io_uring: don't use 'fd' for openat/openat2/statx

[linux/fpc-iii.git] / fs / iomap / buffered-io.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2010 Red Hat, Inc.
 * Copyright (C) 2016-2019 Christoph Hellwig.
 */
#include <linux/module.h>
#include <linux/compiler.h>
#include <linux/fs.h>
#include <linux/iomap.h>
#include <linux/pagemap.h>
#include <linux/uio.h>
#include <linux/buffer_head.h>
#include <linux/dax.h>
#include <linux/writeback.h>
#include <linux/list_sort.h>
#include <linux/swap.h>
#include <linux/bio.h>
#include <linux/sched/signal.h>
#include <linux/migrate.h>
#include "trace.h"

#include "../internal.h"

/*
 * Structure allocated for each page when block size < PAGE_SIZE to track
 * sub-page uptodate status and I/O completions.
 */
struct iomap_page {
        atomic_t                read_count;
        atomic_t                write_count;
        spinlock_t              uptodate_lock;
        DECLARE_BITMAP(uptodate, PAGE_SIZE / 512);
};

static inline struct iomap_page *to_iomap_page(struct page *page)
{
        if (page_has_private(page))
                return (struct iomap_page *)page_private(page);
        return NULL;
}

static struct bio_set iomap_ioend_bioset;

static struct iomap_page *
iomap_page_create(struct inode *inode, struct page *page)
{
        struct iomap_page *iop = to_iomap_page(page);

        if (iop || i_blocksize(inode) == PAGE_SIZE)
                return iop;

        iop = kmalloc(sizeof(*iop), GFP_NOFS | __GFP_NOFAIL);
        atomic_set(&iop->read_count, 0);
        atomic_set(&iop->write_count, 0);
        spin_lock_init(&iop->uptodate_lock);
        bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);

        /*
         * migrate_page_move_mapping() assumes that pages with private data have
         * their count elevated by 1.
         */
        get_page(page);
        set_page_private(page, (unsigned long)iop);
        SetPagePrivate(page);
        return iop;
}

static void
iomap_page_release(struct page *page)
{
        struct iomap_page *iop = to_iomap_page(page);

        if (!iop)
                return;
        WARN_ON_ONCE(atomic_read(&iop->read_count));
        WARN_ON_ONCE(atomic_read(&iop->write_count));
        ClearPagePrivate(page);
        set_page_private(page, 0);
        put_page(page);
        kfree(iop);
}

/*
 * Calculate the range inside the page that we actually need to read.
 */
static void
iomap_adjust_read_range(struct inode *inode, struct iomap_page *iop,
                loff_t *pos, loff_t length, unsigned *offp, unsigned *lenp)
{
        loff_t orig_pos = *pos;
        loff_t isize = i_size_read(inode);
        unsigned block_bits = inode->i_blkbits;
        unsigned block_size = (1 << block_bits);
        unsigned poff = offset_in_page(*pos);
        unsigned plen = min_t(loff_t, PAGE_SIZE - poff, length);
        unsigned first = poff >> block_bits;
        unsigned last = (poff + plen - 1) >> block_bits;

        /*
         * If the block size is smaller than the page size we need to check the
         * per-block uptodate status and adjust the offset and length if needed
         * to avoid reading in already uptodate ranges.
         */
        if (iop) {
                unsigned int i;

                /* move forward for each leading block marked uptodate */
                for (i = first; i <= last; i++) {
                        if (!test_bit(i, iop->uptodate))
                                break;
                        *pos += block_size;
                        poff += block_size;
                        plen -= block_size;
                        first++;
                }

                /* truncate len if we find any trailing uptodate block(s) */
                for ( ; i <= last; i++) {
                        if (test_bit(i, iop->uptodate)) {
                                plen -= (last - i + 1) * block_size;
                                last = i - 1;
                                break;
                        }
                }
        }

        /*
         * If the extent spans the block that contains the i_size we need to
         * handle both halves separately so that we properly zero data in the
         * page cache for blocks that are entirely outside of i_size.
         */
        if (orig_pos <= isize && orig_pos + length > isize) {
                unsigned end = offset_in_page(isize - 1) >> block_bits;

                if (first <= end && last > end)
                        plen -= (last - end) * block_size;
        }

        *offp = poff;
        *lenp = plen;
}

static void
iomap_iop_set_range_uptodate(struct page *page, unsigned off, unsigned len)
{
        struct iomap_page *iop = to_iomap_page(page);
        struct inode *inode = page->mapping->host;
        unsigned first = off >> inode->i_blkbits;
        unsigned last = (off + len - 1) >> inode->i_blkbits;
        bool uptodate = true;
        unsigned long flags;
        unsigned int i;

        spin_lock_irqsave(&iop->uptodate_lock, flags);
        for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
                if (i >= first && i <= last)
                        set_bit(i, iop->uptodate);
                else if (!test_bit(i, iop->uptodate))
                        uptodate = false;
        }

        if (uptodate)
                SetPageUptodate(page);
        spin_unlock_irqrestore(&iop->uptodate_lock, flags);
}

static void
iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len)
{
        if (PageError(page))
                return;

        if (page_has_private(page))
                iomap_iop_set_range_uptodate(page, off, len);
        else
                SetPageUptodate(page);
}

static void
iomap_read_finish(struct iomap_page *iop, struct page *page)
{
        if (!iop || atomic_dec_and_test(&iop->read_count))
                unlock_page(page);
}

static void
iomap_read_page_end_io(struct bio_vec *bvec, int error)
{
        struct page *page = bvec->bv_page;
        struct iomap_page *iop = to_iomap_page(page);

        if (unlikely(error)) {
                ClearPageUptodate(page);
                SetPageError(page);
        } else {
                iomap_set_range_uptodate(page, bvec->bv_offset, bvec->bv_len);
        }

        iomap_read_finish(iop, page);
}

static void
iomap_read_end_io(struct bio *bio)
{
        int error = blk_status_to_errno(bio->bi_status);
        struct bio_vec *bvec;
        struct bvec_iter_all iter_all;

        bio_for_each_segment_all(bvec, bio, iter_all)
                iomap_read_page_end_io(bvec, error);
        bio_put(bio);
}

struct iomap_readpage_ctx {
        struct page             *cur_page;
        bool                    cur_page_in_bio;
        bool                    is_readahead;
        struct bio              *bio;
        struct list_head        *pages;
};

static void
iomap_read_inline_data(struct inode *inode, struct page *page,
                struct iomap *iomap)
{
        size_t size = i_size_read(inode);
        void *addr;

        if (PageUptodate(page))
                return;

        BUG_ON(page->index);
        BUG_ON(size > PAGE_SIZE - offset_in_page(iomap->inline_data));

        addr = kmap_atomic(page);
        memcpy(addr, iomap->inline_data, size);
        memset(addr + size, 0, PAGE_SIZE - size);
        kunmap_atomic(addr);
        SetPageUptodate(page);
}

static inline bool iomap_block_needs_zeroing(struct inode *inode,
                struct iomap *iomap, loff_t pos)
{
        return iomap->type != IOMAP_MAPPED ||
                (iomap->flags & IOMAP_F_NEW) ||
                pos >= i_size_read(inode);
}

static loff_t
iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
                struct iomap *iomap, struct iomap *srcmap)
{
        struct iomap_readpage_ctx *ctx = data;
        struct page *page = ctx->cur_page;
        struct iomap_page *iop = iomap_page_create(inode, page);
        bool same_page = false, is_contig = false;
        loff_t orig_pos = pos;
        unsigned poff, plen;
        sector_t sector;

        if (iomap->type == IOMAP_INLINE) {
                WARN_ON_ONCE(pos);
                iomap_read_inline_data(inode, page, iomap);
                return PAGE_SIZE;
        }

        /* zero post-eof blocks as the page may be mapped */
        iomap_adjust_read_range(inode, iop, &pos, length, &poff, &plen);
        if (plen == 0)
                goto done;

        if (iomap_block_needs_zeroing(inode, iomap, pos)) {
                zero_user(page, poff, plen);
                iomap_set_range_uptodate(page, poff, plen);
                goto done;
        }

        ctx->cur_page_in_bio = true;

        /*
         * Try to merge into a previous segment if we can.
         */
        sector = iomap_sector(iomap, pos);
        if (ctx->bio && bio_end_sector(ctx->bio) == sector)
                is_contig = true;

        if (is_contig &&
            __bio_try_merge_page(ctx->bio, page, plen, poff, &same_page)) {
                if (!same_page && iop)
                        atomic_inc(&iop->read_count);
                goto done;
        }

        /*
         * If we start a new segment we need to increase the read count, and we
         * need to do so before submitting any previous full bio to make sure
         * that we don't prematurely unlock the page.
         */
        if (iop)
                atomic_inc(&iop->read_count);

        if (!ctx->bio || !is_contig || bio_full(ctx->bio, plen)) {
                gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
                int nr_vecs = (length + PAGE_SIZE - 1) >> PAGE_SHIFT;

                if (ctx->bio)
                        submit_bio(ctx->bio);

                if (ctx->is_readahead) /* same as readahead_gfp_mask */
                        gfp |= __GFP_NORETRY | __GFP_NOWARN;
                ctx->bio = bio_alloc(gfp, min(BIO_MAX_PAGES, nr_vecs));
                ctx->bio->bi_opf = REQ_OP_READ;
                if (ctx->is_readahead)
                        ctx->bio->bi_opf |= REQ_RAHEAD;
                ctx->bio->bi_iter.bi_sector = sector;
                bio_set_dev(ctx->bio, iomap->bdev);
                ctx->bio->bi_end_io = iomap_read_end_io;
        }

        bio_add_page(ctx->bio, page, plen, poff);
done:
        /*
         * Move the caller beyond our range so that it keeps making progress.
         * For that we have to include any leading non-uptodate ranges, but
         * we can skip trailing ones as they will be handled in the next
         * iteration.
         */
        return pos - orig_pos + plen;
}

int
iomap_readpage(struct page *page, const struct iomap_ops *ops)
{
        struct iomap_readpage_ctx ctx = { .cur_page = page };
        struct inode *inode = page->mapping->host;
        unsigned poff;
        loff_t ret;

        trace_iomap_readpage(page->mapping->host, 1);

        for (poff = 0; poff < PAGE_SIZE; poff += ret) {
                ret = iomap_apply(inode, page_offset(page) + poff,
                                PAGE_SIZE - poff, 0, ops, &ctx,
                                iomap_readpage_actor);
                if (ret <= 0) {
                        WARN_ON_ONCE(ret == 0);
                        SetPageError(page);
                        break;
                }
        }

        if (ctx.bio) {
                submit_bio(ctx.bio);
                WARN_ON_ONCE(!ctx.cur_page_in_bio);
        } else {
                WARN_ON_ONCE(ctx.cur_page_in_bio);
                unlock_page(page);
        }

        /*
         * Just like mpage_readpages and block_read_full_page we always
         * return 0 and just mark the page as PageError on errors.  This
         * should be cleaned up all through the stack eventually.
         */
        return 0;
}
EXPORT_SYMBOL_GPL(iomap_readpage);

static struct page *
iomap_next_page(struct inode *inode, struct list_head *pages, loff_t pos,
                loff_t length, loff_t *done)
{
        while (!list_empty(pages)) {
                struct page *page = lru_to_page(pages);

                if (page_offset(page) >= (u64)pos + length)
                        break;

                list_del(&page->lru);
                if (!add_to_page_cache_lru(page, inode->i_mapping, page->index,
                                GFP_NOFS))
                        return page;

                /*
                 * If we already have a page in the page cache at index we are
                 * done.  Upper layers don't care if it is uptodate after the
                 * readpages call itself as every page gets checked again once
                 * actually needed.
                 */
                *done += PAGE_SIZE;
                put_page(page);
        }

        return NULL;
}

static loff_t
iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
                void *data, struct iomap *iomap, struct iomap *srcmap)
{
        struct iomap_readpage_ctx *ctx = data;
        loff_t done, ret;

        for (done = 0; done < length; done += ret) {
                if (ctx->cur_page && offset_in_page(pos + done) == 0) {
                        if (!ctx->cur_page_in_bio)
                                unlock_page(ctx->cur_page);
                        put_page(ctx->cur_page);
                        ctx->cur_page = NULL;
                }
                if (!ctx->cur_page) {
                        ctx->cur_page = iomap_next_page(inode, ctx->pages,
                                        pos, length, &done);
                        if (!ctx->cur_page)
                                break;
                        ctx->cur_page_in_bio = false;
                }
                ret = iomap_readpage_actor(inode, pos + done, length - done,
                                ctx, iomap, srcmap);
        }

        return done;
}

int
iomap_readpages(struct address_space *mapping, struct list_head *pages,
                unsigned nr_pages, const struct iomap_ops *ops)
{
        struct iomap_readpage_ctx ctx = {
                .pages          = pages,
                .is_readahead   = true,
        };
        loff_t pos = page_offset(list_entry(pages->prev, struct page, lru));
        loff_t last = page_offset(list_entry(pages->next, struct page, lru));
        loff_t length = last - pos + PAGE_SIZE, ret = 0;

        trace_iomap_readpages(mapping->host, nr_pages);

        while (length > 0) {
                ret = iomap_apply(mapping->host, pos, length, 0, ops,
                                &ctx, iomap_readpages_actor);
                if (ret <= 0) {
                        WARN_ON_ONCE(ret == 0);
                        goto done;
                }
                pos += ret;
                length -= ret;
        }
        ret = 0;
done:
        if (ctx.bio)
                submit_bio(ctx.bio);
        if (ctx.cur_page) {
                if (!ctx.cur_page_in_bio)
                        unlock_page(ctx.cur_page);
                put_page(ctx.cur_page);
        }

        /*
         * Check that we didn't lose a page due to the arcance calling
         * conventions..
         */
        WARN_ON_ONCE(!ret && !list_empty(ctx.pages));
        return ret;
}
EXPORT_SYMBOL_GPL(iomap_readpages);

/*
 * iomap_is_partially_uptodate checks whether blocks within a page are
 * uptodate or not.
 *
 * Returns true if all blocks which correspond to a file portion
 * we want to read within the page are uptodate.
 */
int
iomap_is_partially_uptodate(struct page *page, unsigned long from,
                unsigned long count)
{
        struct iomap_page *iop = to_iomap_page(page);
        struct inode *inode = page->mapping->host;
        unsigned len, first, last;
        unsigned i;

        /* Limit range to one page */
        len = min_t(unsigned, PAGE_SIZE - from, count);

        /* First and last blocks in range within page */
        first = from >> inode->i_blkbits;
        last = (from + len - 1) >> inode->i_blkbits;

        if (iop) {
                for (i = first; i <= last; i++)
                        if (!test_bit(i, iop->uptodate))
                                return 0;
                return 1;
        }

        return 0;
}
EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);

int
iomap_releasepage(struct page *page, gfp_t gfp_mask)
{
        trace_iomap_releasepage(page->mapping->host, page, 0, 0);

        /*
         * mm accommodates an old ext3 case where clean pages might not have had
         * the dirty bit cleared. Thus, it can send actual dirty pages to
         * ->releasepage() via shrink_active_list(), skip those here.
         */
        if (PageDirty(page) || PageWriteback(page))
                return 0;
        iomap_page_release(page);
        return 1;
}
EXPORT_SYMBOL_GPL(iomap_releasepage);

void
iomap_invalidatepage(struct page *page, unsigned int offset, unsigned int len)
{
        trace_iomap_invalidatepage(page->mapping->host, page, offset, len);

        /*
         * If we are invalidating the entire page, clear the dirty state from it
         * and release it to avoid unnecessary buildup of the LRU.
         */
        if (offset == 0 && len == PAGE_SIZE) {
                WARN_ON_ONCE(PageWriteback(page));
                cancel_dirty_page(page);
                iomap_page_release(page);
        }
}
EXPORT_SYMBOL_GPL(iomap_invalidatepage);

#ifdef CONFIG_MIGRATION
int
iomap_migrate_page(struct address_space *mapping, struct page *newpage,
                struct page *page, enum migrate_mode mode)
{
        int ret;

        ret = migrate_page_move_mapping(mapping, newpage, page, 0);
        if (ret != MIGRATEPAGE_SUCCESS)
                return ret;

        if (page_has_private(page)) {
                ClearPagePrivate(page);
                get_page(newpage);
                set_page_private(newpage, page_private(page));
                set_page_private(page, 0);
                put_page(page);
                SetPagePrivate(newpage);
        }

        if (mode != MIGRATE_SYNC_NO_COPY)
                migrate_page_copy(newpage, page);
        else
                migrate_page_states(newpage, page);
        return MIGRATEPAGE_SUCCESS;
}
EXPORT_SYMBOL_GPL(iomap_migrate_page);
#endif /* CONFIG_MIGRATION */

enum {
        IOMAP_WRITE_F_UNSHARE           = (1 << 0),
};

static void
iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
{
        loff_t i_size = i_size_read(inode);

        /*
         * Only truncate newly allocated pages beyoned EOF, even if the
         * write started inside the existing inode size.
         */
        if (pos + len > i_size)
                truncate_pagecache_range(inode, max(pos, i_size), pos + len);
}

static int
iomap_read_page_sync(loff_t block_start, struct page *page, unsigned poff,
                unsigned plen, struct iomap *iomap)
{
        struct bio_vec bvec;
        struct bio bio;

        bio_init(&bio, &bvec, 1);
        bio.bi_opf = REQ_OP_READ;
        bio.bi_iter.bi_sector = iomap_sector(iomap, block_start);
        bio_set_dev(&bio, iomap->bdev);
        __bio_add_page(&bio, page, plen, poff);
        return submit_bio_wait(&bio);
}

static int
__iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
                struct page *page, struct iomap *srcmap)
{
        struct iomap_page *iop = iomap_page_create(inode, page);
        loff_t block_size = i_blocksize(inode);
        loff_t block_start = pos & ~(block_size - 1);
        loff_t block_end = (pos + len + block_size - 1) & ~(block_size - 1);
        unsigned from = offset_in_page(pos), to = from + len, poff, plen;
        int status;

        if (PageUptodate(page))
                return 0;

        do {
                iomap_adjust_read_range(inode, iop, &block_start,
                                block_end - block_start, &poff, &plen);
                if (plen == 0)
                        break;

                if (!(flags & IOMAP_WRITE_F_UNSHARE) &&
                    (from <= poff || from >= poff + plen) &&
                    (to <= poff || to >= poff + plen))
                        continue;

                if (iomap_block_needs_zeroing(inode, srcmap, block_start)) {
                        if (WARN_ON_ONCE(flags & IOMAP_WRITE_F_UNSHARE))
                                return -EIO;
                        zero_user_segments(page, poff, from, to, poff + plen);
                        iomap_set_range_uptodate(page, poff, plen);
                        continue;
                }

                status = iomap_read_page_sync(block_start, page, poff, plen,
                                srcmap);
                if (status)
                        return status;
        } while ((block_start += plen) < block_end);

        return 0;
}

static int
iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
                struct page **pagep, struct iomap *iomap, struct iomap *srcmap)
{
        const struct iomap_page_ops *page_ops = iomap->page_ops;
        struct page *page;
        int status = 0;

        BUG_ON(pos + len > iomap->offset + iomap->length);
        if (srcmap != iomap)
                BUG_ON(pos + len > srcmap->offset + srcmap->length);

        if (fatal_signal_pending(current))
                return -EINTR;

        if (page_ops && page_ops->page_prepare) {
                status = page_ops->page_prepare(inode, pos, len, iomap);
                if (status)
                        return status;
        }

        page = grab_cache_page_write_begin(inode->i_mapping, pos >> PAGE_SHIFT,
                        AOP_FLAG_NOFS);
        if (!page) {
                status = -ENOMEM;
                goto out_no_page;
        }

        if (srcmap->type == IOMAP_INLINE)
                iomap_read_inline_data(inode, page, srcmap);
        else if (iomap->flags & IOMAP_F_BUFFER_HEAD)
                status = __block_write_begin_int(page, pos, len, NULL, srcmap);
        else
                status = __iomap_write_begin(inode, pos, len, flags, page,
                                srcmap);

        if (unlikely(status))
                goto out_unlock;

        *pagep = page;
        return 0;

out_unlock:
        unlock_page(page);
        put_page(page);
        iomap_write_failed(inode, pos, len);

out_no_page:
        if (page_ops && page_ops->page_done)
                page_ops->page_done(inode, pos, 0, NULL, iomap);
        return status;
}

int
iomap_set_page_dirty(struct page *page)
{
        struct address_space *mapping = page_mapping(page);
        int newly_dirty;

        if (unlikely(!mapping))
                return !TestSetPageDirty(page);

        /*
         * Lock out page->mem_cgroup migration to keep PageDirty
         * synchronized with per-memcg dirty page counters.
         */
        lock_page_memcg(page);
        newly_dirty = !TestSetPageDirty(page);
        if (newly_dirty)
                __set_page_dirty(page, mapping, 0);
        unlock_page_memcg(page);

        if (newly_dirty)
                __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
        return newly_dirty;
}
EXPORT_SYMBOL_GPL(iomap_set_page_dirty);

static int
__iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
                unsigned copied, struct page *page)
{
        flush_dcache_page(page);

        /*
         * The blocks that were entirely written will now be uptodate, so we
         * don't have to worry about a readpage reading them and overwriting a
         * partial write.  However if we have encountered a short write and only
         * partially written into a block, it will not be marked uptodate, so a
         * readpage might come in and destroy our partial write.
         *
         * Do the simplest thing, and just treat any short write to a non
         * uptodate page as a zero-length write, and force the caller to redo
         * the whole thing.
         */
        if (unlikely(copied < len && !PageUptodate(page)))
                return 0;
        iomap_set_range_uptodate(page, offset_in_page(pos), len);
        iomap_set_page_dirty(page);
        return copied;
}

static int
iomap_write_end_inline(struct inode *inode, struct page *page,
                struct iomap *iomap, loff_t pos, unsigned copied)
{
        void *addr;

        WARN_ON_ONCE(!PageUptodate(page));
        BUG_ON(pos + copied > PAGE_SIZE - offset_in_page(iomap->inline_data));

        addr = kmap_atomic(page);
        memcpy(iomap->inline_data + pos, addr + pos, copied);
        kunmap_atomic(addr);

        mark_inode_dirty(inode);
        return copied;
}

static int
iomap_write_end(struct inode *inode, loff_t pos, unsigned len, unsigned copied,
                struct page *page, struct iomap *iomap, struct iomap *srcmap)
{
        const struct iomap_page_ops *page_ops = iomap->page_ops;
        loff_t old_size = inode->i_size;
        int ret;

        if (srcmap->type == IOMAP_INLINE) {
                ret = iomap_write_end_inline(inode, page, iomap, pos, copied);
        } else if (srcmap->flags & IOMAP_F_BUFFER_HEAD) {
                ret = block_write_end(NULL, inode->i_mapping, pos, len, copied,
                                page, NULL);
        } else {
                ret = __iomap_write_end(inode, pos, len, copied, page);
        }

        /*
         * Update the in-memory inode size after copying the data into the page
         * cache.  It's up to the file system to write the updated size to disk,
         * preferably after I/O completion so that no stale data is exposed.
         */
        if (pos + ret > old_size) {
                i_size_write(inode, pos + ret);
                iomap->flags |= IOMAP_F_SIZE_CHANGED;
        }
        unlock_page(page);

        if (old_size < pos)
                pagecache_isize_extended(inode, old_size, pos);
        if (page_ops && page_ops->page_done)
                page_ops->page_done(inode, pos, ret, page, iomap);
        put_page(page);

        if (ret < len)
                iomap_write_failed(inode, pos, len);
        return ret;
}

static loff_t
iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
                struct iomap *iomap, struct iomap *srcmap)
{
        struct iov_iter *i = data;
        long status = 0;
        ssize_t written = 0;

        do {
                struct page *page;
                unsigned long offset;   /* Offset into pagecache page */
                unsigned long bytes;    /* Bytes to write to page */
                size_t copied;          /* Bytes copied from user */

                offset = offset_in_page(pos);
                bytes = min_t(unsigned long, PAGE_SIZE - offset,
                                                iov_iter_count(i));
again:
                if (bytes > length)
                        bytes = length;

                /*
                 * Bring in the user page that we will copy from _first_.
                 * Otherwise there's a nasty deadlock on copying from the
                 * same page as we're writing to, without it being marked
                 * up-to-date.
                 *
                 * Not only is this an optimisation, but it is also required
                 * to check that the address is actually valid, when atomic
                 * usercopies are used, below.
                 */
                if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
                        status = -EFAULT;
                        break;
                }

                status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap,
                                srcmap);
                if (unlikely(status))
                        break;

                if (mapping_writably_mapped(inode->i_mapping))
                        flush_dcache_page(page);

                copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);

                flush_dcache_page(page);

                status = iomap_write_end(inode, pos, bytes, copied, page, iomap,
                                srcmap);
                if (unlikely(status < 0))
                        break;
                copied = status;

                cond_resched();

                iov_iter_advance(i, copied);
                if (unlikely(copied == 0)) {
                        /*
                         * If we were unable to copy any data at all, we must
                         * fall back to a single segment length write.
                         *
                         * If we didn't fallback here, we could livelock
                         * because not all segments in the iov can be copied at
                         * once without a pagefault.
                         */
                        bytes = min_t(unsigned long, PAGE_SIZE - offset,
                                                iov_iter_single_seg_count(i));
                        goto again;
                }
                pos += copied;
                written += copied;
                length -= copied;

                balance_dirty_pages_ratelimited(inode->i_mapping);
        } while (iov_iter_count(i) && length);

        return written ? written : status;
}

ssize_t
iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *iter,
                const struct iomap_ops *ops)
{
        struct inode *inode = iocb->ki_filp->f_mapping->host;
        loff_t pos = iocb->ki_pos, ret = 0, written = 0;

        while (iov_iter_count(iter)) {
                ret = iomap_apply(inode, pos, iov_iter_count(iter),
                                IOMAP_WRITE, ops, iter, iomap_write_actor);
                if (ret <= 0)
                        break;
                pos += ret;
                written += ret;
        }

        return written ? written : ret;
}
EXPORT_SYMBOL_GPL(iomap_file_buffered_write);

static loff_t
iomap_unshare_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
                struct iomap *iomap, struct iomap *srcmap)
{
        long status = 0;
        ssize_t written = 0;

        /* don't bother with blocks that are not shared to start with */
        if (!(iomap->flags & IOMAP_F_SHARED))
                return length;
        /* don't bother with holes or unwritten extents */
        if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
                return length;

        do {
                unsigned long offset = offset_in_page(pos);
                unsigned long bytes = min_t(loff_t, PAGE_SIZE - offset, length);
                struct page *page;

                status = iomap_write_begin(inode, pos, bytes,
                                IOMAP_WRITE_F_UNSHARE, &page, iomap, srcmap);
                if (unlikely(status))
                        return status;

                status = iomap_write_end(inode, pos, bytes, bytes, page, iomap,
                                srcmap);
                if (unlikely(status <= 0)) {
                        if (WARN_ON_ONCE(status == 0))
                                return -EIO;
                        return status;
                }

                cond_resched();

                pos += status;
                written += status;
                length -= status;

                balance_dirty_pages_ratelimited(inode->i_mapping);
        } while (length);

        return written;
}

int
iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
                const struct iomap_ops *ops)
{
        loff_t ret;

        while (len) {
                ret = iomap_apply(inode, pos, len, IOMAP_WRITE, ops, NULL,
                                iomap_unshare_actor);
                if (ret <= 0)
                        return ret;
                pos += ret;
                len -= ret;
        }

        return 0;
}
EXPORT_SYMBOL_GPL(iomap_file_unshare);

static int iomap_zero(struct inode *inode, loff_t pos, unsigned offset,
                unsigned bytes, struct iomap *iomap, struct iomap *srcmap)
{
        struct page *page;
        int status;

        status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap, srcmap);
        if (status)
                return status;

        zero_user(page, offset, bytes);
        mark_page_accessed(page);

        return iomap_write_end(inode, pos, bytes, bytes, page, iomap, srcmap);
}

static int iomap_dax_zero(loff_t pos, unsigned offset, unsigned bytes,
                struct iomap *iomap)
{
        return __dax_zero_page_range(iomap->bdev, iomap->dax_dev,
                        iomap_sector(iomap, pos & PAGE_MASK), offset, bytes);
}

static loff_t
iomap_zero_range_actor(struct inode *inode, loff_t pos, loff_t count,
                void *data, struct iomap *iomap, struct iomap *srcmap)
{
        bool *did_zero = data;
        loff_t written = 0;
        int status;

        /* already zeroed?  we're done. */
        if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
                return count;

        do {
                unsigned offset, bytes;

                offset = offset_in_page(pos);
                bytes = min_t(loff_t, PAGE_SIZE - offset, count);

                if (IS_DAX(inode))
                        status = iomap_dax_zero(pos, offset, bytes, iomap);
                else
                        status = iomap_zero(inode, pos, offset, bytes, iomap,
                                        srcmap);
                if (status < 0)
                        return status;

                pos += bytes;
                count -= bytes;
                written += bytes;
                if (did_zero)
                        *did_zero = true;
        } while (count > 0);

        return written;
}

int
iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
                const struct iomap_ops *ops)
{
        loff_t ret;

        while (len > 0) {
                ret = iomap_apply(inode, pos, len, IOMAP_ZERO,
                                ops, did_zero, iomap_zero_range_actor);
                if (ret <= 0)
                        return ret;

                pos += ret;
                len -= ret;
        }

        return 0;
}
EXPORT_SYMBOL_GPL(iomap_zero_range);

int
iomap_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
                const struct iomap_ops *ops)
{
        unsigned int blocksize = i_blocksize(inode);
        unsigned int off = pos & (blocksize - 1);

        /* Block boundary? Nothing to do */
        if (!off)
                return 0;
        return iomap_zero_range(inode, pos, blocksize - off, did_zero, ops);
}
EXPORT_SYMBOL_GPL(iomap_truncate_page);

static loff_t
iomap_page_mkwrite_actor(struct inode *inode, loff_t pos, loff_t length,
                void *data, struct iomap *iomap, struct iomap *srcmap)
{
        struct page *page = data;
        int ret;

        if (iomap->flags & IOMAP_F_BUFFER_HEAD) {
                ret = __block_write_begin_int(page, pos, length, NULL, iomap);
                if (ret)
                        return ret;
                block_commit_write(page, 0, length);
        } else {
                WARN_ON_ONCE(!PageUptodate(page));
                iomap_page_create(inode, page);
                set_page_dirty(page);
        }

        return length;
}

vm_fault_t iomap_page_mkwrite(struct vm_fault *vmf, const struct iomap_ops *ops)
{
        struct page *page = vmf->page;
        struct inode *inode = file_inode(vmf->vma->vm_file);
        unsigned long length;
        loff_t offset;
        ssize_t ret;

        lock_page(page);
        ret = page_mkwrite_check_truncate(page, inode);
        if (ret < 0)
                goto out_unlock;
        length = ret;

        offset = page_offset(page);
        while (length > 0) {
                ret = iomap_apply(inode, offset, length,
                                IOMAP_WRITE | IOMAP_FAULT, ops, page,
                                iomap_page_mkwrite_actor);
                if (unlikely(ret <= 0))
                        goto out_unlock;
                offset += ret;
                length -= ret;
        }

        wait_for_stable_page(page);
        return VM_FAULT_LOCKED;
out_unlock:
        unlock_page(page);
        return block_page_mkwrite_return(ret);
}
EXPORT_SYMBOL_GPL(iomap_page_mkwrite);

static void
iomap_finish_page_writeback(struct inode *inode, struct page *page,
                int error)
{
        struct iomap_page *iop = to_iomap_page(page);

        if (error) {
                SetPageError(page);
                mapping_set_error(inode->i_mapping, -EIO);
        }

        WARN_ON_ONCE(i_blocksize(inode) < PAGE_SIZE && !iop);
        WARN_ON_ONCE(iop && atomic_read(&iop->write_count) <= 0);

        if (!iop || atomic_dec_and_test(&iop->write_count))
                end_page_writeback(page);
}

/*
 * We're now finished for good with this ioend structure.  Update the page
 * state, release holds on bios, and finally free up memory.  Do not use the
 * ioend after this.
 */
static void
iomap_finish_ioend(struct iomap_ioend *ioend, int error)
{
        struct inode *inode = ioend->io_inode;
        struct bio *bio = &ioend->io_inline_bio;
        struct bio *last = ioend->io_bio, *next;
        u64 start = bio->bi_iter.bi_sector;
        loff_t offset = ioend->io_offset;
        bool quiet = bio_flagged(bio, BIO_QUIET);

        for (bio = &ioend->io_inline_bio; bio; bio = next) {
                struct bio_vec *bv;
                struct bvec_iter_all iter_all;

                /*
                 * For the last bio, bi_private points to the ioend, so we
                 * need to explicitly end the iteration here.
                 */
                if (bio == last)
                        next = NULL;
                else
                        next = bio->bi_private;

                /* walk each page on bio, ending page IO on them */
                bio_for_each_segment_all(bv, bio, iter_all)
                        iomap_finish_page_writeback(inode, bv->bv_page, error);
                bio_put(bio);
        }
        /* The ioend has been freed by bio_put() */

        if (unlikely(error && !quiet)) {
                printk_ratelimited(KERN_ERR
"%s: writeback error on inode %lu, offset %lld, sector %llu",
                        inode->i_sb->s_id, inode->i_ino, offset, start);
        }
}

void
iomap_finish_ioends(struct iomap_ioend *ioend, int error)
{
        struct list_head tmp;

        list_replace_init(&ioend->io_list, &tmp);
        iomap_finish_ioend(ioend, error);

        while (!list_empty(&tmp)) {
                ioend = list_first_entry(&tmp, struct iomap_ioend, io_list);
                list_del_init(&ioend->io_list);
                iomap_finish_ioend(ioend, error);
        }
}
EXPORT_SYMBOL_GPL(iomap_finish_ioends);

/*
 * We can merge two adjacent ioends if they have the same set of work to do.
 */
static bool
iomap_ioend_can_merge(struct iomap_ioend *ioend, struct iomap_ioend *next)
{
        if (ioend->io_bio->bi_status != next->io_bio->bi_status)
                return false;
        if ((ioend->io_flags & IOMAP_F_SHARED) ^
            (next->io_flags & IOMAP_F_SHARED))
                return false;
        if ((ioend->io_type == IOMAP_UNWRITTEN) ^
            (next->io_type == IOMAP_UNWRITTEN))
                return false;
        if (ioend->io_offset + ioend->io_size != next->io_offset)
                return false;
        return true;
}

void
iomap_ioend_try_merge(struct iomap_ioend *ioend, struct list_head *more_ioends,
                void (*merge_private)(struct iomap_ioend *ioend,
                                struct iomap_ioend *next))
{
        struct iomap_ioend *next;

        INIT_LIST_HEAD(&ioend->io_list);

        while ((next = list_first_entry_or_null(more_ioends, struct iomap_ioend,
                        io_list))) {
                if (!iomap_ioend_can_merge(ioend, next))
                        break;
                list_move_tail(&next->io_list, &ioend->io_list);
                ioend->io_size += next->io_size;
                if (next->io_private && merge_private)
                        merge_private(ioend, next);
        }
}
EXPORT_SYMBOL_GPL(iomap_ioend_try_merge);

static int
iomap_ioend_compare(void *priv, struct list_head *a, struct list_head *b)
{
        struct iomap_ioend *ia = container_of(a, struct iomap_ioend, io_list);
        struct iomap_ioend *ib = container_of(b, struct iomap_ioend, io_list);

        if (ia->io_offset < ib->io_offset)
                return -1;
        if (ia->io_offset > ib->io_offset)
                return 1;
        return 0;
}

void
iomap_sort_ioends(struct list_head *ioend_list)
{
        list_sort(NULL, ioend_list, iomap_ioend_compare);
}
EXPORT_SYMBOL_GPL(iomap_sort_ioends);

static void iomap_writepage_end_bio(struct bio *bio)
{
        struct iomap_ioend *ioend = bio->bi_private;

        iomap_finish_ioend(ioend, blk_status_to_errno(bio->bi_status));
}

/*
 * Submit the final bio for an ioend.
 *
 * If @error is non-zero, it means that we have a situation where some part of
 * the submission process has failed after we have marked paged for writeback
 * and unlocked them.  In this situation, we need to fail the bio instead of
 * submitting it.  This typically only happens on a filesystem shutdown.
 */
static int
iomap_submit_ioend(struct iomap_writepage_ctx *wpc, struct iomap_ioend *ioend,
                int error)
{
        ioend->io_bio->bi_private = ioend;
        ioend->io_bio->bi_end_io = iomap_writepage_end_bio;

        if (wpc->ops->prepare_ioend)
                error = wpc->ops->prepare_ioend(ioend, error);
        if (error) {
                /*
                 * If we are failing the IO now, just mark the ioend with an
                 * error and finish it.  This will run IO completion immediately
                 * as there is only one reference to the ioend at this point in
                 * time.
                 */
                ioend->io_bio->bi_status = errno_to_blk_status(error);
                bio_endio(ioend->io_bio);
                return error;
        }

        submit_bio(ioend->io_bio);
        return 0;
}

static struct iomap_ioend *
iomap_alloc_ioend(struct inode *inode, struct iomap_writepage_ctx *wpc,
                loff_t offset, sector_t sector, struct writeback_control *wbc)
{
        struct iomap_ioend *ioend;
        struct bio *bio;

        bio = bio_alloc_bioset(GFP_NOFS, BIO_MAX_PAGES, &iomap_ioend_bioset);
        bio_set_dev(bio, wpc->iomap.bdev);
        bio->bi_iter.bi_sector = sector;
        bio->bi_opf = REQ_OP_WRITE | wbc_to_write_flags(wbc);
        bio->bi_write_hint = inode->i_write_hint;
        wbc_init_bio(wbc, bio);

        ioend = container_of(bio, struct iomap_ioend, io_inline_bio);
        INIT_LIST_HEAD(&ioend->io_list);
        ioend->io_type = wpc->iomap.type;
        ioend->io_flags = wpc->iomap.flags;
        ioend->io_inode = inode;
        ioend->io_size = 0;
        ioend->io_offset = offset;
        ioend->io_private = NULL;
        ioend->io_bio = bio;
        return ioend;
}

/*
 * Allocate a new bio, and chain the old bio to the new one.
 *
 * Note that we have to do perform the chaining in this unintuitive order
 * so that the bi_private linkage is set up in the right direction for the
 * traversal in iomap_finish_ioend().
 */
static struct bio *
iomap_chain_bio(struct bio *prev)
{
        struct bio *new;

        new = bio_alloc(GFP_NOFS, BIO_MAX_PAGES);
        bio_copy_dev(new, prev);/* also copies over blkcg information */
        new->bi_iter.bi_sector = bio_end_sector(prev);
        new->bi_opf = prev->bi_opf;
        new->bi_write_hint = prev->bi_write_hint;

        bio_chain(prev, new);
        bio_get(prev);          /* for iomap_finish_ioend */
        submit_bio(prev);
        return new;
}

static bool
iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t offset,
                sector_t sector)
{
        if ((wpc->iomap.flags & IOMAP_F_SHARED) !=
            (wpc->ioend->io_flags & IOMAP_F_SHARED))
                return false;
        if (wpc->iomap.type != wpc->ioend->io_type)
                return false;
        if (offset != wpc->ioend->io_offset + wpc->ioend->io_size)
                return false;
        if (sector != bio_end_sector(wpc->ioend->io_bio))
                return false;
        return true;
}

/*
 * Test to see if we have an existing ioend structure that we could append to
 * first, otherwise finish off the current ioend and start another.
 */
static void
iomap_add_to_ioend(struct inode *inode, loff_t offset, struct page *page,
                struct iomap_page *iop, struct iomap_writepage_ctx *wpc,
                struct writeback_control *wbc, struct list_head *iolist)
{
        sector_t sector = iomap_sector(&wpc->iomap, offset);
        unsigned len = i_blocksize(inode);
        unsigned poff = offset & (PAGE_SIZE - 1);
        bool merged, same_page = false;

        if (!wpc->ioend || !iomap_can_add_to_ioend(wpc, offset, sector)) {
                if (wpc->ioend)
                        list_add(&wpc->ioend->io_list, iolist);
                wpc->ioend = iomap_alloc_ioend(inode, wpc, offset, sector, wbc);
        }

        merged = __bio_try_merge_page(wpc->ioend->io_bio, page, len, poff,
                        &same_page);
        if (iop && !same_page)
                atomic_inc(&iop->write_count);

        if (!merged) {
                if (bio_full(wpc->ioend->io_bio, len)) {
                        wpc->ioend->io_bio =
                                iomap_chain_bio(wpc->ioend->io_bio);
                }
                bio_add_page(wpc->ioend->io_bio, page, len, poff);
        }

        wpc->ioend->io_size += len;
        wbc_account_cgroup_owner(wbc, page, len);
}

/*
 * We implement an immediate ioend submission policy here to avoid needing to
 * chain multiple ioends and hence nest mempool allocations which can violate
 * forward progress guarantees we need to provide. The current ioend we are
 * adding blocks to is cached on the writepage context, and if the new block
 * does not append to the cached ioend it will create a new ioend and cache that
 * instead.
 *
 * If a new ioend is created and cached, the old ioend is returned and queued
 * locally for submission once the entire page is processed or an error has been
 * detected.  While ioends are submitted immediately after they are completed,
 * batching optimisations are provided by higher level block plugging.
 *
 * At the end of a writeback pass, there will be a cached ioend remaining on the
 * writepage context that the caller will need to submit.
 */
static int
iomap_writepage_map(struct iomap_writepage_ctx *wpc,
                struct writeback_control *wbc, struct inode *inode,
                struct page *page, u64 end_offset)
{
        struct iomap_page *iop = to_iomap_page(page);
        struct iomap_ioend *ioend, *next;
        unsigned len = i_blocksize(inode);
        u64 file_offset; /* file offset of page */
        int error = 0, count = 0, i;
        LIST_HEAD(submit_list);

        WARN_ON_ONCE(i_blocksize(inode) < PAGE_SIZE && !iop);
        WARN_ON_ONCE(iop && atomic_read(&iop->write_count) != 0);

        /*
         * Walk through the page to find areas to write back. If we run off the
         * end of the current map or find the current map invalid, grab a new
         * one.
         */
        for (i = 0, file_offset = page_offset(page);
             i < (PAGE_SIZE >> inode->i_blkbits) && file_offset < end_offset;
             i++, file_offset += len) {
                if (iop && !test_bit(i, iop->uptodate))
                        continue;

                error = wpc->ops->map_blocks(wpc, inode, file_offset);
                if (error)
                        break;
                if (WARN_ON_ONCE(wpc->iomap.type == IOMAP_INLINE))
                        continue;
                if (wpc->iomap.type == IOMAP_HOLE)
                        continue;
                iomap_add_to_ioend(inode, file_offset, page, iop, wpc, wbc,
                                 &submit_list);
                count++;
        }

        WARN_ON_ONCE(!wpc->ioend && !list_empty(&submit_list));
        WARN_ON_ONCE(!PageLocked(page));
        WARN_ON_ONCE(PageWriteback(page));

        /*
         * We cannot cancel the ioend directly here on error.  We may have
         * already set other pages under writeback and hence we have to run I/O
         * completion to mark the error state of the pages under writeback
         * appropriately.
         */
        if (unlikely(error)) {
                if (!count) {
                        /*
                         * If the current page hasn't been added to ioend, it
                         * won't be affected by I/O completions and we must
                         * discard and unlock it right here.
                         */
                        if (wpc->ops->discard_page)
                                wpc->ops->discard_page(page);
                        ClearPageUptodate(page);
                        unlock_page(page);
                        goto done;
                }

                /*
                 * If the page was not fully cleaned, we need to ensure that the
                 * higher layers come back to it correctly.  That means we need
                 * to keep the page dirty, and for WB_SYNC_ALL writeback we need
                 * to ensure the PAGECACHE_TAG_TOWRITE index mark is not removed
                 * so another attempt to write this page in this writeback sweep
                 * will be made.
                 */
                set_page_writeback_keepwrite(page);
        } else {
                clear_page_dirty_for_io(page);
                set_page_writeback(page);
        }

        unlock_page(page);

        /*
         * Preserve the original error if there was one, otherwise catch
         * submission errors here and propagate into subsequent ioend
         * submissions.
         */
        list_for_each_entry_safe(ioend, next, &submit_list, io_list) {
                int error2;

                list_del_init(&ioend->io_list);
                error2 = iomap_submit_ioend(wpc, ioend, error);
                if (error2 && !error)
                        error = error2;
        }

        /*
         * We can end up here with no error and nothing to write only if we race
         * with a partial page truncate on a sub-page block sized filesystem.
         */
        if (!count)
                end_page_writeback(page);
done:
        mapping_set_error(page->mapping, error);
        return error;
}

/*
 * Write out a dirty page.
 *
 * For delalloc space on the page we need to allocate space and flush it.
 * For unwritten space on the page we need to start the conversion to
 * regular allocated space.
 */
static int
iomap_do_writepage(struct page *page, struct writeback_control *wbc, void *data)
{
        struct iomap_writepage_ctx *wpc = data;
        struct inode *inode = page->mapping->host;
        pgoff_t end_index;
        u64 end_offset;
        loff_t offset;

        trace_iomap_writepage(inode, page, 0, 0);

        /*
         * Refuse to write the page out if we are called from reclaim context.
         *
         * This avoids stack overflows when called from deeply used stacks in
         * random callers for direct reclaim or memcg reclaim.  We explicitly
         * allow reclaim from kswapd as the stack usage there is relatively low.
         *
         * This should never happen except in the case of a VM regression so
         * warn about it.
         */
        if (WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD)) ==
                        PF_MEMALLOC))
                goto redirty;

        /*
         * Given that we do not allow direct reclaim to call us, we should
         * never be called in a recursive filesystem reclaim context.
         */
        if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
                goto redirty;

        /*
         * Is this page beyond the end of the file?
         *
         * The page index is less than the end_index, adjust the end_offset
         * to the highest offset that this page should represent.
         * -----------------------------------------------------
         * |                    file mapping           | <EOF> |
         * -----------------------------------------------------
         * | Page ... | Page N-2 | Page N-1 |  Page N  |       |
         * ^--------------------------------^----------|--------
         * |     desired writeback range    |      see else    |
         * ---------------------------------^------------------|
         */
        offset = i_size_read(inode);
        end_index = offset >> PAGE_SHIFT;
        if (page->index < end_index)
                end_offset = (loff_t)(page->index + 1) << PAGE_SHIFT;
        else {
                /*
                 * Check whether the page to write out is beyond or straddles
                 * i_size or not.
                 * -------------------------------------------------------
                 * |            file mapping                    | <EOF>  |
                 * -------------------------------------------------------
                 * | Page ... | Page N-2 | Page N-1 |  Page N   | Beyond |
                 * ^--------------------------------^-----------|---------
                 * |                                |      Straddles     |
                 * ---------------------------------^-----------|--------|
                 */
                unsigned offset_into_page = offset & (PAGE_SIZE - 1);

                /*
                 * Skip the page if it is fully outside i_size, e.g. due to a
                 * truncate operation that is in progress. We must redirty the
                 * page so that reclaim stops reclaiming it. Otherwise
                 * iomap_vm_releasepage() is called on it and gets confused.
                 *
                 * Note that the end_index is unsigned long, it would overflow
                 * if the given offset is greater than 16TB on 32-bit system
                 * and if we do check the page is fully outside i_size or not
                 * via "if (page->index >= end_index + 1)" as "end_index + 1"
                 * will be evaluated to 0.  Hence this page will be redirtied
                 * and be written out repeatedly which would result in an
                 * infinite loop, the user program that perform this operation
                 * will hang.  Instead, we can verify this situation by checking
                 * if the page to write is totally beyond the i_size or if it's
                 * offset is just equal to the EOF.
                 */
                if (page->index > end_index ||
                    (page->index == end_index && offset_into_page == 0))
                        goto redirty;

                /*
                 * 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."
                 */
                zero_user_segment(page, offset_into_page, PAGE_SIZE);

                /* Adjust the end_offset to the end of file */
                end_offset = offset;
        }

        return iomap_writepage_map(wpc, wbc, inode, page, end_offset);

redirty:
        redirty_page_for_writepage(wbc, page);
        unlock_page(page);
        return 0;
}

int
iomap_writepage(struct page *page, struct writeback_control *wbc,
                struct iomap_writepage_ctx *wpc,
                const struct iomap_writeback_ops *ops)
{
        int ret;

        wpc->ops = ops;
        ret = iomap_do_writepage(page, wbc, wpc);
        if (!wpc->ioend)
                return ret;
        return iomap_submit_ioend(wpc, wpc->ioend, ret);
}
EXPORT_SYMBOL_GPL(iomap_writepage);

int
iomap_writepages(struct address_space *mapping, struct writeback_control *wbc,
                struct iomap_writepage_ctx *wpc,
                const struct iomap_writeback_ops *ops)
{
        int                     ret;

        wpc->ops = ops;
        ret = write_cache_pages(mapping, wbc, iomap_do_writepage, wpc);
        if (!wpc->ioend)
                return ret;
        return iomap_submit_ioend(wpc, wpc->ioend, ret);
}
EXPORT_SYMBOL_GPL(iomap_writepages);

static int __init iomap_init(void)
{
        return bioset_init(&iomap_ioend_bioset, 4 * (PAGE_SIZE / SECTOR_SIZE),
                           offsetof(struct iomap_ioend, io_inline_bio),
                           BIOSET_NEED_BVECS);
}
fs_initcall(iomap_init);