offset + binfo_size +
                (!sci->sc_blk_cnt ? sizeof(struct nilfs_finfo) : 0) >
                blocksize;
}

static void nilfs_segctor_begin_finfo(struct nilfs_sc_info *sci,
                                      struct inode *inode)
{
        sci->sc_curseg->sb_sum.nfinfo++;
        sci->sc_binfo_ptr = sci->sc_finfo_ptr;
        nilfs_segctor_map_segsum_entry(
                sci, &sci->sc_binfo_ptr, sizeof(struct nilfs_finfo));

        if (NILFS_I(inode)->i_root &&
            !test_bit(NILFS_SC_HAVE_DELTA, &sci->sc_flags))
                set_bit(NILFS_SC_HAVE_DELTA, &sci->sc_flags);
        /* skip finfo */
}

static void nilfs_segctor_end_finfo(struct nilfs_sc_info *sci,
                                    struct inode *inode)
{
        struct nilfs_finfo *finfo;
        struct nilfs_inode_info *ii;
        struct nilfs_segment_buffer *segbuf;
        __u64 cno;

        if (sci->sc_blk_cnt == 0)
                return;

        ii = NILFS_I(inode);

        if (ii->i_type & NILFS_I_TYPE_GC)
                cno = ii->i_cno;
        else if (NILFS_ROOT_METADATA_FILE(inode->i_ino))
                cno = 0;
        else
                cno = sci->sc_cno;

        finfo = nilfs_segctor_map_segsum_entry(sci, &sci->sc_finfo_ptr,
                                                 sizeof(*finfo));
        finfo->fi_ino = cpu_to_le64(inode->i_ino);
        finfo->fi_nblocks = cpu_to_le32(sci->sc_blk_cnt);
        finfo->fi_ndatablk = cpu_to_le32(sci->sc_datablk_cnt);
        finfo->fi_cno = cpu_to_le64(cno);

        segbuf = sci->sc_curseg;
        segbuf->sb_sum.sumbytes = sci->sc_binfo_ptr.offset +
                sci->sc_super->s_blocksize * (segbuf->sb_sum.nsumblk - 1);
        sci->sc_finfo_ptr = sci->sc_binfo_ptr;
        sci->sc_blk_cnt = sci->sc_datablk_cnt = 0;
}

static int nilfs_segctor_add_file_block(struct nilfs_sc_info *sci,
                                        struct buffer_head *bh,
                                        struct inode *inode,
                                        unsigned int binfo_size)
{
        struct nilfs_segment_buffer *segbuf;
        int required, err = 0;

 retry:
        segbuf = sci->sc_curseg;
        required = nilfs_segctor_segsum_block_required(
                sci, &sci->sc_binfo_ptr, binfo_size);
        if (segbuf->sb_sum.nblocks + required + 1 > segbuf->sb_rest_blocks) {
                nilfs_segctor_end_finfo(sci, inode);
                err = nilfs_segctor_feed_segment(sci);
                if (err)
                        return err;
                goto retry;
        }
        if (unlikely(required)) {
                nilfs_segctor_zeropad_segsum(sci);
                err = nilfs_segbuf_extend_segsum(segbuf);
                if (unlikely(err))
                        goto failed;
        }
        if (sci->sc_blk_cnt == 0)
                nilfs_segctor_begin_finfo(sci, inode);

        nilfs_segctor_map_segsum_entry(sci, &sci->sc_binfo_ptr, binfo_size);
        /* Substitution to vblocknr is delayed until update_blocknr() */
        nilfs_segbuf_add_file_buffer(segbuf, bh);
        sci->sc_blk_cnt++;
 failed:
        return err;
}

/*
 * Callback functions that enumerate, mark, and collect dirty blocks
 */
static int nilfs_collect_file_data(struct nilfs_sc_info *sci,
                                   struct buffer_head *bh, struct inode *inode)
{
        int err;

        err = nilfs_bmap_propagate(NILFS_I(inode)->i_bmap, bh);
        if (err < 0)
                return err;

        err = nilfs_segctor_add_file_block(sci, bh, inode,
                                           sizeof(struct nilfs_binfo_v));
        if (!err)
                sci->sc_datablk_cnt++;
        return err;
}

static int nilfs_collect_file_node(struct nilfs_sc_info *sci,
                                   struct buffer_head *bh,
                                   struct inode *inode)
{
        return nilfs_bmap_propagate(NILFS_I(inode)->i_bmap, bh);
}

static int nilfs_collect_file_bmap(struct nilfs_sc_info *sci,
                                   struct buffer_head *bh,
                                   struct inode *inode)
{
        WARN_ON(!buffer_dirty(bh));
        return nilfs_segctor_add_file_block(sci, bh, inode, sizeof(__le64));
}

static void nilfs_write_file_data_binfo(struct nilfs_sc_info *sci,
                                        struct nilfs_segsum_pointer *ssp,
                                        union nilfs_binfo *binfo)
{
        struct nilfs_binfo_v *binfo_v = nilfs_segctor_map_segsum_entry(
                sci, ssp, sizeof(*binfo_v));
        *binfo_v = binfo->bi_v;
}

static void nilfs_write_file_node_binfo(struct nilfs_sc_info *sci,
                                        struct nilfs_segsum_pointer *ssp,
                                        union nilfs_binfo *binfo)
{
        __le64 *vblocknr = nilfs_segctor_map_segsum_entry(
                sci, ssp, sizeof(*vblocknr));
        *vblocknr = binfo->bi_v.bi_vblocknr;
}

static const struct nilfs_sc_operations nilfs_sc_file_ops = {
        .collect_data = nilfs_collect_file_data,
        .collect_node = nilfs_collect_file_node,
        .collect_bmap = nilfs_collect_file_bmap,
        .write_data_binfo = nilfs_write_file_data_binfo,
        .write_node_binfo = nilfs_write_file_node_binfo,
};

static int nilfs_collect_dat_data(struct nilfs_sc_info *sci,
                                  struct buffer_head *bh, struct inode *inode)
{
        int err;

        err = nilfs_bmap_propagate(NILFS_I(inode)->i_bmap, bh);
        if (err < 0)
                return err;

        err = nilfs_segctor_add_file_block(sci, bh, inode, sizeof(__le64));
        if (!err)
                sci->sc_datablk_cnt++;
        return err;
}

static int nilfs_collect_dat_bmap(struct nilfs_sc_info *sci,
                                  struct buffer_head *bh, struct inode *inode)
{
        WARN_ON(!buffer_dirty(bh));
        return nilfs_segctor_add_file_block(sci, bh, inode,
                                            sizeof(struct nilfs_binfo_dat));
}

static void nilfs_write_dat_data_binfo(struct nilfs_sc_info *sci,
                                       struct nilfs_segsum_pointer *ssp,
                                       union nilfs_binfo *binfo)
{
        __le64 *blkoff = nilfs_segctor_map_segsum_entry(sci, ssp,
                                                          sizeof(*blkoff));
        *blkoff = binfo->bi_dat.bi_blkoff;
}

static void nilfs_write_dat_node_binfo(struct nilfs_sc_info *sci,
                                       struct nilfs_segsum_pointer *ssp,
                                       union nilfs_binfo *binfo)
{
        struct nilfs_binfo_dat *binfo_dat =
                nilfs_segctor_map_segsum_entry(sci, ssp, sizeof(*binfo_dat));
        *binfo_dat = binfo->bi_dat;
}

static const struct nilfs_sc_operations nilfs_sc_dat_ops = {
        .collect_data = nilfs_collect_dat_data,
        .collect_node = nilfs_collect_file_node,
        .collect_bmap = nilfs_collect_dat_bmap,
        .write_data_binfo = nilfs_write_dat_data_binfo,
        .write_node_binfo = nilfs_write_dat_node_binfo,
};

static const struct nilfs_sc_operations nilfs_sc_dsync_ops = {
        .collect_data = nilfs_collect_file_data,
        .collect_node = NULL,
        .collect_bmap = NULL,
        .write_data_binfo = nilfs_write_file_data_binfo,
        .write_node_binfo = NULL,
};

static size_t nilfs_lookup_dirty_data_buffers(struct inode *inode,
                                              struct list_head *listp,
                                              size_t nlimit,
                                              loff_t start, loff_t end)
{
        struct address_space *mapping = inode->i_mapping;
        struct folio_batch fbatch;
        pgoff_t index = 0, last = ULONG_MAX;
        size_t ndirties = 0;
        int i;

        if (unlikely(start != 0 || end != LLONG_MAX)) {
                /*
                 * A valid range is given for sync-ing data pages. The
                 * range is rounded to per-page; extra dirty buffers
                 * may be included if blocksize < pagesize.
                 */
                index = start >> PAGE_SHIFT;
                last = end >> PAGE_SHIFT;
        }
        folio_batch_init(&fbatch);
 repeat:
        if (unlikely(index > last) ||
              !filemap_get_folios_tag(mapping, &index, last,
                      PAGECACHE_TAG_DIRTY, &fbatch))
                return ndirties;

        for (i = 0; i < folio_batch_count(&fbatch); i++) {
                struct buffer_head *bh, *head;
                struct folio *folio = fbatch.folios[i];

                folio_lock(folio);
                if (unlikely(folio->mapping != mapping)) {
                        /* Exclude folios removed from the address space */
                        folio_unlock(folio);
                        continue;
                }
                head = folio_buffers(folio);
                if (!head)
                        head = create_empty_buffers(folio,
                                        i_blocksize(inode), 0);

                bh = head;
                do {
                        if (!buffer_dirty(bh) || buffer_async_write(bh))
                                continue;
                        get_bh(bh);
                        list_add_tail(&bh->b_assoc_buffers, listp);
                        ndirties++;
                        if (unlikely(ndirties >= nlimit)) {
                                folio_unlock(folio);
                                folio_batch_release(&fbatch);
                                cond_resched();
                                return ndirties;
                        }
                } while (bh = bh->b_this_page, bh != head);

                folio_unlock(folio);
        }
        folio_batch_release(&fbatch);
        cond_resched();
        goto repeat;
}

static void nilfs_lookup_dirty_node_buffers(struct inode *inode,
                                            struct list_head *listp)
{
        struct nilfs_inode_info *ii = NILFS_I(inode);
        struct inode *btnc_inode = ii->i_assoc_inode;
        struct folio_batch fbatch;
        struct buffer_head *bh, *head;
        unsigned int i;
        pgoff_t index = 0;

        if (!btnc_inode)
                return;
        folio_batch_init(&fbatch);

        while (filemap_get_folios_tag(btnc_inode->i_mapping, &index,
                                (pgoff_t)-1, PAGECACHE_TAG_DIRTY, &fbatch)) {
                for (i = 0; i < folio_batch_count(&fbatch); i++) {
                        bh = head = folio_buffers(fbatch.folios[i]);
                        do {
                                if (buffer_dirty(bh) &&
                                                !buffer_async_write(bh)) {
                                        get_bh(bh);
                                        list_add_tail(&bh->b_assoc_buffers,
                                                      listp);
                                }
                                bh = bh->b_this_page;
                        } while (bh != head);
                }
                folio_batch_release(&fbatch);
                cond_resched();
        }
}

static void nilfs_dispose_list(struct the_nilfs *nilfs,
                               struct list_head *head, int force)
{
        struct nilfs_inode_info *ii, *n;
        struct nilfs_inode_info *ivec[SC_N_INODEVEC], **pii;
        unsigned int nv = 0;

        while (!list_empty(head)) {
                spin_lock(&nilfs->ns_inode_lock);
                list_for_each_entry_safe(ii, n, head, i_dirty) {
                        list_del_init(&ii->i_dirty);
                        if (force) {
                                if (unlikely(ii->i_bh)) {
                                        brelse(ii->i_bh);
                                        ii->i_bh = NULL;
                                }
                        } else if (test_bit(NILFS_I_DIRTY, &ii->i_state)) {
                                set_bit(NILFS_I_QUEUED, &ii->i_state);
                                list_add_tail(&ii->i_dirty,
                                              &nilfs->ns_dirty_files);
                                continue;
                        }
                        ivec[nv++] = ii;
                        if (nv == SC_N_INODEVEC)
                                break;
                }
                spin_unlock(&nilfs->ns_inode_lock);

                for (pii = ivec; nv > 0; pii++, nv--)
                        iput(&(*pii)->vfs_inode);
        }
}

static void nilfs_iput_work_func(struct work_struct *work)
{
        struct nilfs_sc_info *sci = container_of(work, struct nilfs_sc_info,
                                                 sc_iput_work);
        struct the_nilfs *nilfs = sci->sc_super->s_fs_info;

        nilfs_dispose_list(nilfs, &sci->sc_iput_queue, 0);
}

static int nilfs_test_metadata_dirty(struct the_nilfs *nilfs,
                                     struct nilfs_root *root)
{
        int ret = 0;

        if (nilfs_mdt_fetch_dirty(root->ifile))
                ret++;
        if (nilfs_mdt_fetch_dirty(nilfs->ns_cpfile))
                ret++;
        if (nilfs_mdt_fetch_dirty(nilfs->ns_sufile))
                ret++;
        if ((ret || nilfs_doing_gc()) && nilfs_mdt_fetch_dirty(nilfs->ns_dat))
                ret++;
        return ret;
}

static int nilfs_segctor_clean(struct nilfs_sc_info *sci)
{
        return list_empty(&sci->sc_dirty_files) &&
                !test_bit(NILFS_SC_DIRTY, &sci->sc_flags) &&
                sci->sc_nfreesegs == 0 &&
                (!nilfs_doing_gc() || list_empty(&sci->sc_gc_inodes));
}

static int nilfs_segctor_confirm(struct nilfs_sc_info *sci)
{
        struct the_nilfs *nilfs = sci->sc_super->s_fs_info;
        int ret = 0;

        if (nilfs_test_metadata_dirty(nilfs, sci->sc_root))
                set_bit(NILFS_SC_DIRTY, &sci->sc_flags);

        spin_lock(&nilfs->ns_inode_lock);
        if (list_empty(&nilfs->ns_dirty_files) && nilfs_segctor_clean(sci))
                ret++;

        spin_unlock(&nilfs->ns_inode_lock);
        return ret;
}

static void nilfs_segctor_clear_metadata_dirty(struct nilfs_sc_info *sci)
{
        struct the_nilfs *nilfs = sci->sc_super->s_fs_info;

        nilfs_mdt_clear_dirty(sci->sc_root->ifile);
        nilfs_mdt_clear_dirty(nilfs->ns_cpfile);
        nilfs_mdt_clear_dirty(nilfs->ns_sufile);
        nilfs_mdt_clear_dirty(nilfs->ns_dat);
}

static void nilfs_fill_in_file_bmap(struct inode *ifile,
                                    struct nilfs_inode_info *ii)

{
        struct buffer_head *ibh;
        struct nilfs_inode *raw_inode;

        if (test_bit(NILFS_I_BMAP, &ii->i_state)) {
                ibh = ii->i_bh;
                BUG_ON(!ibh);
                raw_inode = nilfs_ifile_map_inode(ifile, ii->vfs_inode.i_ino,
                                                  ibh);
                nilfs_bmap_write(ii->i_bmap, raw_inode);
                nilfs_ifile_unmap_inode(raw_inode);
        }
}

static void nilfs_segctor_fill_in_file_bmap(struct nilfs_sc_info *sci)
{
        struct nilfs_inode_info *ii;

        list_for_each_entry(ii, &sci->sc_dirty_files, i_dirty) {
                nilfs_fill_in_file_bmap(sci->sc_root->ifile, ii);
                set_bit(NILFS_I_COLLECTED, &ii->i_state);
        }
}

/**
 * nilfs_write_root_mdt_inode - export root metadata inode information to
 *                              the on-disk inode
 * @inode:     inode object of the root metadata file
 * @raw_inode: on-disk inode
 *
 * nilfs_write_root_mdt_inode() writes inode information and bmap data of
 * @inode to the inode area of the metadata file allocated on the super root
 * block created to finalize the log.  Since super root blocks are configured
 * each time, this function zero-fills the unused area of @raw_inode.
 */
static void nilfs_write_root_mdt_inode(struct inode *inode,
                                       struct nilfs_inode *raw_inode)
{
        struct the_nilfs *nilfs = inode->i_sb->s_fs_info;

        nilfs_write_inode_common(inode, raw_inode);

        /* zero-fill unused portion of raw_inode */
        raw_inode->i_xattr = 0;
        raw_inode->i_pad = 0;
        memset((void *)raw_inode + sizeof(*raw_inode), 0,
               nilfs->ns_inode_size - sizeof(*raw_inode));

        nilfs_bmap_write(NILFS_I(inode)->i_bmap, raw_inode);
}

static void nilfs_segctor_fill_in_super_root(struct nilfs_sc_info *sci,
                                             struct the_nilfs *nilfs)
{
        struct buffer_head *bh_sr;
        struct nilfs_super_root *raw_sr;
        unsigned int isz, srsz;

        bh_sr = NILFS_LAST_SEGBUF(&sci->sc_segbufs)->sb_super_root;

        lock_buffer(bh_sr);
        raw_sr = (struct nilfs_super_root *)bh_sr->b_data;
        isz = nilfs->ns_inode_size;
        srsz = NILFS_SR_BYTES(isz);

        raw_sr->sr_sum = 0;  /* Ensure initialization within this update */
        raw_sr->sr_bytes = cpu_to_le16(srsz);
        raw_sr->sr_nongc_ctime
                = cpu_to_le64(nilfs_doing_gc() ?
                              nilfs->ns_nongc_ctime : sci->sc_seg_ctime);
        raw_sr->sr_flags = 0;

        nilfs_write_root_mdt_inode(nilfs->ns_dat, (void *)raw_sr +
                                   NILFS_SR_DAT_OFFSET(isz));
        nilfs_write_root_mdt_inode(nilfs->ns_cpfile, (void *)raw_sr +
                                   NILFS_SR_CPFILE_OFFSET(isz));
        nilfs_write_root_mdt_inode(nilfs->ns_sufile, (void *)raw_sr +
                                   NILFS_SR_SUFILE_OFFSET(isz));

        memset((void *)raw_sr + srsz, 0, nilfs->ns_blocksize - srsz);
        set_buffer_uptodate(bh_sr);
        unlock_buffer(bh_sr);
}

static void nilfs_redirty_inodes(struct list_head *head)
{
        struct nilfs_inode_info *ii;

        list_for_each_entry(ii, head, i_dirty) {
                if (test_bit(NILFS_I_COLLECTED, &ii->i_state))
                        clear_bit(NILFS_I_COLLECTED, &ii->i_state);
        }
}

static void nilfs_drop_collected_inodes(struct list_head *head)
{
        struct nilfs_inode_info *ii;

        list_for_each_entry(ii, head, i_dirty) {
                if (!test_and_clear_bit(NILFS_I_COLLECTED, &ii->i_state))
                        continue;

                clear_bit(NILFS_I_INODE_SYNC, &ii->i_state);
                set_bit(NILFS_I_UPDATED, &ii->i_state);
        }
}

static int nilfs_segctor_apply_buffers(struct nilfs_sc_info *sci,
                                       struct inode *inode,
                                       struct list_head *listp,
                                       int (*collect)(struct nilfs_sc_info *,
                                                      struct buffer_head *,
                                                      struct inode *))
{
        struct buffer_head *bh, *n;
        int err = 0;

        if (collect) {
                list_for_each_entry_safe(bh, n, listp, b_assoc_buffers) {
                        list_del_init(&bh->b_assoc_buffers);
                        err = collect(sci, bh, inode);
                        brelse(bh);
                        if (unlikely(err))
                                goto dispose_buffers;
                }
                return 0;
        }

 dispose_buffers:
        while (!list_empty(listp)) {
                bh = list_first_entry(listp, struct buffer_head,
                                      b_assoc_buffers);
                list_del_init(&bh->b_assoc_buffers);
                brelse(bh);
        }
        return err;
}

static size_t nilfs_segctor_buffer_rest(struct nilfs_sc_info *sci)
{
        /* Remaining number of blocks within segment buffer */
        return sci->sc_segbuf_nblocks -
                (sci->sc_nblk_this_inc + sci->sc_curseg->sb_sum.nblocks);
}

static int nilfs_segctor_scan_file(struct nilfs_sc_info *sci,
                                   struct inode *inode,
                                   const struct nilfs_sc_operations *sc_ops)
{
        LIST_HEAD(data_buffers);
        LIST_HEAD(node_buffers);
        int err;

        if (!(sci->sc_stage.flags & NILFS_CF_NODE)) {
                size_t n, rest = nilfs_segctor_buffer_rest(sci);

                n = nilfs_lookup_dirty_data_buffers(
                        inode, &data_buffers, rest + 1, 0, LLONG_MAX);
                if (n > rest) {
                        err = nilfs_segctor_apply_buffers(
                                sci, inode, &data_buffers,
                                sc_ops->collect_data);
                        BUG_ON(!err); /* always receive -E2BIG or true error */
                        goto break_or_fail;
                }
        }
        nilfs_lookup_dirty_node_buffers(inode, &node_buffers);

        if (!(sci->sc_stage.flags & NILFS_CF_NODE)) {
                err = nilfs_segctor_apply_buffers(
                        sci, inode, &data_buffers, sc_ops->collect_data);
                if (unlikely(err)) {
                        /* dispose node list */
                        nilfs_segctor_apply_buffers(
                                sci, inode, &node_buffers, NULL);
                        goto break_or_fail;
                }
                sci->sc_stage.flags |= NILFS_CF_NODE;
        }
        /* Collect node */
        err = nilfs_segctor_apply_buffers(
                sci, inode, &node_buffers, sc_ops->collect_node);
        if (unlikely(err))
                goto break_or_fail;

        nilfs_bmap_lookup_dirty_buffers(NILFS_I(inode)->i_bmap, &node_buffers);
        err = nilfs_segctor_apply_buffers(
                sci, inode, &node_buffers, sc_ops->collect_bmap);
        if (unlikely(err))
                goto break_or_fail;

        nilfs_segctor_end_finfo(sci, inode);
        sci->sc_stage.flags &= ~NILFS_CF_NODE;

 break_or_fail:
        return err;
}

static int nilfs_segctor_scan_file_dsync(struct nilfs_sc_info *sci,
                                         struct inode *inode)
{
        LIST_HEAD(data_buffers);
        size_t n, rest = nilfs_segctor_buffer_rest(sci);
        int err;

        n = nilfs_lookup_dirty_data_buffers(inode, &data_buffers, rest + 1,
                                            sci->sc_dsync_start,
                                            sci->sc_dsync_end);

        err = nilfs_segctor_apply_buffers(sci, inode, &data_buffers,
                                          nilfs_collect_file_data);
        if (!err) {
                nilfs_segctor_end_finfo(sci, inode);
                BUG_ON(n > rest);
                /* always receive -E2BIG or true error if n > rest */
        }
        return err;
}

/**
 * nilfs_free_segments - free the segments given by an array of segment numbers
 * @nilfs:   nilfs object
 * @segnumv: array of segment numbers to be freed
 * @nsegs:   number of segments to be freed in @segnumv
 *
 * nilfs_free_segments() wraps nilfs_sufile_freev() and
 * nilfs_sufile_cancel_freev(), and edits the segment usage metadata file
 * (sufile) to free all segments given by @segnumv and @nsegs at once.  If
 * it fails midway, it cancels the changes so that none of the segments are
 * freed.  If @nsegs is 0, this function does nothing.
 *
 * The freeing of segments is not finalized until the writing of a log with
 * a super root block containing this sufile change is complete, and it can
 * be canceled with nilfs_sufile_cancel_freev() until then.
 *
 * Return: 0 on success, or one of the following negative error codes on
 * failure:
 * * %-EINVAL   - Invalid segment number.
 * * %-EIO      - I/O error (including metadata corruption).
 * * %-ENOMEM   - Insufficient memory available.
 */
static int nilfs_free_segments(struct the_nilfs *nilfs, __u64 *segnumv,
                               size_t nsegs)
{
        size_t ndone;
        int ret;

        if (!nsegs)
                return 0;

        ret = nilfs_sufile_freev(nilfs->ns_sufile, segnumv, nsegs, &ndone);
        if (unlikely(ret)) {
                nilfs_sufile_cancel_freev(nilfs->ns_sufile, segnumv, ndone,
                                          NULL);
                /*
                 * If a segment usage of the segments to be freed is in a
                 * hole block, nilfs_sufile_freev() will return -ENOENT.
                 * In this case, -EINVAL should be returned to the caller
                 * since there is something wrong with the given segment
                 * number array.  This error can only occur during GC, so
                 * there is no need to worry about it propagating to other
                 * callers (such as fsync).
                 */
                if (ret == -ENOENT) {
                        nilfs_err(nilfs->ns_sb,
                                  "The segment usage entry %llu to be freed is invalid (in a hole)",
                                  (unsigned long long)segnumv[ndone]);
                        ret = -EINVAL;
                }
        }
        return ret;
}

static int nilfs_segctor_collect_blocks(struct nilfs_sc_info *sci, int mode)
{
        struct the_nilfs *nilfs = sci->sc_super->s_fs_info;
        struct list_head *head;
        struct nilfs_inode_info *ii;
        int err = 0;

        switch (nilfs_sc_cstage_get(sci)) {
        case NILFS_ST_INIT:
                /* Pre-processes */
                sci->sc_stage.flags = 0;

                if (!test_bit(NILFS_SC_UNCLOSED, &sci->sc_flags)) {
                        sci->sc_nblk_inc = 0;
                        sci->sc_curseg->sb_sum.flags = NILFS_SS_LOGBGN;
                        if (mode == SC_LSEG_DSYNC) {
                                nilfs_sc_cstage_set(sci, NILFS_ST_DSYNC);
                                goto dsync_mode;
                        }
                }

                sci->sc_stage.dirty_file_ptr = NULL;
                sci->sc_stage.gc_inode_ptr = NULL;
                if (mode == SC_FLUSH_DAT) {
                        nilfs_sc_cstage_set(sci, NILFS_ST_DAT);
                        goto dat_stage;
                }
                nilfs_sc_cstage_inc(sci);
                fallthrough;
        case NILFS_ST_GC:
                if (nilfs_doing_gc()) {
                        head = &sci->sc_gc_inodes;
                        ii = list_prepare_entry(sci->sc_stage.gc_inode_ptr,
                                                head, i_dirty);
                        list_for_each_entry_continue(ii, head, i_dirty) {
                                err = nilfs_segctor_scan_file(
                                        sci, &ii->vfs_inode,
                                        &nilfs_sc_file_ops);
                                if (unlikely(err)) {
                                        sci->sc_stage.gc_inode_ptr = list_entry(
                                                ii->i_dirty.prev,
                                                struct nilfs_inode_info,
                                                i_dirty);
                                        goto break_or_fail;
                                }
                                set_bit(NILFS_I_COLLECTED, &ii->i_state);
                        }
                        sci->sc_stage.gc_inode_ptr = NULL;
                }
                nilfs_sc_cstage_inc(sci);
                fallthrough;
        case NILFS_ST_FILE:
                head = &sci->sc_dirty_files;
                ii = list_prepare_entry(sci->sc_stage.dirty_file_ptr, head,
                                        i_dirty);
                list_for_each_entry_continue(ii, head, i_dirty) {
                        clear_bit(NILFS_I_DIRTY, &ii->i_state);

                        err = nilfs_segctor_scan_file(sci, &ii->vfs_inode,
                                                      &nilfs_sc_file_ops);
                        if (unlikely(err)) {
                                sci->sc_stage.dirty_file_ptr =
                                        list_entry(ii->i_dirty.prev,
                                                   struct nilfs_inode_info,
                                                   i_dirty);
                                goto break_or_fail;
                        }
                        /* sci->sc_stage.dirty_file_ptr = NILFS_I(inode); */
                        /* XXX: required ? */
                }
                sci->sc_stage.dirty_file_ptr = NULL;
                if (mode == SC_FLUSH_FILE) {
                        nilfs_sc_cstage_set(sci, NILFS_ST_DONE);
                        return 0;
                }
                nilfs_sc_cstage_inc(sci);
                sci->sc_stage.flags |= NILFS_CF_IFILE_STARTED;
                fallthrough;
        case NILFS_ST_IFILE:
                err = nilfs_segctor_scan_file(sci, sci->sc_root->ifile,
                                              &nilfs_sc_file_ops);
                if (unlikely(err))
                        break;
                nilfs_sc_cstage_inc(sci);
                /* Creating a checkpoint */
                err = nilfs_cpfile_create_checkpoint(nilfs->ns_cpfile,
                                                     nilfs->ns_cno);
                if (unlikely(err))
                        break;
                fallthrough;
        case NILFS_ST_CPFILE:
                err = nilfs_segctor_scan_file(sci, nilfs->ns_cpfile,
                                              &nilfs_sc_file_ops);
                if (unlikely(err))
                        break;
                nilfs_sc_cstage_inc(sci);
                fallthrough;
        case NILFS_ST_SUFILE:
                err = nilfs_free_segments(nilfs, sci->sc_freesegs,
                                          sci->sc_nfreesegs);
                if (unlikely(err))
                        break;
                sci->sc_stage.flags |= NILFS_CF_SUFREED;

                err = nilfs_segctor_scan_file(sci, nilfs->ns_sufile,
                                              &nilfs_sc_file_ops);
                if (unlikely(err))
                        break;
                nilfs_sc_cstage_inc(sci);
                fallthrough;
        case NILFS_ST_DAT:
 dat_stage:
                err = nilfs_segctor_scan_file(sci, nilfs->ns_dat,
                                              &nilfs_sc_dat_ops);
                if (unlikely(err))
                        break;
                if (mode == SC_FLUSH_DAT) {
                        nilfs_sc_cstage_set(sci, NILFS_ST_DONE);
                        return 0;
                }
                nilfs_sc_cstage_inc(sci);
                fallthrough;
        case NILFS_ST_SR:
                if (mode == SC_LSEG_SR) {
                        /* Appending a super root */
                        err = nilfs_segctor_add_super_root(sci);
                        if (unlikely(err))
                                break;
                }
                /* End of a logical segment */
                sci->sc_curseg->sb_sum.flags |= NILFS_SS_LOGEND;
                nilfs_sc_cstage_set(sci, NILFS_ST_DONE);
                return 0;
        case NILFS_ST_DSYNC:
 dsync_mode:
                sci->sc_curseg->sb_sum.flags |= NILFS_SS_SYNDT;
                ii = sci->sc_dsync_inode;
                if (!test_bit(NILFS_I_BUSY, &ii->i_state))
                        break;

                err = nilfs_segctor_scan_file_dsync(sci, &ii->vfs_inode);
                if (unlikely(err))
                        break;
                sci->sc_curseg->sb_sum.flags |= NILFS_SS_LOGEND;
                nilfs_sc_cstage_set(sci, NILFS_ST_DONE);
                return 0;
        case NILFS_ST_DONE:
                return 0;
        default:
                BUG();
        }

 break_or_fail:
        return err;
}

/**
 * nilfs_segctor_begin_construction - setup segment buffer to make a new log
 * @sci: nilfs_sc_info
 * @nilfs: nilfs object
 *
 * Return: 0 on success, or a negative error code on failure.
 */
static int nilfs_segctor_begin_construction(struct nilfs_sc_info *sci,
                                            struct the_nilfs *nilfs)
{
        struct nilfs_segment_buffer *segbuf, *prev;
        __u64 nextnum;
        int err, alloc = 0;

        segbuf = nilfs_segbuf_new(sci->sc_super);
        if (unlikely(!segbuf))
                return -ENOMEM;

        if (list_empty(&sci->sc_write_logs)) {
                nilfs_segbuf_map(segbuf, nilfs->ns_segnum,
                                 nilfs->ns_pseg_offset, nilfs);
                if (segbuf->sb_rest_blocks < NILFS_PSEG_MIN_BLOCKS) {
                        nilfs_shift_to_next_segment(nilfs);
                        nilfs_segbuf_map(segbuf, nilfs->ns_segnum, 0, nilfs);
                }

                segbuf->sb_sum.seg_seq = nilfs->ns_seg_seq;
                nextnum = nilfs->ns_nextnum;

                if (nilfs->ns_segnum == nilfs->ns_nextnum)
                        /* Start from the head of a new full segment */
                        alloc++;
        } else {
                /* Continue logs */
                prev = NILFS_LAST_SEGBUF(&sci->sc_write_logs);
                nilfs_segbuf_map_cont(segbuf, prev);
                segbuf->sb_sum.seg_seq = prev->sb_sum.seg_seq;
                nextnum = prev->sb_nextnum;

                if (segbuf->sb_rest_blocks < NILFS_PSEG_MIN_BLOCKS) {
                        nilfs_segbuf_map(segbuf, prev->sb_nextnum, 0, nilfs);
                        segbuf->sb_sum.seg_seq++;
                        alloc++;
                }
        }

        err = nilfs_sufile_mark_dirty(nilfs->ns_sufile, segbuf->sb_segnum);
        if (err)
                goto failed;

        if (alloc) {
                err = nilfs_sufile_alloc(nilfs->ns_sufile, &nextnum);
                if (err)
                        goto failed;
        }
        nilfs_segbuf_set_next_segnum(segbuf, nextnum, nilfs);

        BUG_ON(!list_empty(&sci->sc_segbufs));
        list_add_tail(&segbuf->sb_list, &sci->sc_segbufs);
        sci->sc_segbuf_nblocks = segbuf->sb_rest_blocks;
        return 0;

 failed:
        nilfs_segbuf_free(segbuf);
        return err;
}

static int nilfs_segctor_extend_segments(struct nilfs_sc_info *sci,
                                         struct the_nilfs *nilfs, int nadd)
{
        struct nilfs_segment_buffer *segbuf, *prev;
        struct inode *sufile = nilfs->ns_sufile;
        __u64 nextnextnum;
        LIST_HEAD(list);
        int err, ret, i;

        prev = NILFS_LAST_SEGBUF(&sci->sc_segbufs);
        /*
         * Since the segment specified with nextnum might be allocated during
         * the previous construction, the buffer including its segusage may
         * not be dirty.  The following call ensures that the buffer is dirty
         * and will pin the buffer on memory until the sufile is written.
         */
        err = nilfs_sufile_mark_dirty(sufile, prev->sb_nextnum);
        if (unlikely(err))
                return err;

        for (i = 0; i < nadd; i++) {
                /* extend segment info */
                err = -ENOMEM;
                segbuf = nilfs_segbuf_new(sci->sc_super);
                if (unlikely(!segbuf))
                        goto failed;

                /* map this buffer to region of segment on-disk */
                nilfs_segbuf_map(segbuf, prev->sb_nextnum, 0, nilfs);
                sci->sc_segbuf_nblocks += segbuf->sb_rest_blocks;

                /* allocate the next next full segment */
                err = nilfs_sufile_alloc(sufile, &nextnextnum);
                if (unlikely(err))
                        goto failed_segbuf;

                segbuf->sb_sum.seg_seq = prev->sb_sum.seg_seq + 1;
                nilfs_segbuf_set_next_segnum(segbuf, nextnextnum, nilfs);

                list_add_tail(&segbuf->sb_list, &list);
                prev = segbuf;
        }
        list_splice_tail(&list, &sci->sc_segbufs);
        return 0;

 failed_segbuf:
        nilfs_segbuf_free(segbuf);
 failed:
        list_for_each_entry(segbuf, &list, sb_list) {
                ret = nilfs_sufile_free(sufile, segbuf->sb_nextnum);
                WARN_ON(ret); /* never fails */
        }
        nilfs_destroy_logs(&list);
        return err;
}

static void nilfs_free_incomplete_logs(struct list_head *logs,
                                       struct the_nilfs *nilfs)
{
        struct nilfs_segment_buffer *segbuf, *prev;
        struct inode *sufile = nilfs->ns_sufile;
        int ret;

        segbuf = NILFS_FIRST_SEGBUF(logs);
        if (nilfs->ns_nextnum != segbuf->sb_nextnum) {
                ret = nilfs_sufile_free(sufile, segbuf->sb_nextnum);
                WARN_ON(ret); /* never fails */
        }
        if (atomic_read(&segbuf->sb_err)) {
                /* Case 1: The first segment failed */
                if (segbuf->sb_pseg_start != segbuf->sb_fseg_start)
                        /*
                         * Case 1a:  Partial segment appended into an existing
                         * segment
                         */
                        nilfs_terminate_segment(nilfs, segbuf->sb_fseg_start,
                                                segbuf->sb_fseg_end);
                else /* Case 1b:  New full segment */
                        set_nilfs_discontinued(nilfs);
        }

        prev = segbuf;
        list_for_each_entry_continue(segbuf, logs, sb_list) {
                if (prev->sb_nextnum != segbuf->sb_nextnum) {
                        ret = nilfs_sufile_free(sufile, segbuf->sb_nextnum);
                        WARN_ON(ret); /* never fails */
                }
                if (atomic_read(&segbuf->sb_err) &&
                    segbuf->sb_segnum != nilfs->ns_nextnum)
                        /* Case 2: extended segment (!= next) failed */
                        nilfs_sufile_set_error(sufile, segbuf->sb_segnum);
                prev = segbuf;
        }
}

static void nilfs_segctor_update_segusage(struct nilfs_sc_info *sci,
                                          struct inode *sufile)
{
        struct nilfs_segment_buffer *segbuf;
        unsigned long live_blocks;
        int ret;

        list_for_each_entry(segbuf, &sci->sc_segbufs, sb_list) {
                live_blocks = segbuf->sb_sum.nblocks +
                        (segbuf->sb_pseg_start - segbuf->sb_fseg_start);
                ret = nilfs_sufile_set_segment_usage(sufile, segbuf->sb_segnum,
                                                     live_blocks,
                                                     sci->sc_seg_ctime);
                WARN_ON(ret); /* always succeed because the segusage is dirty */
        }
}

static void nilfs_cancel_segusage(struct list_head *logs, struct inode *sufile)
{
        struct nilfs_segment_buffer *segbuf;
        int ret;

        segbuf = NILFS_FIRST_SEGBUF(logs);
        ret = nilfs_sufile_set_segment_usage(sufile, segbuf->sb_segnum,
                                             segbuf->sb_pseg_start -
                                             segbuf->sb_fseg_start, 0);
        WARN_ON(ret); /* always succeed because the segusage is dirty */

        list_for_each_entry_continue(segbuf, logs, sb_list) {
                ret = nilfs_sufile_set_segment_usage(sufile, segbuf->sb_segnum,
                                                     0, 0);
                WARN_ON(ret); /* always succeed */
        }
}

static void nilfs_segctor_truncate_segments(struct nilfs_sc_info *sci,
                                            struct nilfs_segment_buffer *last,
                                            struct inode *sufile)
{
        struct nilfs_segment_buffer *segbuf = last;
        int ret;

        list_for_each_entry_continue(segbuf, &sci->sc_segbufs, sb_list) {
                sci->sc_segbuf_nblocks -= segbuf->sb_rest_blocks;
                ret = nilfs_sufile_free(sufile, segbuf->sb_nextnum);
                WARN_ON(ret);
        }
        nilfs_truncate_logs(&sci->sc_segbufs, last);
}


static int nilfs_segctor_collect(struct nilfs_sc_info *sci,
                                 struct the_nilfs *nilfs, int mode)
{
        struct nilfs_cstage prev_stage = sci->sc_stage;
        int err, nadd = 1;

        /* Collection retry loop */
        for (;;) {
                sci->sc_nblk_this_inc = 0;
                sci->sc_curseg = NILFS_FIRST_SEGBUF(&sci->sc_segbufs);

                err = nilfs_segctor_reset_segment_buffer(sci);
                if (unlikely(err))
                        goto failed;

                err = nilfs_segctor_collect_blocks(sci, mode);
                sci->sc_nblk_this_inc += sci->sc_curseg->sb_sum.nblocks;
                if (!err)
                        break;

                if (unlikely(err != -E2BIG))
                        goto failed;

                /* The current segment is filled up */
                if (mode != SC_LSEG_SR ||
                    nilfs_sc_cstage_get(sci) < NILFS_ST_CPFILE)
                        break;

                nilfs_clear_logs(&sci->sc_segbufs);

                if (sci->sc_stage.flags & NILFS_CF_SUFREED) {
                        err = nilfs_sufile_cancel_freev(nilfs->ns_sufile,
                                                        sci->sc_freesegs,
                                                        sci->sc_nfreesegs,
                                                        NULL);
                        WARN_ON(err); /* do not happen */
                        sci->sc_stage.flags &= ~NILFS_CF_SUFREED;
                }

                err = nilfs_segctor_extend_segments(sci, nilfs, nadd);
                if (unlikely(err))
                        return err;

                nadd = min_t(int, nadd << 1, SC_MAX_SEGDELTA);
                sci->sc_stage = prev_stage;
        }
        nilfs_segctor_zeropad_segsum(sci);
        nilfs_segctor_truncate_segments(sci, sci->sc_curseg, nilfs->ns_sufile);
        return 0;

 failed:
        return err;
}

static void nilfs_list_replace_buffer(struct buffer_head *old_bh,
                                      struct buffer_head *new_bh)
{
        BUG_ON(!list_empty(&new_bh->b_assoc_buffers));

        list_replace_init(&old_bh->b_assoc_buffers, &new_bh->b_assoc_buffers);
        /* The caller must release old_bh */
}

static int
nilfs_segctor_update_payload_blocknr(struct nilfs_sc_info *sci,
                                     struct nilfs_segment_buffer *segbuf,
                                     int mode)
{
        struct inode *inode = NULL;
        sector_t blocknr;
        unsigned long nfinfo = segbuf->sb_sum.nfinfo;
        unsigned long nblocks = 0, ndatablk = 0;
        const struct nilfs_sc_operations *sc_op = NULL;
        struct nilfs_segsum_pointer ssp;
        struct nilfs_finfo *finfo = NULL;
        union nilfs_binfo binfo;
        struct buffer_head *bh, *bh_org;
        ino_t ino = 0;
        int err = 0;

        if (!nfinfo)
                goto out;

        blocknr = segbuf->sb_pseg_start + segbuf->sb_sum.nsumblk;
        ssp.bh = NILFS_SEGBUF_FIRST_BH(&segbuf->sb_segsum_buffers);
        ssp.offset = sizeof(struct nilfs_segment_summary);

        list_for_each_entry(bh, &segbuf->sb_payload_buffers, b_assoc_buffers) {
                if (bh == segbuf->sb_super_root)
                        break;
                if (!finfo) {
                        finfo = nilfs_segctor_map_segsum_entry(
                                sci, &ssp, sizeof(*finfo));
                        ino = le64_to_cpu(finfo->fi_ino);
                        nblocks = le32_to_cpu(finfo->fi_nblocks);
                        ndatablk = le32_to_cpu(finfo->fi_ndatablk);

                        inode = bh->b_folio->mapping->host;

                        if (mode == SC_LSEG_DSYNC)
                                sc_op = &nilfs_sc_dsync_ops;
                        else if (ino == NILFS_DAT_INO)
                                sc_op = &nilfs_sc_dat_ops;
                        else /* file blocks */
                                sc_op = &nilfs_sc_file_ops;
                }
                bh_org = bh;
                get_bh(bh_org);
                err = nilfs_bmap_assign(NILFS_I(inode)->i_bmap, &bh, blocknr,
                                        &binfo);
                if (bh != bh_org)
                        nilfs_list_replace_buffer(bh_org, bh);
                brelse(bh_org);
                if (unlikely(err))
                        goto failed_bmap;

                if (ndatablk > 0)
                        sc_op->write_data_binfo(sci, &ssp, &binfo);
                else
                        sc_op->write_node_binfo(sci, &ssp, &binfo);

                blocknr++;
                if (--nblocks == 0) {
                        finfo = NULL;
                        if (--nfinfo == 0)
                                break;
                } else if (ndatablk > 0)
                        ndatablk--;
        }
 out:
        return 0;

 failed_bmap:
        return err;
}

static int nilfs_segctor_assign(struct nilfs_sc_info *sci, int mode)
{
        struct nilfs_segment_buffer *segbuf;
        int err;

        list_for_each_entry(segbuf, &sci->sc_segbufs, sb_list) {
                err = nilfs_segctor_update_payload_blocknr(sci, segbuf, mode);
                if (unlikely(err))
                        return err;
                nilfs_segbuf_fill_in_segsum(segbuf);
        }
        return 0;
}

static void nilfs_begin_folio_io(struct folio *folio)
{
        if (!folio || folio_test_writeback(folio))
                /*
                 * For split b-tree node pages, this function may be called
                 * twice.  We ignore the 2nd or later calls by this check.
                 */
                return;

        folio_lock(folio);
        folio_clear_dirty_for_io(folio);
        folio_start_writeback(folio);
        folio_unlock(folio);
}

/**
 * nilfs_prepare_write_logs - prepare to write logs
 * @logs: logs to prepare for writing
 * @seed: checksum seed value
 *
 * nilfs_prepare_write_logs() adds checksums and prepares the block
 * buffers/folios for writing logs.  In order to stabilize folios of
 * memory-mapped file blocks by putting them in writeback state before
 * calculating the checksums, first prepare to write payload blocks other
 * than segment summary and super root blocks in which the checksums will
 * be embedded.
 */
static void nilfs_prepare_write_logs(struct list_head *logs, u32 seed)
{
        struct nilfs_segment_buffer *segbuf;
        struct folio *bd_folio = NULL, *fs_folio = NULL;
        struct buffer_head *bh;

        /* Prepare to write payload blocks */
        list_for_each_entry(segbuf, logs, sb_list) {
                list_for_each_entry(bh, &segbuf->sb_payload_buffers,
                                    b_assoc_buffers) {
                        if (bh == segbuf->sb_super_root)
                                break;
                        set_buffer_async_write(bh);
                        if (bh->b_folio != fs_folio) {
                                nilfs_begin_folio_io(fs_folio);
                                fs_folio = bh->b_folio;
                        }
                }
        }
        nilfs_begin_folio_io(fs_folio);

        nilfs_add_checksums_on_logs(logs, seed);

        /* Prepare to write segment summary blocks */
        list_for_each_entry(segbuf, logs, sb_list) {
                list_for_each_entry(bh, &segbuf->sb_segsum_buffers,
                                    b_assoc_buffers) {
                        mark_buffer_dirty(bh);
                        if (bh->b_folio == bd_folio)
                                continue;
                        if (bd_folio) {
                                folio_lock(bd_folio);
                                folio_wait_writeback(bd_folio);
                                folio_clear_dirty_for_io(bd_folio);
                                folio_start_writeback(bd_folio);
                                folio_unlock(bd_folio);
                        }
                        bd_folio = bh->b_folio;
                }
        }

        /* Prepare to write super root block */
        bh = NILFS_LAST_SEGBUF(logs)->sb_super_root;
        if (bh) {
                mark_buffer_dirty(bh);
                if (bh->b_folio != bd_folio) {
                        folio_lock(bd_folio);
                        folio_wait_writeback(bd_folio);
                        folio_clear_dirty_for_io(bd_folio);
                        folio_start_writeback(bd_folio);
                        folio_unlock(bd_folio);
                        bd_folio = bh->b_folio;
                }
        }

        if (bd_folio) {
                folio_lock(bd_folio);
                folio_wait_writeback(bd_folio);
                folio_clear_dirty_for_io(bd_folio);
                folio_start_writeback(bd_folio);
                folio_unlock(bd_folio);
        }
}

static int nilfs_segctor_write(struct nilfs_sc_info *sci,
                               struct the_nilfs *nilfs)
{
        int ret;

        ret = nilfs_write_logs(&sci->sc_segbufs, nilfs);
        list_splice_tail_init(&sci->sc_segbufs, &sci->sc_write_logs);
        return ret;
}

static void nilfs_end_folio_io(struct folio *folio, int err)
{
        if (!folio)
                return;

        if (buffer_nilfs_node(folio_buffers(folio)) &&
                        !folio_test_writeback(folio)) {
                /*
                 * For b-tree node pages, this function may be called twice
                 * or more because they might be split in a segment.
                 */
                if (folio_test_dirty(folio)) {
                        /*
                         * For pages holding split b-tree node buffers, dirty
                         * flag on the buffers may be cleared discretely.
                         * In that case, the page is once redirtied for
                         * remaining buffers, and it must be cancelled if
                         * all the buffers get cleaned later.
                         */
                        folio_lock(folio);
                        if (nilfs_folio_buffers_clean(folio))
                                __nilfs_clear_folio_dirty(folio);
                        folio_unlock(folio);
                }
                return;
        }

        if (err || !nilfs_folio_buffers_clean(folio))
                filemap_dirty_folio(folio->mapping, folio);

        folio_end_writeback(folio);
}

static void nilfs_abort_logs(struct list_head *logs, int err)
{
        struct nilfs_segment_buffer *segbuf;
        struct folio *bd_folio = NULL, *fs_folio = NULL;
        struct buffer_head *bh;

        if (list_empty(logs))
                return;

        list_for_each_entry(segbuf, logs, sb_list) {
                list_for_each_entry(bh, &segbuf->sb_segsum_buffers,
                                    b_assoc_buffers) {
                        clear_buffer_uptodate(bh);
                        if (bh->b_folio != bd_folio) {
                                if (bd_folio)
                                        folio_end_writeback(bd_folio);
                                bd_folio = bh->b_folio;
                        }
                }

                list_for_each_entry(bh, &segbuf->sb_payload_buffers,
                                    b_assoc_buffers) {
                        if (bh == segbuf->sb_super_root) {
                                clear_buffer_uptodate(bh);
                                if (bh->b_folio != bd_folio) {
                                        folio_end_writeback(bd_folio);
                                        bd_folio = bh->b_folio;
                                }
                                break;
                        }
                        clear_buffer_async_write(bh);
                        if (bh->b_folio != fs_folio) {
                                nilfs_end_folio_io(fs_folio, err);
                                fs_folio = bh->b_folio;
                        }
                }
        }
        if (bd_folio)
                folio_end_writeback(bd_folio);

        nilfs_end_folio_io(fs_folio, err);
}

static void nilfs_segctor_abort_construction(struct nilfs_sc_info *sci,
                                             struct the_nilfs *nilfs, int err)
{
        LIST_HEAD(logs);
        int ret;

        list_splice_tail_init(&sci->sc_write_logs, &logs);
        ret = nilfs_wait_on_logs(&logs);
        nilfs_abort_logs(&logs, ret ? : err);

        list_splice_tail_init(&sci->sc_segbufs, &logs);
        if (list_empty(&logs))
                return; /* if the first segment buffer preparation failed */

        nilfs_cancel_segusage(&logs, nilfs->ns_sufile);
        nilfs_free_incomplete_logs(&logs, nilfs);

        if (sci->sc_stage.flags & NILFS_CF_SUFREED) {
                ret = nilfs_sufile_cancel_freev(nilfs->ns_sufile,
                                                sci->sc_freesegs,
                                                sci->sc_nfreesegs,
                                                NULL);
                WARN_ON(ret); /* do not happen */
        }

        nilfs_destroy_logs(&logs);
}

static void nilfs_set_next_segment(struct the_nilfs *nilfs,
                                   struct nilfs_segment_buffer *segbuf)
{
        nilfs->ns_segnum = segbuf->sb_segnum;
        nilfs->ns_nextnum = segbuf->sb_nextnum;
        nilfs->ns_pseg_offset = segbuf->sb_pseg_start - segbuf->sb_fseg_start
                + segbuf->sb_sum.nblocks;
        nilfs->ns_seg_seq = segbuf->sb_sum.seg_seq;
        nilfs->ns_ctime = segbuf->sb_sum.ctime;
}

static void nilfs_segctor_complete_write(struct nilfs_sc_info *sci)
{
        struct nilfs_segment_buffer *segbuf;
        struct folio *bd_folio = NULL, *fs_folio = NULL;
        struct the_nilfs *nilfs = sci->sc_super->s_fs_info;
        int update_sr = false;

        list_for_each_entry(segbuf, &sci->sc_write_logs, sb_list) {
                struct buffer_head *bh;

                list_for_each_entry(bh, &segbuf->sb_segsum_buffers,
                                    b_assoc_buffers) {
                        set_buffer_uptodate(bh);
                        clear_buffer_dirty(bh);
                        if (bh->b_folio != bd_folio) {
                                if (bd_folio)
                                        folio_end_writeback(bd_folio);
                                bd_folio = bh->b_folio;
                        }
                }
                /*
                 * We assume that the buffers which belong to the same folio
                 * continue over the buffer list.
                 * Under this assumption, the last BHs of folios is
                 * identifiable by the discontinuity of bh->b_folio
                 * (folio != fs_folio).
                 *
                 * For B-tree node blocks, however, this assumption is not
                 * guaranteed.  The cleanup code of B-tree node folios needs
                 * special care.
                 */
                list_for_each_entry(bh, &segbuf->sb_payload_buffers,
                                    b_assoc_buffers) {
                        const unsigned long set_bits = BIT(BH_Uptodate);
                        const unsigned long clear_bits =
                                (BIT(BH_Dirty) | BIT(BH_Async_Write) |
                                 BIT(BH_Delay) | BIT(BH_NILFS_Volatile) |
                                 BIT(BH_NILFS_Redirected));

                        if (bh == segbuf->sb_super_root) {
                                set_buffer_uptodate(bh);
                                clear_buffer_dirty(bh);
                                if (bh->b_folio != bd_folio) {
                                        folio_end_writeback(bd_folio);
                                        bd_folio = bh->b_folio;
                                }
                                update_sr = true;
                                break;
                        }
                        set_mask_bits(&bh->b_state, clear_bits, set_bits);
                        if (bh->b_folio != fs_folio) {
                                nilfs_end_folio_io(fs_folio, 0);
                                fs_folio = bh->b_folio;
                        }
                }

                if (!nilfs_segbuf_simplex(segbuf)) {
                        if (segbuf->sb_sum.flags & NILFS_SS_LOGBGN) {
                                set_bit(NILFS_SC_UNCLOSED, &sci->sc_flags);
                                sci->sc_lseg_stime = jiffies;
                        }
                        if (segbuf->sb_sum.flags & NILFS_SS_LOGEND)
                                clear_bit(NILFS_SC_UNCLOSED, &sci->sc_flags);
                }
        }
        /*
         * Since folios may continue over multiple segment buffers,
         * end of the last folio must be checked outside of the loop.
         */
        if (bd_folio)
                folio_end_writeback(bd_folio);

        nilfs_end_folio_io(fs_folio, 0);

        nilfs_drop_collected_inodes(&sci->sc_dirty_files);

        if (nilfs_doing_gc())
                nilfs_drop_collected_inodes(&sci->sc_gc_inodes);
        else
                nilfs->ns_nongc_ctime = sci->sc_seg_ctime;

        sci->sc_nblk_inc += sci->sc_nblk_this_inc;

        segbuf = NILFS_LAST_SEGBUF(&sci->sc_write_logs);
        nilfs_set_next_segment(nilfs, segbuf);

        if (update_sr) {
                nilfs->ns_flushed_device = 0;
                nilfs_set_last_segment(nilfs, segbuf->sb_pseg_start,
                                       segbuf->sb_sum.seg_seq, nilfs->ns_cno++);

                clear_bit(NILFS_SC_HAVE_DELTA, &sci->sc_flags);
                clear_bit(NILFS_SC_DIRTY, &sci->sc_flags);
                set_bit(NILFS_SC_SUPER_ROOT, &sci->sc_flags);
                nilfs_segctor_clear_metadata_dirty(sci);
        } else
                clear_bit(NILFS_SC_SUPER_ROOT, &sci->sc_flags);
}

static int nilfs_segctor_wait(struct nilfs_sc_info *sci)
{
        int ret;

        ret = nilfs_wait_on_logs(&sci->sc_write_logs);
        if (!ret) {
                nilfs_segctor_complete_write(sci);
                nilfs_destroy_logs(&sci->sc_write_logs);
        }
        return ret;
}

static int nilfs_segctor_collect_dirty_files(struct nilfs_sc_info *sci,
                                             struct the_nilfs *nilfs)
{
        struct nilfs_inode_info *ii, *n;
        struct inode *ifile = sci->sc_root->ifile;

        spin_lock(&nilfs->ns_inode_lock);
 retry:
        list_for_each_entry_safe(ii, n, &nilfs->ns_dirty_files, i_dirty) {
                if (!ii->i_bh) {
                        struct buffer_head *ibh;
                        int err;

                        spin_unlock(&nilfs->ns_inode_lock);
                        err = nilfs_ifile_get_inode_block(
                                ifile, ii->vfs_inode.i_ino, &ibh);
                        if (unlikely(err)) {
                                nilfs_warn(sci->sc_super,
                                           "log writer: error %d getting inode block (ino=%lu)",
                                           err, ii->vfs_inode.i_ino);
                                return err;
                        }
                        spin_lock(&nilfs->ns_inode_lock);
                        if (likely(!ii->i_bh))
                                ii->i_bh = ibh;
                        else
                                brelse(ibh);
                        goto retry;
                }

                // Always redirty the buffer to avoid race condition
                mark_buffer_dirty(ii->i_bh);
                nilfs_mdt_mark_dirty(ifile);

                clear_bit(NILFS_I_QUEUED, &ii->i_state);
                set_bit(NILFS_I_BUSY, &ii->i_state);
                list_move_tail(&ii->i_dirty, &sci->sc_dirty_files);
        }
        spin_unlock(&nilfs->ns_inode_lock);

        return 0;
}

static void nilfs_segctor_drop_written_files(struct nilfs_sc_info *sci,
                                             struct the_nilfs *nilfs)
{
        struct nilfs_inode_info *ii, *n;
        int during_mount = !(sci->sc_super->s_flags & SB_ACTIVE);
        int defer_iput = false;

        spin_lock(&nilfs->ns_inode_lock);
        list_for_each_entry_safe(ii, n, &sci->sc_dirty_files, i_dirty) {
                if (!test_and_clear_bit(NILFS_I_UPDATED, &ii->i_state) ||
                    test_bit(NILFS_I_DIRTY, &ii->i_state))
                        continue;

                clear_bit(NILFS_I_BUSY, &ii->i_state);
                brelse(ii->i_bh);
                ii->i_bh = NULL;
                list_del_init(&ii->i_dirty);
                if (!ii->vfs_inode.i_nlink || during_mount) {
                        /*
                         * Defer calling iput() to avoid deadlocks if
                         * i_nlink == 0 or mount is not yet finished.
                         */
                        list_add_tail(&ii->i_dirty, &sci->sc_iput_queue);
                        defer_iput = true;
                } else {
                        spin_unlock(&nilfs->ns_inode_lock);
                        iput(&ii->vfs_inode);
                        spin_lock(&nilfs->ns_inode_lock);
                }
        }
        spin_unlock(&nilfs->ns_inode_lock);

        if (defer_iput)
                schedule_work(&sci->sc_iput_work);
}

/*
 * Main procedure of segment constructor
 */
static int nilfs_segctor_do_construct(struct nilfs_sc_info *sci, int mode)
{
        struct the_nilfs *nilfs = sci->sc_super->s_fs_info;
        int err;

        if (sb_rdonly(sci->sc_super))
                return -EROFS;

        nilfs_sc_cstage_set(sci, NILFS_ST_INIT);
        sci->sc_cno = nilfs->ns_cno;

        err = nilfs_segctor_collect_dirty_files(sci, nilfs);
        if (unlikely(err))
                goto out;

        if (nilfs_test_metadata_dirty(nilfs, sci->sc_root))
                set_bit(NILFS_SC_DIRTY, &sci->sc_flags);

        if (nilfs_segctor_clean(sci))
                goto out;

        do {
                sci->sc_stage.flags &= ~NILFS_CF_HISTORY_MASK;

                err = nilfs_segctor_begin_construction(sci, nilfs);
                if (unlikely(err))
                        goto failed;

                /* Update time stamp */
                sci->sc_seg_ctime = ktime_get_real_seconds();

                err = nilfs_segctor_collect(sci, nilfs, mode);
                if (unlikely(err))
                        goto failed;

                /* Avoid empty segment */
                if (nilfs_sc_cstage_get(sci) == NILFS_ST_DONE &&
                    nilfs_segbuf_empty(sci->sc_curseg)) {
                        nilfs_segctor_abort_construction(sci, nilfs, 1);
                        goto out;
                }

                err = nilfs_segctor_assign(sci, mode);
                if (unlikely(err))
                        goto failed;

                if (sci->sc_stage.flags & NILFS_CF_IFILE_STARTED)
                        nilfs_segctor_fill_in_file_bmap(sci);

                if (mode == SC_LSEG_SR &&
                    nilfs_sc_cstage_get(sci) >= NILFS_ST_CPFILE) {
                        err = nilfs_cpfile_finalize_checkpoint(
                                nilfs->ns_cpfile, nilfs->ns_cno, sci->sc_root,
                                sci->sc_nblk_inc + sci->sc_nblk_this_inc,
                                sci->sc_seg_ctime,
                                !test_bit(NILFS_SC_HAVE_DELTA, &sci->sc_flags));
                        if (unlikely(err))
                                goto failed_to_write;

                        nilfs_segctor_fill_in_super_root(sci, nilfs);
                }
                nilfs_segctor_update_segusage(sci, nilfs->ns_sufile);

                /* Write partial segments */
                nilfs_prepare_write_logs(&sci->sc_segbufs, nilfs->ns_crc_seed);

                err = nilfs_segctor_write(sci, nilfs);
                if (unlikely(err))
                        goto failed_to_write;

                if (nilfs_sc_cstage_get(sci) == NILFS_ST_DONE ||
                    nilfs->ns_blocksize_bits != PAGE_SHIFT) {
                        /*
                         * At this point, we avoid double buffering
                         * for blocksize < pagesize because page dirty
                         * flag is turned off during write and dirty
                         * buffers are not properly collected for
                         * pages crossing over segments.
                         */
                        err = nilfs_segctor_wait(sci);
                        if (err)
                                goto failed_to_write;
                }
        } while (nilfs_sc_cstage_get(sci) != NILFS_ST_DONE);

 out:
        nilfs_segctor_drop_written_files(sci, nilfs);
        return err;

 failed_to_write:
 failed:
        if (mode == SC_LSEG_SR && nilfs_sc_cstage_get(sci) >= NILFS_ST_IFILE)
                nilfs_redirty_inodes(&sci->sc_dirty_files);
        if (nilfs_doing_gc())
                nilfs_redirty_inodes(&sci->sc_gc_inodes);
        nilfs_segctor_abort_construction(sci, nilfs, err);
        goto out;
}

/**
 * nilfs_segctor_start_timer - set timer of background write
 * @sci: nilfs_sc_info
 *
 * If the timer has already been set, it ignores the new request.
 * This function MUST be called within a section locking the segment
 * semaphore.
 */
static void nilfs_segctor_start_timer(struct nilfs_sc_info *sci)
{
        spin_lock(&sci->sc_state_lock);
        if (!(sci->sc_state & NILFS_SEGCTOR_COMMIT)) {
                if (sci->sc_task) {
                        sci->sc_timer.expires = jiffies + sci->sc_interval;
                        add_timer(&sci->sc_timer);
                }
                sci->sc_state |= NILFS_SEGCTOR_COMMIT;
        }
        spin_unlock(&sci->sc_state_lock);
}

static void nilfs_segctor_do_flush(struct nilfs_sc_info *sci, int bn)
{
        spin_lock(&sci->sc_state_lock);
        if (!(sci->sc_flush_request & BIT(bn))) {
                unsigned long prev_req = sci->sc_flush_request;

                sci->sc_flush_request |= BIT(bn);
                if (!prev_req)
                        wake_up(&sci->sc_wait_daemon);
        }
        spin_unlock(&sci->sc_state_lock);
}

struct nilfs_segctor_wait_request {
        wait_queue_entry_t      wq;
        __u32           seq;
        int             err;
        atomic_t        done;
};

static int nilfs_segctor_sync(struct nilfs_sc_info *sci)
{
        struct nilfs_segctor_wait_request wait_req;
        int err = 0;

        init_wait(&wait_req.wq);
        wait_req.err = 0;
        atomic_set(&wait_req.done, 0);
        init_waitqueue_entry(&wait_req.wq, current);

        /*
         * To prevent a race issue where completion notifications from the
         * log writer thread are missed, increment the request sequence count
         * "sc_seq_request" and insert a wait queue entry using the current
         * sequence number into the "sc_wait_request" queue at the same time
         * within the lock section of "sc_state_lock".
         */
        spin_lock(&sci->sc_state_lock);
        wait_req.seq = ++sci->sc_seq_request;
        add_wait_queue(&sci->sc_wait_request, &wait_req.wq);
        spin_unlock(&sci->sc_state_lock);

        wake_up(&sci->sc_wait_daemon);

        for (;;) {
                set_current_state(TASK_INTERRUPTIBLE);

                /*
                 * Synchronize only while the log writer thread is alive.
                 * Leave flushing out after the log writer thread exits to
                 * the cleanup work in nilfs_segctor_destroy().
                 */
                if (!sci->sc_task)
                        break;

                if (atomic_read(&wait_req.done)) {
                        err = wait_req.err;
                        break;
                }
                if (!signal_pending(current)) {
                        schedule();
                        continue;
                }
                err = -ERESTARTSYS;
                break;
        }
        finish_wait(&sci->sc_wait_request, &wait_req.wq);
        return err;
}

static void nilfs_segctor_wakeup(struct nilfs_sc_info *sci, int err, bool force)
{
        struct nilfs_segctor_wait_request *wrq, *n;
        unsigned long flags;

        spin_lock_irqsave(&sci->sc_wait_request.lock, flags);
        list_for_each_entry_safe(wrq, n, &sci->sc_wait_request.head, wq.entry) {
                if (!atomic_read(&wrq->done) &&
                    (force || nilfs_cnt32_ge(sci->sc_seq_done, wrq->seq))) {
                        wrq->err = err;
                        atomic_set(&wrq->done, 1);
                }
                if (atomic_read(&wrq->done)) {
                        wrq->wq.func(&wrq->wq,
                                     TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE,
                                     0, NULL);
                }
        }
        spin_unlock_irqrestore(&sci->sc_wait_request.lock, flags);
}

/**
 * nilfs_construct_segment - construct a logical segment
 * @sb: super block
 *
 * Return: 0 on success, or one of the following negative error codes on
 * failure:
 * * %-EIO              - I/O error (including metadata corruption).
 * * %-ENOMEM           - Insufficient memory available.
 * * %-ENOSPC           - No space left on device (only in a panic state).
 * * %-ERESTARTSYS      - Interrupted.
 * * %-EROFS            - Read only filesystem.
 */
int nilfs_construct_segment(struct super_block *sb)
{
        struct the_nilfs *nilfs = sb->s_fs_info;
        struct nilfs_sc_info *sci = nilfs->ns_writer;
        struct nilfs_transaction_info *ti;

        if (sb_rdonly(sb) || unlikely(!sci))
                return -EROFS;

        /* A call inside transactions causes a deadlock. */
        BUG_ON((ti = current->journal_info) && ti->ti_magic == NILFS_TI_MAGIC);

        return nilfs_segctor_sync(sci);
}

/**
 * nilfs_construct_dsync_segment - construct a data-only logical segment
 * @sb: super block
 * @inode: inode whose data blocks should be written out
 * @start: start byte offset
 * @end: end byte offset (inclusive)
 *
 * Return: 0 on success, or one of the following negative error codes on
 * failure:
 * * %-EIO              - I/O error (including metadata corruption).
 * * %-ENOMEM           - Insufficient memory available.
 * * %-ENOSPC           - No space left on device (only in a panic state).
 * * %-ERESTARTSYS      - Interrupted.
 * * %-EROFS            - Read only filesystem.
 */
int nilfs_construct_dsync_segment(struct super_block *sb, struct inode *inode,
                                  loff_t start, loff_t end)
{
        struct the_nilfs *nilfs = sb->s_fs_info;
        struct nilfs_sc_info *sci = nilfs->ns_writer;
        struct nilfs_inode_info *ii;
        struct nilfs_transaction_info ti;
        int err = 0;

        if (sb_rdonly(sb) || unlikely(!sci))
                return -EROFS;

        nilfs_transaction_lock(sb, &ti, 0);

        ii = NILFS_I(inode);
        if (test_bit(NILFS_I_INODE_SYNC, &ii->i_state) ||
            nilfs_test_opt(nilfs, STRICT_ORDER) ||
            test_bit(NILFS_SC_UNCLOSED, &sci->sc_flags) ||
            nilfs_discontinued(nilfs)) {
                nilfs_transaction_unlock(sb);
                err = nilfs_segctor_sync(sci);
                return err;
        }

        spin_lock(&nilfs->ns_inode_lock);
        if (!test_bit(NILFS_I_QUEUED, &ii->i_state) &&
            !test_bit(NILFS_I_BUSY, &ii->i_state)) {
                spin_unlock(&nilfs->ns_inode_lock);
                nilfs_transaction_unlock(sb);
                return 0;
        }
        spin_unlock(&nilfs->ns_inode_lock);
        sci->sc_dsync_inode = ii;
        sci->sc_dsync_start = start;
        sci->sc_dsync_end = end;

        err = nilfs_segctor_do_construct(sci, SC_LSEG_DSYNC);
        if (!err)
                nilfs->ns_flushed_device = 0;

        nilfs_transaction_unlock(sb);
        return err;
}

#define FLUSH_FILE_BIT  (0x1) /* data file only */
#define FLUSH_DAT_BIT   BIT(NILFS_DAT_INO) /* DAT only */

/**
 * nilfs_segctor_accept - record accepted sequence count of log-write requests
 * @sci: segment constructor object
 */
static void nilfs_segctor_accept(struct nilfs_sc_info *sci)
{
        bool thread_is_alive;

        spin_lock(&sci->sc_state_lock);
        sci->sc_seq_accepted = sci->sc_seq_request;
        thread_is_alive = (bool)sci->sc_task;
        spin_unlock(&sci->sc_state_lock);

        /*
         * This function does not race with the log writer thread's
         * termination.  Therefore, deleting sc_timer, which should not be
         * done after the log writer thread exits, can be done safely outside
         * the area protected by sc_state_lock.
         */
        if (thread_is_alive)
                timer_delete_sync(&sci->sc_timer);
}

/**
 * nilfs_segctor_notify - notify the result of request to caller threads
 * @sci: segment constructor object
 * @mode: mode of log forming
 * @err: error code to be notified
 */
static void nilfs_segctor_notify(struct nilfs_sc_info *sci, int mode, int err)
{
        /* Clear requests (even when the construction failed) */
        spin_lock(&sci->sc_state_lock);

        if (mode == SC_LSEG_SR) {
                sci->sc_state &= ~NILFS_SEGCTOR_COMMIT;
                sci->sc_seq_done = sci->sc_seq_accepted;
                nilfs_segctor_wakeup(sci, err, false);
                sci->sc_flush_request = 0;
        } else {
                if (mode == SC_FLUSH_FILE)
                        sci->sc_flush_request &= ~FLUSH_FILE_BIT;
                else if (mode == SC_FLUSH_DAT)
                        sci->sc_flush_request &= ~FLUSH_DAT_BIT;

                /* re-enable timer if checkpoint creation was not done */
                if ((sci->sc_state & NILFS_SEGCTOR_COMMIT) && sci->sc_task &&
                    time_before(jiffies, sci->sc_timer.expires))
                        add_timer(&sci->sc_timer);
        }
        spin_unlock(&sci->sc_state_lock);
}

/**
 * nilfs_segctor_construct - form logs and write them to disk
 * @sci: segment constructor object
 * @mode: mode of log forming
 *
 * Return: 0 on success, or a negative error code on failure.
 */
static int nilfs_segctor_construct(struct nilfs_sc_info *sci, int mode)
{
        struct the_nilfs *nilfs = sci->sc_super->s_fs_info;
        struct nilfs_super_block **sbp;
        int err = 0;

        nilfs_segctor_accept(sci);

        if (nilfs_discontinued(nilfs))
                mode = SC_LSEG_SR;
        if (!nilfs_segctor_confirm(sci))
                err = nilfs_segctor_do_construct(sci, mode);

        if (likely(!err)) {
                if (mode != SC_FLUSH_DAT)
                        atomic_set(&nilfs->ns_ndirtyblks, 0);
                if (test_bit(NILFS_SC_SUPER_ROOT, &sci->sc_flags) &&
                    nilfs_discontinued(nilfs)) {
                        down_write(&nilfs->ns_sem);
                        err = -EIO;
                        sbp = nilfs_prepare_super(sci->sc_super,
                                                  nilfs_sb_will_flip(nilfs));
                        if (likely(sbp)) {
                                nilfs_set_log_cursor(sbp[0], nilfs);
                                err = nilfs_commit_super(sci->sc_super,
                                                         NILFS_SB_COMMIT);
                        }
                        up_write(&nilfs->ns_sem);
                }
        }

        nilfs_segctor_notify(sci, mode, err);
        return err;
}

static void nilfs_construction_timeout(struct timer_list *t)
{
        struct nilfs_sc_info *sci = timer_container_of(sci, t, sc_timer);

        wake_up_process(sci->sc_task);
}

static void
nilfs_remove_written_gcinodes(struct the_nilfs *nilfs, struct list_head *head)
{
        struct nilfs_inode_info *ii, *n;

        list_for_each_entry_safe(ii, n, head, i_dirty) {
                if (!test_bit(NILFS_I_UPDATED, &ii->i_state))
                        continue;
                list_del_init(&ii->i_dirty);
                truncate_inode_pages(&ii->vfs_inode.i_data, 0);
                nilfs_btnode_cache_clear(ii->i_assoc_inode->i_mapping);
                iput(&ii->vfs_inode);
        }
}

int nilfs_clean_segments(struct super_block *sb, struct nilfs_argv *argv,
                         void **kbufs)
{
        struct the_nilfs *nilfs = sb->s_fs_info;
        struct nilfs_sc_info *sci = nilfs->ns_writer;
        struct nilfs_transaction_info ti;
        int err;

        if (unlikely(!sci))
                return -EROFS;

        nilfs_transaction_lock(sb, &ti, 1);

        err = nilfs_mdt_save_to_shadow_map(nilfs->ns_dat);
        if (unlikely(err))
                goto out_unlock;

        err = nilfs_ioctl_prepare_clean_segments(nilfs, argv, kbufs);
        if (unlikely(err)) {
                nilfs_mdt_restore_from_shadow_map(nilfs->ns_dat);
                goto out_unlock;
        }

        sci->sc_freesegs = kbufs[4];
        sci->sc_nfreesegs = argv[4].v_nmembs;
        list_splice_tail_init(&nilfs->ns_gc_inodes, &sci->sc_gc_inodes);

        for (;;) {
                err = nilfs_segctor_construct(sci, SC_LSEG_SR);
                nilfs_remove_written_gcinodes(nilfs, &sci->sc_gc_inodes);

                if (likely(!err))
                        break;

                nilfs_warn(sb, "error %d cleaning segments", err);
                set_current_state(TASK_INTERRUPTIBLE);
                schedule_timeout(sci->sc_interval);
        }
        if (nilfs_test_opt(nilfs, DISCARD)) {
                int ret = nilfs_discard_segments(nilfs, sci->sc_freesegs,
                                                 sci->sc_nfreesegs);
                if (ret) {
                        nilfs_warn(sb,
                                   "error %d on discard request, turning discards off for the device",
                                   ret);
                        nilfs_clear_opt(nilfs, DISCARD);
                }
        }

 out_unlock:
        sci->sc_freesegs = NULL;
        sci->sc_nfreesegs = 0;
        nilfs_mdt_clear_shadow_map(nilfs->ns_dat);
        nilfs_transaction_unlock(sb);
        return err;
}

static void nilfs_segctor_thread_construct(struct nilfs_sc_info *sci, int mode)
{
        struct nilfs_transaction_info ti;

        nilfs_transaction_lock(sci->sc_super, &ti, 0);
        nilfs_segctor_construct(sci, mode);

        /*
         * Unclosed segment should be retried.  We do this using sc_timer.
         * Timeout of sc_timer will invoke complete construction which leads
         * to close the current logical segment.
         */
        if (test_bit(NILFS_SC_UNCLOSED, &sci->sc_flags))
                nilfs_segctor_start_timer(sci);

        nilfs_transaction_unlock(sci->sc_super);
}

static void nilfs_segctor_do_immediate_flush(struct nilfs_sc_info *sci)
{
        int mode = 0;

        spin_lock(&sci->sc_state_lock);
        mode = (sci->sc_flush_request & FLUSH_DAT_BIT) ?
                SC_FLUSH_DAT : SC_FLUSH_FILE;
        spin_unlock(&sci->sc_state_lock);

        if (mode) {
                nilfs_segctor_do_construct(sci, mode);

                spin_lock(&sci->sc_state_lock);
                sci->sc_flush_request &= (mode == SC_FLUSH_FILE) ?
                        ~FLUSH_FILE_BIT : ~FLUSH_DAT_BIT;
                spin_unlock(&sci->sc_state_lock);
        }
        clear_bit(NILFS_SC_PRIOR_FLUSH, &sci->sc_flags);
}

static int nilfs_segctor_flush_mode(struct nilfs_sc_info *sci)
{
        if (!test_bit(NILFS_SC_UNCLOSED, &sci->sc_flags) ||
            time_before(jiffies, sci->sc_lseg_stime + sci->sc_mjcp_freq)) {
                if (!(sci->sc_flush_request & ~FLUSH_FILE_BIT))
                        return SC_FLUSH_FILE;
                else if (!(sci->sc_flush_request & ~FLUSH_DAT_BIT))
                        return SC_FLUSH_DAT;
        }
        return SC_LSEG_SR;
}

/**
 * nilfs_log_write_required - determine whether log writing is required
 * @sci:   nilfs_sc_info struct
 * @modep: location for storing log writing mode
 *
 * Return: true if log writing is required, false otherwise.  If log writing
 * is required, the mode is stored in the location pointed to by @modep.
 */
static bool nilfs_log_write_required(struct nilfs_sc_info *sci, int *modep)
{
        bool timedout, ret = true;

        spin_lock(&sci->sc_state_lock);
        timedout = ((sci->sc_state & NILFS_SEGCTOR_COMMIT) &&
                   time_after_eq(jiffies, sci->sc_timer.expires));
        if (timedout || sci->sc_seq_request != sci->sc_seq_done)
                *modep = SC_LSEG_SR;
        else if (sci->sc_flush_request)
                *modep = nilfs_segctor_flush_mode(sci);
        else
                ret = false;

        spin_unlock(&sci->sc_state_lock);
        return ret;
}

/**
 * nilfs_segctor_thread - main loop of the log writer thread
 * @arg: pointer to a struct nilfs_sc_info.
 *
 * nilfs_segctor_thread() is the main loop function of the log writer kernel
 * thread, which determines whether log writing is necessary, and if so,
 * performs the log write in the background, or waits if not.  It is also
 * used to decide the background writeback of the superblock.
 *
 * Return: Always 0.
 */
static int nilfs_segctor_thread(void *arg)
{
        struct nilfs_sc_info *sci = (struct nilfs_sc_info *)arg;
        struct the_nilfs *nilfs = sci->sc_super->s_fs_info;

        nilfs_info(sci->sc_super,
                   "segctord starting. Construction interval = %lu seconds, CP frequency < %lu seconds",
                   sci->sc_interval / HZ, sci->sc_mjcp_freq / HZ);

        set_freezable();

        while (!kthread_should_stop()) {
                DEFINE_WAIT(wait);
                bool should_write;
                int mode;

                if (freezing(current)) {
                        try_to_freeze();
                        continue;
                }

                prepare_to_wait(&sci->sc_wait_daemon, &wait,
                                TASK_INTERRUPTIBLE);
                should_write = nilfs_log_write_required(sci, &mode);
                if (!should_write)
                        schedule();
                finish_wait(&sci->sc_wait_daemon, &wait);

                if (nilfs_sb_dirty(nilfs) && nilfs_sb_need_update(nilfs))
                        set_nilfs_discontinued(nilfs);

                if (should_write)
                        nilfs_segctor_thread_construct(sci, mode);
        }

        /* end sync. */
        spin_lock(&sci->sc_state_lock);
        sci->sc_task = NULL;
        timer_shutdown_sync(&sci->sc_timer);
        spin_unlock(&sci->sc_state_lock);
        return 0;
}

/*
 * Setup & clean-up functions
 */
static struct nilfs_sc_info *nilfs_segctor_new(struct super_block *sb,
                                               struct nilfs_root *root)
{
        struct the_nilfs *nilfs = sb->s_fs_info;
        struct nilfs_sc_info *sci;

        sci = kzalloc_obj(*sci);
        if (!sci)
                return NULL;

        sci->sc_super = sb;

        nilfs_get_root(root);
        sci->sc_root = root;

        init_waitqueue_head(&sci->sc_wait_request);
        init_waitqueue_head(&sci->sc_wait_daemon);
        spin_lock_init(&sci->sc_state_lock);
        INIT_LIST_HEAD(&sci->sc_dirty_files);
        INIT_LIST_HEAD(&sci->sc_segbufs);
        INIT_LIST_HEAD(&sci->sc_write_logs);
        INIT_LIST_HEAD(&sci->sc_gc_inodes);
        INIT_LIST_HEAD(&sci->sc_iput_queue);
        INIT_WORK(&sci->sc_iput_work, nilfs_iput_work_func);

        sci->sc_interval = HZ * NILFS_SC_DEFAULT_TIMEOUT;
        sci->sc_mjcp_freq = HZ * NILFS_SC_DEFAULT_SR_FREQ;
        sci->sc_watermark = NILFS_SC_DEFAULT_WATERMARK;

        if (nilfs->ns_interval)
                sci->sc_interval = HZ * nilfs->ns_interval;
        if (nilfs->ns_watermark)
                sci->sc_watermark = nilfs->ns_watermark;
        return sci;
}

static void nilfs_segctor_write_out(struct nilfs_sc_info *sci)
{
        int ret, retrycount = NILFS_SC_CLEANUP_RETRY;

        /*
         * The segctord thread was stopped and its timer was removed.
         * But some tasks remain.
         */
        do {
                struct nilfs_transaction_info ti;

                nilfs_transaction_lock(sci->sc_super, &ti, 0);
                ret = nilfs_segctor_construct(sci, SC_LSEG_SR);
                nilfs_transaction_unlock(sci->sc_super);

                flush_work(&sci->sc_iput_work);

        } while (ret && ret != -EROFS && retrycount-- > 0);
}

/**
 * nilfs_segctor_destroy - destroy the segment constructor.
 * @sci: nilfs_sc_info
 *
 * nilfs_segctor_destroy() kills the segctord thread and frees
 * the nilfs_sc_info struct.
 * Caller must hold the segment semaphore.
 */
static void nilfs_segctor_destroy(struct nilfs_sc_info *sci)
{
        struct the_nilfs *nilfs = sci->sc_super->s_fs_info;
        int flag;

        up_write(&nilfs->ns_segctor_sem);

        if (sci->sc_task) {
                wake_up(&sci->sc_wait_daemon);
                if (kthread_stop(sci->sc_task)) {
                        spin_lock(&sci->sc_state_lock);
                        sci->sc_task = NULL;
                        timer_shutdown_sync(&sci->sc_timer);
                        spin_unlock(&sci->sc_state_lock);
                }
        }

        spin_lock(&sci->sc_state_lock);
        flag = ((sci->sc_state & NILFS_SEGCTOR_COMMIT) || sci->sc_flush_request
                || sci->sc_seq_request != sci->sc_seq_done);
        spin_unlock(&sci->sc_state_lock);

        /*
         * Forcibly wake up tasks waiting in nilfs_segctor_sync(), which can
         * be called from delayed iput() via nilfs_evict_inode() and can race
         * with the above log writer thread termination.
         */
        nilfs_segctor_wakeup(sci, 0, true);

        if (flush_work(&sci->sc_iput_work))
                flag = true;

        if (flag || !nilfs_segctor_confirm(sci))
                nilfs_segctor_write_out(sci);

        if (!list_empty(&sci->sc_dirty_files)) {
                nilfs_warn(sci->sc_super,
                           "disposed unprocessed dirty file(s) when stopping log writer");
                nilfs_dispose_list(nilfs, &sci->sc_dirty_files, 1);
        }

        if (!list_empty(&sci->sc_iput_queue)) {
                nilfs_warn(sci->sc_super,
                           "disposed unprocessed inode(s) in iput queue when stopping log writer");
                nilfs_dispose_list(nilfs, &sci->sc_iput_queue, 1);
        }

        WARN_ON(!list_empty(&sci->sc_segbufs));
        WARN_ON(!list_empty(&sci->sc_write_logs));

        nilfs_put_root(sci->sc_root);

        down_write(&nilfs->ns_segctor_sem);

        kfree(sci);
}

/**
 * nilfs_attach_log_writer - attach log writer
 * @sb: super block instance
 * @root: root object of the current filesystem tree
 *
 * This allocates a log writer object, initializes it, and starts the
 * log writer.
 *
 * Return: 0 on success, or one of the following negative error codes on
 * failure:
 * * %-EINTR    - Log writer thread creation failed due to interruption.
 * * %-ENOMEM   - Insufficient memory available.
 */
int nilfs_attach_log_writer(struct super_block *sb, struct nilfs_root *root)
{
        struct the_nilfs *nilfs = sb->s_fs_info;
        struct nilfs_sc_info *sci;
        struct task_struct *t;
        int err;

        if (nilfs->ns_writer) {
                /*
                 * This happens if the filesystem is made read-only by
                 * __nilfs_error or nilfs_remount and then remounted
                 * read/write.  In these cases, reuse the existing
                 * writer.
                 */
                return 0;
        }

        sci = nilfs_segctor_new(sb, root);
        if (unlikely(!sci))
                return -ENOMEM;

        nilfs->ns_writer = sci;
        t = kthread_create(nilfs_segctor_thread, sci, "segctord");
        if (IS_ERR(t)) {
                err = PTR_ERR(t);
                nilfs_err(sb, "error %d creating segctord thread", err);
                nilfs_detach_log_writer(sb);
                return err;
        }
        sci->sc_task = t;
        timer_setup(&sci->sc_timer, nilfs_construction_timeout, 0);

        wake_up_process(sci->sc_task);
        return 0;
}

/**
 * nilfs_detach_log_writer - destroy log writer
 * @sb: super block instance
 *
 * This kills log writer daemon, frees the log writer object, and
 * destroys list of dirty files.
 */
void nilfs_detach_log_writer(struct super_block *sb)
{
        struct the_nilfs *nilfs = sb->s_fs_info;
        LIST_HEAD(garbage_list);

        down_write(&nilfs->ns_segctor_sem);
        if (nilfs->ns_writer) {
                nilfs_segctor_destroy(nilfs->ns_writer);
                nilfs->ns_writer = NULL;
        }
        set_nilfs_purging(nilfs);

        /* Force to free the list of dirty files */
        spin_lock(&nilfs->ns_inode_lock);
        if (!list_empty(&nilfs->ns_dirty_files)) {
                list_splice_init(&nilfs->ns_dirty_files, &garbage_list);
                nilfs_warn(sb,
                           "disposed unprocessed dirty file(s) when detaching log writer");
        }
        spin_unlock(&nilfs->ns_inode_lock);
        up_write(&nilfs->ns_segctor_sem);

        nilfs_dispose_list(nilfs, &garbage_list, 1);
        clear_nilfs_purging(nilfs);
}