// SPDX-License-Identifier: GPL-2.0
/*
 * fs/mpage.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * Contains functions related to preparing and submitting BIOs which contain
 * multiple pagecache pages.
 *
 * 15May2002    Andrew Morton
 *              Initial version
 * 27Jun2002    axboe@suse.de
 *              use bio_add_page() to build bio's just the right size
 */

#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/mm.h>
#include <linux/kdev_t.h>
#include <linux/gfp.h>
#include <linux/bio.h>
#include <linux/fs.h>
#include <linux/buffer_head.h>
#include <linux/blkdev.h>
#include <linux/highmem.h>
#include <linux/prefetch.h>
#include <linux/mpage.h>
#include <linux/mm_inline.h>
#include <linux/writeback.h>
#include <linux/backing-dev.h>
#include <linux/pagevec.h>
#include "internal.h"

/*
 * I/O completion handler for multipage BIOs.
 *
 * The mpage code never puts partial pages into a BIO (except for end-of-file).
 * If a page does not map to a contiguous run of blocks then it simply falls
 * back to block_read_full_folio().
 *
 * Why is this?  If a page's completion depends on a number of different BIOs
 * which can complete in any order (or at the same time) then determining the
 * status of that page is hard.  See end_buffer_async_read() for the details.
 * There is no point in duplicating all that complexity.
 */
static void mpage_read_end_io(struct bio *bio)
{
        struct folio_iter fi;
        int err = blk_status_to_errno(bio->bi_status);

        bio_for_each_folio_all(fi, bio)
                folio_end_read(fi.folio, err == 0);

        bio_put(bio);
}

static void mpage_write_end_io(struct bio *bio)
{
        struct folio_iter fi;
        int err = blk_status_to_errno(bio->bi_status);

        bio_for_each_folio_all(fi, bio) {
                if (err)
                        mapping_set_error(fi.folio->mapping, err);
                folio_end_writeback(fi.folio);
        }

        bio_put(bio);
}

static struct bio *mpage_bio_submit_read(struct bio *bio)
{
        bio->bi_end_io = mpage_read_end_io;
        guard_bio_eod(bio);
        submit_bio(bio);
        return NULL;
}

static struct bio *mpage_bio_submit_write(struct bio *bio)
{
        bio->bi_end_io = mpage_write_end_io;
        guard_bio_eod(bio);
        submit_bio(bio);
        return NULL;
}

/*
 * support function for mpage_readahead.  The fs supplied get_block might
 * return an up to date buffer.  This is used to map that buffer into
 * the page, which allows read_folio to avoid triggering a duplicate call
 * to get_block.
 *
 * The idea is to avoid adding buffers to pages that don't already have
 * them.  So when the buffer is up to date and the page size == block size,
 * this marks the page up to date instead of adding new buffers.
 */
static void map_buffer_to_folio(struct folio *folio, struct buffer_head *bh,
                int page_block)
{
        struct inode *inode = folio->mapping->host;
        struct buffer_head *page_bh, *head;
        int block = 0;

        head = folio_buffers(folio);
        if (!head) {
                /*
                 * don't make any buffers if there is only one buffer on
                 * the folio and the folio just needs to be set up to date
                 */
                if (inode->i_blkbits == folio_shift(folio) &&
                    buffer_uptodate(bh)) {
                        folio_mark_uptodate(folio);
                        return;
                }
                head = create_empty_buffers(folio, i_blocksize(inode), 0);
        }

        page_bh = head;
        do {
                if (block == page_block) {
                        page_bh->b_state = bh->b_state;
                        page_bh->b_bdev = bh->b_bdev;
                        page_bh->b_blocknr = bh->b_blocknr;
                        break;
                }
                page_bh = page_bh->b_this_page;
                block++;
        } while (page_bh != head);
}

struct mpage_readpage_args {
        struct bio *bio;
        struct folio *folio;
        unsigned int nr_pages;
        bool is_readahead;
        sector_t last_block_in_bio;
        struct buffer_head map_bh;
        unsigned long first_logical_block;
        get_block_t *get_block;
};

/*
 * This is the worker routine which does all the work of mapping the disk
 * blocks and constructs largest possible bios, submits them for IO if the
 * blocks are not contiguous on the disk.
 *
 * We pass a buffer_head back and forth and use its buffer_mapped() flag to
 * represent the validity of its disk mapping and to decide when to do the next
 * get_block() call.
 */
static void do_mpage_readpage(struct mpage_readpage_args *args)
{
        struct folio *folio = args->folio;
        struct inode *inode = folio->mapping->host;
        const unsigned blkbits = inode->i_blkbits;
        const unsigned blocks_per_folio = folio_size(folio) >> blkbits;
        const unsigned blocksize = 1 << blkbits;
        struct buffer_head *map_bh = &args->map_bh;
        sector_t block_in_file;
        sector_t last_block;
        sector_t last_block_in_file;
        sector_t first_block;
        unsigned page_block;
        unsigned first_hole = blocks_per_folio;
        struct block_device *bdev = NULL;
        int length;
        int fully_mapped = 1;
        blk_opf_t opf = REQ_OP_READ;
        unsigned nblocks;
        unsigned relative_block;
        gfp_t gfp = mapping_gfp_constraint(folio->mapping, GFP_KERNEL);

        if (args->is_readahead) {
                opf |= REQ_RAHEAD;
                gfp |= __GFP_NORETRY | __GFP_NOWARN;
        }

        if (folio_buffers(folio))
                goto confused;

        block_in_file = folio_pos(folio) >> blkbits;
        last_block = block_in_file + ((args->nr_pages * PAGE_SIZE) >> blkbits);
        last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
        if (last_block > last_block_in_file)
                last_block = last_block_in_file;
        page_block = 0;

        /*
         * Map blocks using the result from the previous get_blocks call first.
         */
        nblocks = map_bh->b_size >> blkbits;
        if (buffer_mapped(map_bh) &&
                        block_in_file > args->first_logical_block &&
                        block_in_file < (args->first_logical_block + nblocks)) {
                unsigned map_offset = block_in_file - args->first_logical_block;
                unsigned last = nblocks - map_offset;

                first_block = map_bh->b_blocknr + map_offset;
                for (relative_block = 0; ; relative_block++) {
                        if (relative_block == last) {
                                clear_buffer_mapped(map_bh);
                                break;
                        }
                        if (page_block == blocks_per_folio)
                                break;
                        page_block++;
                        block_in_file++;
                }
                bdev = map_bh->b_bdev;
        }

        /*
         * Then do more get_blocks calls until we are done with this folio.
         */
        map_bh->b_folio = folio;
        while (page_block < blocks_per_folio) {
                map_bh->b_state = 0;
                map_bh->b_size = 0;

                if (block_in_file < last_block) {
                        map_bh->b_size = (last_block-block_in_file) << blkbits;
                        if (args->get_block(inode, block_in_file, map_bh, 0))
                                goto confused;
                        args->first_logical_block = block_in_file;
                }

                if (!buffer_mapped(map_bh)) {
                        fully_mapped = 0;
                        if (first_hole == blocks_per_folio)
                                first_hole = page_block;
                        page_block++;
                        block_in_file++;
                        continue;
                }

                /* some filesystems will copy data into the page during
                 * the get_block call, in which case we don't want to
                 * read it again.  map_buffer_to_folio copies the data
                 * we just collected from get_block into the folio's buffers
                 * so read_folio doesn't have to repeat the get_block call
                 */
                if (buffer_uptodate(map_bh)) {
                        map_buffer_to_folio(folio, map_bh, page_block);
                        goto confused;
                }
        
                if (first_hole != blocks_per_folio)
                        goto confused;          /* hole -> non-hole */

                /* Contiguous blocks? */
                if (!page_block)
                        first_block = map_bh->b_blocknr;
                else if (first_block + page_block != map_bh->b_blocknr)
                        goto confused;
                nblocks = map_bh->b_size >> blkbits;
                for (relative_block = 0; ; relative_block++) {
                        if (relative_block == nblocks) {
                                clear_buffer_mapped(map_bh);
                                break;
                        } else if (page_block == blocks_per_folio)
                                break;
                        page_block++;
                        block_in_file++;
                }
                bdev = map_bh->b_bdev;
        }

        if (first_hole != blocks_per_folio) {
                folio_zero_segment(folio, first_hole << blkbits, folio_size(folio));
                if (first_hole == 0) {
                        folio_mark_uptodate(folio);
                        folio_unlock(folio);
                        goto out;
                }
        } else if (fully_mapped) {
                folio_set_mappedtodisk(folio);
        }

        /*
         * This folio will go to BIO.  Do we need to send this BIO off first?
         */
        if (args->bio && (args->last_block_in_bio != first_block - 1))
                args->bio = mpage_bio_submit_read(args->bio);

alloc_new:
        if (args->bio == NULL) {
                args->bio = bio_alloc(bdev, bio_max_segs(args->nr_pages), opf,
                                      gfp);
                if (args->bio == NULL)
                        goto confused;
                args->bio->bi_iter.bi_sector = first_block << (blkbits - 9);
        }

        length = first_hole << blkbits;
        if (!bio_add_folio(args->bio, folio, length, 0)) {
                args->bio = mpage_bio_submit_read(args->bio);
                goto alloc_new;
        }

        relative_block = block_in_file - args->first_logical_block;
        nblocks = map_bh->b_size >> blkbits;
        if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
            (first_hole != blocks_per_folio))
                args->bio = mpage_bio_submit_read(args->bio);
        else
                args->last_block_in_bio = first_block + blocks_per_folio - 1;
out:
        return;

confused:
        if (args->bio)
                args->bio = mpage_bio_submit_read(args->bio);
        if (!folio_test_uptodate(folio))
                block_read_full_folio(folio, args->get_block);
        else
                folio_unlock(folio);
        goto out;
}

/**
 * mpage_readahead - start reads against pages
 * @rac: Describes which pages to read.
 * @get_block: The filesystem's block mapper function.
 *
 * This function walks the pages and the blocks within each page, building and
 * emitting large BIOs.
 *
 * If anything unusual happens, such as:
 *
 * - encountering a page which has buffers
 * - encountering a page which has a non-hole after a hole
 * - encountering a page with non-contiguous blocks
 *
 * then this code just gives up and calls the buffer_head-based read function.
 * It does handle a page which has holes at the end - that is a common case:
 * the end-of-file on blocksize < PAGE_SIZE setups.
 *
 * BH_Boundary explanation:
 *
 * There is a problem.  The mpage read code assembles several pages, gets all
 * their disk mappings, and then submits them all.  That's fine, but obtaining
 * the disk mappings may require I/O.  Reads of indirect blocks, for example.
 *
 * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
 * submitted in the following order:
 *
 *      12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
 *
 * because the indirect block has to be read to get the mappings of blocks
 * 13,14,15,16.  Obviously, this impacts performance.
 *
 * So what we do it to allow the filesystem's get_block() function to set
 * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
 * after this one will require I/O against a block which is probably close to
 * this one.  So you should push what I/O you have currently accumulated.
 *
 * This all causes the disk requests to be issued in the correct order.
 */
void mpage_readahead(struct readahead_control *rac, get_block_t get_block)
{
        struct folio *folio;
        struct mpage_readpage_args args = {
                .get_block = get_block,
                .is_readahead = true,
        };

        while ((folio = readahead_folio(rac))) {
                prefetchw(&folio->flags);
                args.folio = folio;
                args.nr_pages = readahead_count(rac);
                do_mpage_readpage(&args);
                /*
                 * If read ahead failed synchronously, it may cause by removed
                 * device, or some filesystem metadata error.
                 */
                if (!folio_test_locked(folio) && !folio_test_uptodate(folio))
                        break;
        }
        if (args.bio)
                mpage_bio_submit_read(args.bio);
}
EXPORT_SYMBOL(mpage_readahead);

/*
 * This isn't called much at all
 */
int mpage_read_folio(struct folio *folio, get_block_t get_block)
{
        struct mpage_readpage_args args = {
                .folio = folio,
                .nr_pages = folio_nr_pages(folio),
                .get_block = get_block,
        };

        do_mpage_readpage(&args);
        if (args.bio)
                mpage_bio_submit_read(args.bio);
        return 0;
}
EXPORT_SYMBOL(mpage_read_folio);

/*
 * Writing is not so simple.
 *
 * If the page has buffers then they will be used for obtaining the disk
 * mapping.  We only support pages which are fully mapped-and-dirty, with a
 * special case for pages which are unmapped at the end: end-of-file.
 *
 * If the page has no buffers (preferred) then the page is mapped here.
 *
 * If all blocks are found to be contiguous then the page can go into the
 * BIO.  Otherwise fall back to the mapping's writepage().
 * 
 * FIXME: This code wants an estimate of how many pages are still to be
 * written, so it can intelligently allocate a suitably-sized BIO.  For now,
 * just allocate full-size (16-page) BIOs.
 */

struct mpage_data {
        struct bio *bio;
        sector_t last_block_in_bio;
        get_block_t *get_block;
};

/*
 * We have our BIO, so we can now mark the buffers clean.  Make
 * sure to only clean buffers which we know we'll be writing.
 */
static void clean_buffers(struct folio *folio, unsigned first_unmapped)
{
        unsigned buffer_counter = 0;
        struct buffer_head *bh, *head = folio_buffers(folio);

        if (!head)
                return;
        bh = head;

        do {
                if (buffer_counter++ == first_unmapped)
                        break;
                clear_buffer_dirty(bh);
                bh = bh->b_this_page;
        } while (bh != head);

        /*
         * we cannot drop the bh if the page is not uptodate or a concurrent
         * read_folio would fail to serialize with the bh and it would read from
         * disk before we reach the platter.
         */
        if (buffer_heads_over_limit && folio_test_uptodate(folio))
                try_to_free_buffers(folio);
}

static int mpage_write_folio(struct writeback_control *wbc, struct folio *folio,
                struct mpage_data *mpd)
{
        struct bio *bio = mpd->bio;
        struct address_space *mapping = folio->mapping;
        struct inode *inode = mapping->host;
        const unsigned blkbits = inode->i_blkbits;
        const unsigned blocks_per_folio = folio_size(folio) >> blkbits;
        sector_t last_block;
        sector_t block_in_file;
        sector_t first_block;
        unsigned page_block;
        unsigned first_unmapped = blocks_per_folio;
        struct block_device *bdev = NULL;
        int boundary = 0;
        sector_t boundary_block = 0;
        struct block_device *boundary_bdev = NULL;
        size_t length;
        struct buffer_head map_bh;
        loff_t i_size = i_size_read(inode);
        int ret = 0;
        struct buffer_head *head = folio_buffers(folio);

        if (head) {
                struct buffer_head *bh = head;

                /* If they're all mapped and dirty, do it */
                page_block = 0;
                do {
                        BUG_ON(buffer_locked(bh));
                        if (!buffer_mapped(bh)) {
                                /*
                                 * unmapped dirty buffers are created by
                                 * block_dirty_folio -> mmapped data
                                 */
                                if (buffer_dirty(bh))
                                        goto confused;
                                if (first_unmapped == blocks_per_folio)
                                        first_unmapped = page_block;
                                continue;
                        }

                        if (first_unmapped != blocks_per_folio)
                                goto confused;  /* hole -> non-hole */

                        if (!buffer_dirty(bh) || !buffer_uptodate(bh))
                                goto confused;
                        if (page_block) {
                                if (bh->b_blocknr != first_block + page_block)
                                        goto confused;
                        } else {
                                first_block = bh->b_blocknr;
                        }
                        page_block++;
                        boundary = buffer_boundary(bh);
                        if (boundary) {
                                boundary_block = bh->b_blocknr;
                                boundary_bdev = bh->b_bdev;
                        }
                        bdev = bh->b_bdev;
                } while ((bh = bh->b_this_page) != head);

                if (first_unmapped)
                        goto page_is_mapped;

                /*
                 * Page has buffers, but they are all unmapped. The page was
                 * created by pagein or read over a hole which was handled by
                 * block_read_full_folio().  If this address_space is also
                 * using mpage_readahead then this can rarely happen.
                 */
                goto confused;
        }

        /*
         * The page has no buffers: map it to disk
         */
        BUG_ON(!folio_test_uptodate(folio));
        block_in_file = folio_pos(folio) >> blkbits;
        /*
         * Whole page beyond EOF? Skip allocating blocks to avoid leaking
         * space.
         */
        if (block_in_file >= (i_size + (1 << blkbits) - 1) >> blkbits)
                goto page_is_mapped;
        last_block = (i_size - 1) >> blkbits;
        map_bh.b_folio = folio;
        for (page_block = 0; page_block < blocks_per_folio; ) {

                map_bh.b_state = 0;
                map_bh.b_size = 1 << blkbits;
                if (mpd->get_block(inode, block_in_file, &map_bh, 1))
                        goto confused;
                if (!buffer_mapped(&map_bh))
                        goto confused;
                if (buffer_new(&map_bh))
                        clean_bdev_bh_alias(&map_bh);
                if (buffer_boundary(&map_bh)) {
                        boundary_block = map_bh.b_blocknr;
                        boundary_bdev = map_bh.b_bdev;
                }
                if (page_block) {
                        if (map_bh.b_blocknr != first_block + page_block)
                                goto confused;
                } else {
                        first_block = map_bh.b_blocknr;
                }
                page_block++;
                boundary = buffer_boundary(&map_bh);
                bdev = map_bh.b_bdev;
                if (block_in_file == last_block)
                        break;
                block_in_file++;
        }
        BUG_ON(page_block == 0);

        first_unmapped = page_block;

page_is_mapped:
        /* Don't bother writing beyond EOF, truncate will discard the folio */
        if (folio_pos(folio) >= i_size)
                goto confused;
        length = folio_size(folio);
        if (folio_pos(folio) + length > i_size) {
                /*
                 * The page straddles i_size.  It must be zeroed out on each
                 * and every writepage invocation because it may be mmapped.
                 * "A file is mapped in multiples of the page size.  For a file
                 * that is not a multiple of the page size, the remaining memory
                 * is zeroed when mapped, and writes to that region are not
                 * written out to the file."
                 */
                length = i_size - folio_pos(folio);
                folio_zero_segment(folio, length, folio_size(folio));
        }

        /*
         * This page will go to BIO.  Do we need to send this BIO off first?
         */
        if (bio && mpd->last_block_in_bio != first_block - 1)
                bio = mpage_bio_submit_write(bio);

alloc_new:
        if (bio == NULL) {
                bio = bio_alloc(bdev, BIO_MAX_VECS,
                                REQ_OP_WRITE | wbc_to_write_flags(wbc),
                                GFP_NOFS);
                bio->bi_iter.bi_sector = first_block << (blkbits - 9);
                wbc_init_bio(wbc, bio);
                bio->bi_write_hint = inode->i_write_hint;
        }

        /*
         * Must try to add the page before marking the buffer clean or
         * the confused fail path above (OOM) will be very confused when
         * it finds all bh marked clean (i.e. it will not write anything)
         */
        wbc_account_cgroup_owner(wbc, folio, folio_size(folio));
        length = first_unmapped << blkbits;
        if (!bio_add_folio(bio, folio, length, 0)) {
                bio = mpage_bio_submit_write(bio);
                goto alloc_new;
        }

        clean_buffers(folio, first_unmapped);

        BUG_ON(folio_test_writeback(folio));
        folio_start_writeback(folio);
        folio_unlock(folio);
        if (boundary || (first_unmapped != blocks_per_folio)) {
                bio = mpage_bio_submit_write(bio);
                if (boundary_block) {
                        write_boundary_block(boundary_bdev,
                                        boundary_block, 1 << blkbits);
                }
        } else {
                mpd->last_block_in_bio = first_block + blocks_per_folio - 1;
        }
        goto out;

confused:
        if (bio)
                bio = mpage_bio_submit_write(bio);

        /*
         * The caller has a ref on the inode, so *mapping is stable
         */
        ret = block_write_full_folio(folio, wbc, mpd->get_block);
        mapping_set_error(mapping, ret);
out:
        mpd->bio = bio;
        return ret;
}

/**
 * __mpage_writepages - walk the list of dirty pages of the given address space
 *                      & writepage() all of them
 * @mapping: address space structure to write
 * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 * @get_block: the filesystem's block mapper function.
 * @write_folio: handler to call for each folio before calling
 *               mpage_write_folio()
 *
 * This is a library function, which implements the writepages()
 * address_space_operation. It calls @write_folio handler for each folio. If
 * the handler returns value > 0, it calls mpage_write_folio() to do the
 * folio writeback.
 */
int
__mpage_writepages(struct address_space *mapping,
                   struct writeback_control *wbc, get_block_t get_block,
                   int (*write_folio)(struct folio *folio,
                                      struct writeback_control *wbc))
{
        struct mpage_data mpd = {
                .get_block      = get_block,
        };
        struct folio *folio = NULL;
        struct blk_plug plug;
        int error;

        blk_start_plug(&plug);
        while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
                if (write_folio) {
                        error = write_folio(folio, wbc);
                        /*
                         * == 0 means folio is handled, < 0 means error. In
                         * both cases hand back control to writeback_iter()
                         */
                        if (error <= 0)
                                continue;
                        /* Let mpage_write_folio() handle the folio. */
                }
                error = mpage_write_folio(wbc, folio, &mpd);
        }
        if (mpd.bio)
                mpage_bio_submit_write(mpd.bio);
        blk_finish_plug(&plug);
        return error;
}
EXPORT_SYMBOL(__mpage_writepages);