// SPDX-License-Identifier: GPL-2.0-or-later
/* Network filesystem high-level buffered read support.
 *
 * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 */

#include <linux/export.h>
#include <linux/task_io_accounting_ops.h>
#include "internal.h"

static void netfs_cache_expand_readahead(struct netfs_io_request *rreq,
                                         unsigned long long *_start,
                                         unsigned long long *_len,
                                         unsigned long long i_size)
{
        struct netfs_cache_resources *cres = &rreq->cache_resources;

        if (cres->ops && cres->ops->expand_readahead)
                cres->ops->expand_readahead(cres, _start, _len, i_size);
}

static void netfs_rreq_expand(struct netfs_io_request *rreq,
                              struct readahead_control *ractl)
{
        /* Give the cache a chance to change the request parameters.  The
         * resultant request must contain the original region.
         */
        netfs_cache_expand_readahead(rreq, &rreq->start, &rreq->len, rreq->i_size);

        /* Give the netfs a chance to change the request parameters.  The
         * resultant request must contain the original region.
         */
        if (rreq->netfs_ops->expand_readahead)
                rreq->netfs_ops->expand_readahead(rreq);

        /* Expand the request if the cache wants it to start earlier.  Note
         * that the expansion may get further extended if the VM wishes to
         * insert THPs and the preferred start and/or end wind up in the middle
         * of THPs.
         *
         * If this is the case, however, the THP size should be an integer
         * multiple of the cache granule size, so we get a whole number of
         * granules to deal with.
         */
        if (rreq->start  != readahead_pos(ractl) ||
            rreq->len != readahead_length(ractl)) {
                readahead_expand(ractl, rreq->start, rreq->len);
                rreq->start  = readahead_pos(ractl);
                rreq->len = readahead_length(ractl);

                trace_netfs_read(rreq, readahead_pos(ractl), readahead_length(ractl),
                                 netfs_read_trace_expanded);
        }
}

/*
 * Begin an operation, and fetch the stored zero point value from the cookie if
 * available.
 */
static int netfs_begin_cache_read(struct netfs_io_request *rreq, struct netfs_inode *ctx)
{
        return fscache_begin_read_operation(&rreq->cache_resources, netfs_i_cookie(ctx));
}

/*
 * netfs_prepare_read_iterator - Prepare the subreq iterator for I/O
 * @subreq: The subrequest to be set up
 *
 * Prepare the I/O iterator representing the read buffer on a subrequest for
 * the filesystem to use for I/O (it can be passed directly to a socket).  This
 * is intended to be called from the ->issue_read() method once the filesystem
 * has trimmed the request to the size it wants.
 *
 * Returns the limited size if successful and -ENOMEM if insufficient memory
 * available.
 *
 * [!] NOTE: This must be run in the same thread as ->issue_read() was called
 * in as we access the readahead_control struct.
 */
static ssize_t netfs_prepare_read_iterator(struct netfs_io_subrequest *subreq,
                                           struct readahead_control *ractl)
{
        struct netfs_io_request *rreq = subreq->rreq;
        size_t rsize = subreq->len;

        if (subreq->source == NETFS_DOWNLOAD_FROM_SERVER)
                rsize = umin(rsize, rreq->io_streams[0].sreq_max_len);

        if (ractl) {
                /* If we don't have sufficient folios in the rolling buffer,
                 * extract a folioq's worth from the readahead region at a time
                 * into the buffer.  Note that this acquires a ref on each page
                 * that we will need to release later - but we don't want to do
                 * that until after we've started the I/O.
                 */
                struct folio_batch put_batch;

                folio_batch_init(&put_batch);
                while (rreq->submitted < subreq->start + rsize) {
                        ssize_t added;

                        added = rolling_buffer_load_from_ra(&rreq->buffer, ractl,
                                                            &put_batch);
                        if (added < 0)
                                return added;
                        rreq->submitted += added;
                }
                folio_batch_release(&put_batch);
        }

        subreq->len = rsize;
        if (unlikely(rreq->io_streams[0].sreq_max_segs)) {
                size_t limit = netfs_limit_iter(&rreq->buffer.iter, 0, rsize,
                                                rreq->io_streams[0].sreq_max_segs);

                if (limit < rsize) {
                        subreq->len = limit;
                        trace_netfs_sreq(subreq, netfs_sreq_trace_limited);
                }
        }

        subreq->io_iter = rreq->buffer.iter;

        iov_iter_truncate(&subreq->io_iter, subreq->len);
        rolling_buffer_advance(&rreq->buffer, subreq->len);
        return subreq->len;
}

static enum netfs_io_source netfs_cache_prepare_read(struct netfs_io_request *rreq,
                                                     struct netfs_io_subrequest *subreq,
                                                     loff_t i_size)
{
        struct netfs_cache_resources *cres = &rreq->cache_resources;
        enum netfs_io_source source;

        if (!cres->ops)
                return NETFS_DOWNLOAD_FROM_SERVER;
        source = cres->ops->prepare_read(subreq, i_size);
        trace_netfs_sreq(subreq, netfs_sreq_trace_prepare);
        return source;

}

/*
 * Issue a read against the cache.
 * - Eats the caller's ref on subreq.
 */
static void netfs_read_cache_to_pagecache(struct netfs_io_request *rreq,
                                          struct netfs_io_subrequest *subreq)
{
        struct netfs_cache_resources *cres = &rreq->cache_resources;

        netfs_stat(&netfs_n_rh_read);
        cres->ops->read(cres, subreq->start, &subreq->io_iter, NETFS_READ_HOLE_IGNORE,
                        netfs_cache_read_terminated, subreq);
}

static void netfs_queue_read(struct netfs_io_request *rreq,
                             struct netfs_io_subrequest *subreq,
                             bool last_subreq)
{
        struct netfs_io_stream *stream = &rreq->io_streams[0];

        __set_bit(NETFS_SREQ_IN_PROGRESS, &subreq->flags);

        /* We add to the end of the list whilst the collector may be walking
         * the list.  The collector only goes nextwards and uses the lock to
         * remove entries off of the front.
         */
        spin_lock(&rreq->lock);
        list_add_tail(&subreq->rreq_link, &stream->subrequests);
        if (list_is_first(&subreq->rreq_link, &stream->subrequests)) {
                if (!stream->active) {
                        stream->collected_to = subreq->start;
                        /* Store list pointers before active flag */
                        smp_store_release(&stream->active, true);
                }
        }

        if (last_subreq) {
                smp_wmb(); /* Write lists before ALL_QUEUED. */
                set_bit(NETFS_RREQ_ALL_QUEUED, &rreq->flags);
        }

        spin_unlock(&rreq->lock);
}

static void netfs_issue_read(struct netfs_io_request *rreq,
                             struct netfs_io_subrequest *subreq)
{
        switch (subreq->source) {
        case NETFS_DOWNLOAD_FROM_SERVER:
                rreq->netfs_ops->issue_read(subreq);
                break;
        case NETFS_READ_FROM_CACHE:
                netfs_read_cache_to_pagecache(rreq, subreq);
                break;
        default:
                __set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
                subreq->error = 0;
                iov_iter_zero(subreq->len, &subreq->io_iter);
                subreq->transferred = subreq->len;
                netfs_read_subreq_terminated(subreq);
                break;
        }
}

/*
 * Perform a read to the pagecache from a series of sources of different types,
 * slicing up the region to be read according to available cache blocks and
 * network rsize.
 */
static void netfs_read_to_pagecache(struct netfs_io_request *rreq,
                                    struct readahead_control *ractl)
{
        struct netfs_inode *ictx = netfs_inode(rreq->inode);
        unsigned long long start = rreq->start;
        ssize_t size = rreq->len;
        int ret = 0;

        do {
                struct netfs_io_subrequest *subreq;
                enum netfs_io_source source = NETFS_SOURCE_UNKNOWN;
                ssize_t slice;

                subreq = netfs_alloc_subrequest(rreq);
                if (!subreq) {
                        ret = -ENOMEM;
                        break;
                }

                subreq->start   = start;
                subreq->len     = size;

                source = netfs_cache_prepare_read(rreq, subreq, rreq->i_size);
                subreq->source = source;
                if (source == NETFS_DOWNLOAD_FROM_SERVER) {
                        unsigned long long zp = umin(ictx->zero_point, rreq->i_size);
                        size_t len = subreq->len;

                        if (unlikely(rreq->origin == NETFS_READ_SINGLE))
                                zp = rreq->i_size;
                        if (subreq->start >= zp) {
                                subreq->source = source = NETFS_FILL_WITH_ZEROES;
                                goto fill_with_zeroes;
                        }

                        if (len > zp - subreq->start)
                                len = zp - subreq->start;
                        if (len == 0) {
                                pr_err("ZERO-LEN READ: R=%08x[%x] l=%zx/%zx s=%llx z=%llx i=%llx",
                                       rreq->debug_id, subreq->debug_index,
                                       subreq->len, size,
                                       subreq->start, ictx->zero_point, rreq->i_size);
                                break;
                        }
                        subreq->len = len;

                        netfs_stat(&netfs_n_rh_download);
                        if (rreq->netfs_ops->prepare_read) {
                                ret = rreq->netfs_ops->prepare_read(subreq);
                                if (ret < 0) {
                                        subreq->error = ret;
                                        /* Not queued - release both refs. */
                                        netfs_put_subrequest(subreq,
                                                             netfs_sreq_trace_put_cancel);
                                        netfs_put_subrequest(subreq,
                                                             netfs_sreq_trace_put_cancel);
                                        break;
                                }
                                trace_netfs_sreq(subreq, netfs_sreq_trace_prepare);
                        }
                        goto issue;
                }

        fill_with_zeroes:
                if (source == NETFS_FILL_WITH_ZEROES) {
                        subreq->source = NETFS_FILL_WITH_ZEROES;
                        trace_netfs_sreq(subreq, netfs_sreq_trace_submit);
                        netfs_stat(&netfs_n_rh_zero);
                        goto issue;
                }

                if (source == NETFS_READ_FROM_CACHE) {
                        trace_netfs_sreq(subreq, netfs_sreq_trace_submit);
                        goto issue;
                }

                pr_err("Unexpected read source %u\n", source);
                WARN_ON_ONCE(1);
                break;

        issue:
                slice = netfs_prepare_read_iterator(subreq, ractl);
                if (slice < 0) {
                        ret = slice;
                        subreq->error = ret;
                        trace_netfs_sreq(subreq, netfs_sreq_trace_cancel);
                        /* Not queued - release both refs. */
                        netfs_put_subrequest(subreq, netfs_sreq_trace_put_cancel);
                        netfs_put_subrequest(subreq, netfs_sreq_trace_put_cancel);
                        break;
                }
                size -= slice;
                start += slice;

                netfs_queue_read(rreq, subreq, size <= 0);
                netfs_issue_read(rreq, subreq);
                cond_resched();
        } while (size > 0);

        if (unlikely(size > 0)) {
                smp_wmb(); /* Write lists before ALL_QUEUED. */
                set_bit(NETFS_RREQ_ALL_QUEUED, &rreq->flags);
                netfs_wake_collector(rreq);
        }

        /* Defer error return as we may need to wait for outstanding I/O. */
        cmpxchg(&rreq->error, 0, ret);
}

/**
 * netfs_readahead - Helper to manage a read request
 * @ractl: The description of the readahead request
 *
 * Fulfil a readahead request by drawing data from the cache if possible, or
 * the netfs if not.  Space beyond the EOF is zero-filled.  Multiple I/O
 * requests from different sources will get munged together.  If necessary, the
 * readahead window can be expanded in either direction to a more convenient
 * alighment for RPC efficiency or to make storage in the cache feasible.
 *
 * The calling netfs must initialise a netfs context contiguous to the vfs
 * inode before calling this.
 *
 * This is usable whether or not caching is enabled.
 */
void netfs_readahead(struct readahead_control *ractl)
{
        struct netfs_io_request *rreq;
        struct netfs_inode *ictx = netfs_inode(ractl->mapping->host);
        unsigned long long start = readahead_pos(ractl);
        size_t size = readahead_length(ractl);
        int ret;

        rreq = netfs_alloc_request(ractl->mapping, ractl->file, start, size,
                                   NETFS_READAHEAD);
        if (IS_ERR(rreq))
                return;

        __set_bit(NETFS_RREQ_OFFLOAD_COLLECTION, &rreq->flags);

        ret = netfs_begin_cache_read(rreq, ictx);
        if (ret == -ENOMEM || ret == -EINTR || ret == -ERESTARTSYS)
                goto cleanup_free;

        netfs_stat(&netfs_n_rh_readahead);
        trace_netfs_read(rreq, readahead_pos(ractl), readahead_length(ractl),
                         netfs_read_trace_readahead);

        netfs_rreq_expand(rreq, ractl);

        rreq->submitted = rreq->start;
        if (rolling_buffer_init(&rreq->buffer, rreq->debug_id, ITER_DEST) < 0)
                goto cleanup_free;
        netfs_read_to_pagecache(rreq, ractl);

        return netfs_put_request(rreq, netfs_rreq_trace_put_return);

cleanup_free:
        return netfs_put_failed_request(rreq);
}
EXPORT_SYMBOL(netfs_readahead);

/*
 * Create a rolling buffer with a single occupying folio.
 */
static int netfs_create_singular_buffer(struct netfs_io_request *rreq, struct folio *folio,
                                        unsigned int rollbuf_flags)
{
        ssize_t added;

        if (rolling_buffer_init(&rreq->buffer, rreq->debug_id, ITER_DEST) < 0)
                return -ENOMEM;

        added = rolling_buffer_append(&rreq->buffer, folio, rollbuf_flags);
        if (added < 0)
                return added;
        rreq->submitted = rreq->start + added;
        return 0;
}

/*
 * Read into gaps in a folio partially filled by a streaming write.
 */
static int netfs_read_gaps(struct file *file, struct folio *folio)
{
        struct netfs_io_request *rreq;
        struct address_space *mapping = folio->mapping;
        struct netfs_folio *finfo = netfs_folio_info(folio);
        struct netfs_inode *ctx = netfs_inode(mapping->host);
        struct folio *sink = NULL;
        struct bio_vec *bvec;
        unsigned int from = finfo->dirty_offset;
        unsigned int to = from + finfo->dirty_len;
        unsigned int off = 0, i = 0;
        size_t flen = folio_size(folio);
        size_t nr_bvec = flen / PAGE_SIZE + 2;
        size_t part;
        int ret;

        _enter("%lx", folio->index);

        rreq = netfs_alloc_request(mapping, file, folio_pos(folio), flen, NETFS_READ_GAPS);
        if (IS_ERR(rreq)) {
                ret = PTR_ERR(rreq);
                goto alloc_error;
        }

        ret = netfs_begin_cache_read(rreq, ctx);
        if (ret == -ENOMEM || ret == -EINTR || ret == -ERESTARTSYS)
                goto discard;

        netfs_stat(&netfs_n_rh_read_folio);
        trace_netfs_read(rreq, rreq->start, rreq->len, netfs_read_trace_read_gaps);

        /* Fiddle the buffer so that a gap at the beginning and/or a gap at the
         * end get copied to, but the middle is discarded.
         */
        ret = -ENOMEM;
        bvec = kmalloc_objs(*bvec, nr_bvec);
        if (!bvec)
                goto discard;

        sink = folio_alloc(GFP_KERNEL, 0);
        if (!sink) {
                kfree(bvec);
                goto discard;
        }

        trace_netfs_folio(folio, netfs_folio_trace_read_gaps);

        rreq->direct_bv = bvec;
        rreq->direct_bv_count = nr_bvec;
        if (from > 0) {
                bvec_set_folio(&bvec[i++], folio, from, 0);
                off = from;
        }
        while (off < to) {
                part = min_t(size_t, to - off, PAGE_SIZE);
                bvec_set_folio(&bvec[i++], sink, part, 0);
                off += part;
        }
        if (to < flen)
                bvec_set_folio(&bvec[i++], folio, flen - to, to);
        iov_iter_bvec(&rreq->buffer.iter, ITER_DEST, bvec, i, rreq->len);
        rreq->submitted = rreq->start + flen;

        netfs_read_to_pagecache(rreq, NULL);

        if (sink)
                folio_put(sink);

        ret = netfs_wait_for_read(rreq);
        if (ret >= 0) {
                flush_dcache_folio(folio);
                folio_mark_uptodate(folio);
        }
        folio_unlock(folio);
        netfs_put_request(rreq, netfs_rreq_trace_put_return);
        return ret < 0 ? ret : 0;

discard:
        netfs_put_failed_request(rreq);
alloc_error:
        folio_unlock(folio);
        return ret;
}

/**
 * netfs_read_folio - Helper to manage a read_folio request
 * @file: The file to read from
 * @folio: The folio to read
 *
 * Fulfil a read_folio request by drawing data from the cache if
 * possible, or the netfs if not.  Space beyond the EOF is zero-filled.
 * Multiple I/O requests from different sources will get munged together.
 *
 * The calling netfs must initialise a netfs context contiguous to the vfs
 * inode before calling this.
 *
 * This is usable whether or not caching is enabled.
 */
int netfs_read_folio(struct file *file, struct folio *folio)
{
        struct address_space *mapping = folio->mapping;
        struct netfs_io_request *rreq;
        struct netfs_inode *ctx = netfs_inode(mapping->host);
        int ret;

        if (folio_test_dirty(folio)) {
                trace_netfs_folio(folio, netfs_folio_trace_read_gaps);
                return netfs_read_gaps(file, folio);
        }

        _enter("%lx", folio->index);

        rreq = netfs_alloc_request(mapping, file,
                                   folio_pos(folio), folio_size(folio),
                                   NETFS_READPAGE);
        if (IS_ERR(rreq)) {
                ret = PTR_ERR(rreq);
                goto alloc_error;
        }

        ret = netfs_begin_cache_read(rreq, ctx);
        if (ret == -ENOMEM || ret == -EINTR || ret == -ERESTARTSYS)
                goto discard;

        netfs_stat(&netfs_n_rh_read_folio);
        trace_netfs_read(rreq, rreq->start, rreq->len, netfs_read_trace_readpage);

        /* Set up the output buffer */
        ret = netfs_create_singular_buffer(rreq, folio, 0);
        if (ret < 0)
                goto discard;

        netfs_read_to_pagecache(rreq, NULL);
        ret = netfs_wait_for_read(rreq);
        netfs_put_request(rreq, netfs_rreq_trace_put_return);
        return ret < 0 ? ret : 0;

discard:
        netfs_put_failed_request(rreq);
alloc_error:
        folio_unlock(folio);
        return ret;
}
EXPORT_SYMBOL(netfs_read_folio);

/*
 * Prepare a folio for writing without reading first
 * @folio: The folio being prepared
 * @pos: starting position for the write
 * @len: length of write
 * @always_fill: T if the folio should always be completely filled/cleared
 *
 * In some cases, write_begin doesn't need to read at all:
 * - full folio write
 * - write that lies in a folio that is completely beyond EOF
 * - write that covers the folio from start to EOF or beyond it
 *
 * If any of these criteria are met, then zero out the unwritten parts
 * of the folio and return true. Otherwise, return false.
 */
static bool netfs_skip_folio_read(struct folio *folio, loff_t pos, size_t len,
                                 bool always_fill)
{
        struct inode *inode = folio_inode(folio);
        loff_t i_size = i_size_read(inode);
        size_t offset = offset_in_folio(folio, pos);
        size_t plen = folio_size(folio);

        if (unlikely(always_fill)) {
                if (pos - offset + len <= i_size)
                        return false; /* Page entirely before EOF */
                folio_zero_segment(folio, 0, plen);
                folio_mark_uptodate(folio);
                return true;
        }

        /* Full folio write */
        if (offset == 0 && len >= plen)
                return true;

        /* Page entirely beyond the end of the file */
        if (pos - offset >= i_size)
                goto zero_out;

        /* Write that covers from the start of the folio to EOF or beyond */
        if (offset == 0 && (pos + len) >= i_size)
                goto zero_out;

        return false;
zero_out:
        folio_zero_segments(folio, 0, offset, offset + len, plen);
        return true;
}

/**
 * netfs_write_begin - Helper to prepare for writing [DEPRECATED]
 * @ctx: The netfs context
 * @file: The file to read from
 * @mapping: The mapping to read from
 * @pos: File position at which the write will begin
 * @len: The length of the write (may extend beyond the end of the folio chosen)
 * @_folio: Where to put the resultant folio
 * @_fsdata: Place for the netfs to store a cookie
 *
 * Pre-read data for a write-begin request by drawing data from the cache if
 * possible, or the netfs if not.  Space beyond the EOF is zero-filled.
 * Multiple I/O requests from different sources will get munged together.
 *
 * The calling netfs must provide a table of operations, only one of which,
 * issue_read, is mandatory.
 *
 * The check_write_begin() operation can be provided to check for and flush
 * conflicting writes once the folio is grabbed and locked.  It is passed a
 * pointer to the fsdata cookie that gets returned to the VM to be passed to
 * write_end.  It is permitted to sleep.  It should return 0 if the request
 * should go ahead or it may return an error.  It may also unlock and put the
 * folio, provided it sets ``*foliop`` to NULL, in which case a return of 0
 * will cause the folio to be re-got and the process to be retried.
 *
 * The calling netfs must initialise a netfs context contiguous to the vfs
 * inode before calling this.
 *
 * This is usable whether or not caching is enabled.
 *
 * Note that this should be considered deprecated and netfs_perform_write()
 * used instead.
 */
int netfs_write_begin(struct netfs_inode *ctx,
                      struct file *file, struct address_space *mapping,
                      loff_t pos, unsigned int len, struct folio **_folio,
                      void **_fsdata)
{
        struct netfs_io_request *rreq;
        struct folio *folio;
        pgoff_t index = pos >> PAGE_SHIFT;
        int ret;

retry:
        folio = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN,
                                    mapping_gfp_mask(mapping));
        if (IS_ERR(folio))
                return PTR_ERR(folio);

        if (ctx->ops->check_write_begin) {
                /* Allow the netfs (eg. ceph) to flush conflicts. */
                ret = ctx->ops->check_write_begin(file, pos, len, &folio, _fsdata);
                if (ret < 0) {
                        trace_netfs_failure(NULL, NULL, ret, netfs_fail_check_write_begin);
                        goto error;
                }
                if (!folio)
                        goto retry;
        }

        if (folio_test_uptodate(folio))
                goto have_folio;

        /* If the folio is beyond the EOF, we want to clear it - unless it's
         * within the cache granule containing the EOF, in which case we need
         * to preload the granule.
         */
        if (!netfs_is_cache_enabled(ctx) &&
            netfs_skip_folio_read(folio, pos, len, false)) {
                netfs_stat(&netfs_n_rh_write_zskip);
                goto have_folio_no_wait;
        }

        rreq = netfs_alloc_request(mapping, file,
                                   folio_pos(folio), folio_size(folio),
                                   NETFS_READ_FOR_WRITE);
        if (IS_ERR(rreq)) {
                ret = PTR_ERR(rreq);
                goto error;
        }
        rreq->no_unlock_folio   = folio->index;
        __set_bit(NETFS_RREQ_NO_UNLOCK_FOLIO, &rreq->flags);

        ret = netfs_begin_cache_read(rreq, ctx);
        if (ret == -ENOMEM || ret == -EINTR || ret == -ERESTARTSYS)
                goto error_put;

        netfs_stat(&netfs_n_rh_write_begin);
        trace_netfs_read(rreq, pos, len, netfs_read_trace_write_begin);

        /* Set up the output buffer */
        ret = netfs_create_singular_buffer(rreq, folio, 0);
        if (ret < 0)
                goto error_put;

        netfs_read_to_pagecache(rreq, NULL);
        ret = netfs_wait_for_read(rreq);
        if (ret < 0)
                goto error;
        netfs_put_request(rreq, netfs_rreq_trace_put_return);

have_folio:
        ret = folio_wait_private_2_killable(folio);
        if (ret < 0)
                goto error;
have_folio_no_wait:
        *_folio = folio;
        _leave(" = 0");
        return 0;

error_put:
        netfs_put_failed_request(rreq);
error:
        if (folio) {
                folio_unlock(folio);
                folio_put(folio);
        }
        _leave(" = %d", ret);
        return ret;
}
EXPORT_SYMBOL(netfs_write_begin);

/*
 * Preload the data into a folio we're proposing to write into.
 */
int netfs_prefetch_for_write(struct file *file, struct folio *folio,
                             size_t offset, size_t len)
{
        struct netfs_io_request *rreq;
        struct address_space *mapping = folio->mapping;
        struct netfs_inode *ctx = netfs_inode(mapping->host);
        unsigned long long start = folio_pos(folio);
        size_t flen = folio_size(folio);
        int ret;

        _enter("%zx @%llx", flen, start);

        ret = -ENOMEM;

        rreq = netfs_alloc_request(mapping, file, start, flen,
                                   NETFS_READ_FOR_WRITE);
        if (IS_ERR(rreq)) {
                ret = PTR_ERR(rreq);
                goto error;
        }

        rreq->no_unlock_folio = folio->index;
        __set_bit(NETFS_RREQ_NO_UNLOCK_FOLIO, &rreq->flags);
        ret = netfs_begin_cache_read(rreq, ctx);
        if (ret == -ENOMEM || ret == -EINTR || ret == -ERESTARTSYS)
                goto error_put;

        netfs_stat(&netfs_n_rh_write_begin);
        trace_netfs_read(rreq, start, flen, netfs_read_trace_prefetch_for_write);

        /* Set up the output buffer */
        ret = netfs_create_singular_buffer(rreq, folio, NETFS_ROLLBUF_PAGECACHE_MARK);
        if (ret < 0)
                goto error_put;

        netfs_read_to_pagecache(rreq, NULL);
        ret = netfs_wait_for_read(rreq);
        netfs_put_request(rreq, netfs_rreq_trace_put_return);
        return ret < 0 ? ret : 0;

error_put:
        netfs_put_failed_request(rreq);
error:
        _leave(" = %d", ret);
        return ret;
}

/**
 * netfs_buffered_read_iter - Filesystem buffered I/O read routine
 * @iocb: kernel I/O control block
 * @iter: destination for the data read
 *
 * This is the ->read_iter() routine for all filesystems that can use the page
 * cache directly.
 *
 * The IOCB_NOWAIT flag in iocb->ki_flags indicates that -EAGAIN shall be
 * returned when no data can be read without waiting for I/O requests to
 * complete; it doesn't prevent readahead.
 *
 * The IOCB_NOIO flag in iocb->ki_flags indicates that no new I/O requests
 * shall be made for the read or for readahead.  When no data can be read,
 * -EAGAIN shall be returned.  When readahead would be triggered, a partial,
 * possibly empty read shall be returned.
 *
 * Return:
 * * number of bytes copied, even for partial reads
 * * negative error code (or 0 if IOCB_NOIO) if nothing was read
 */
ssize_t netfs_buffered_read_iter(struct kiocb *iocb, struct iov_iter *iter)
{
        struct inode *inode = file_inode(iocb->ki_filp);
        struct netfs_inode *ictx = netfs_inode(inode);
        ssize_t ret;

        if (WARN_ON_ONCE((iocb->ki_flags & IOCB_DIRECT) ||
                         test_bit(NETFS_ICTX_UNBUFFERED, &ictx->flags)))
                return -EINVAL;

        ret = netfs_start_io_read(inode);
        if (ret == 0) {
                ret = filemap_read(iocb, iter, 0);
                netfs_end_io_read(inode);
        }
        return ret;
}
EXPORT_SYMBOL(netfs_buffered_read_iter);

/**
 * netfs_file_read_iter - Generic filesystem read routine
 * @iocb: kernel I/O control block
 * @iter: destination for the data read
 *
 * This is the ->read_iter() routine for all filesystems that can use the page
 * cache directly.
 *
 * The IOCB_NOWAIT flag in iocb->ki_flags indicates that -EAGAIN shall be
 * returned when no data can be read without waiting for I/O requests to
 * complete; it doesn't prevent readahead.
 *
 * The IOCB_NOIO flag in iocb->ki_flags indicates that no new I/O requests
 * shall be made for the read or for readahead.  When no data can be read,
 * -EAGAIN shall be returned.  When readahead would be triggered, a partial,
 * possibly empty read shall be returned.
 *
 * Return:
 * * number of bytes copied, even for partial reads
 * * negative error code (or 0 if IOCB_NOIO) if nothing was read
 */
ssize_t netfs_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
{
        struct netfs_inode *ictx = netfs_inode(iocb->ki_filp->f_mapping->host);

        if ((iocb->ki_flags & IOCB_DIRECT) ||
            test_bit(NETFS_ICTX_UNBUFFERED, &ictx->flags))
                return netfs_unbuffered_read_iter(iocb, iter);

        return netfs_buffered_read_iter(iocb, iter);
}
EXPORT_SYMBOL(netfs_file_read_iter);