GNU Linux-libre 6.8.7-gnu

[releases.git] / fs / xfs / xfs_reflink.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * Copyright (C) 2016 Oracle.  All Rights Reserved.
 * Author: Darrick J. Wong <darrick.wong@oracle.com>
 */
#include "xfs.h"
#include "xfs_fs.h"
#include "xfs_shared.h"
#include "xfs_format.h"
#include "xfs_log_format.h"
#include "xfs_trans_resv.h"
#include "xfs_mount.h"
#include "xfs_defer.h"
#include "xfs_inode.h"
#include "xfs_trans.h"
#include "xfs_bmap.h"
#include "xfs_bmap_util.h"
#include "xfs_trace.h"
#include "xfs_icache.h"
#include "xfs_btree.h"
#include "xfs_refcount_btree.h"
#include "xfs_refcount.h"
#include "xfs_bmap_btree.h"
#include "xfs_trans_space.h"
#include "xfs_bit.h"
#include "xfs_alloc.h"
#include "xfs_quota.h"
#include "xfs_reflink.h"
#include "xfs_iomap.h"
#include "xfs_ag.h"
#include "xfs_ag_resv.h"

/*
 * Copy on Write of Shared Blocks
 *
 * XFS must preserve "the usual" file semantics even when two files share
 * the same physical blocks.  This means that a write to one file must not
 * alter the blocks in a different file; the way that we'll do that is
 * through the use of a copy-on-write mechanism.  At a high level, that
 * means that when we want to write to a shared block, we allocate a new
 * block, write the data to the new block, and if that succeeds we map the
 * new block into the file.
 *
 * XFS provides a "delayed allocation" mechanism that defers the allocation
 * of disk blocks to dirty-but-not-yet-mapped file blocks as long as
 * possible.  This reduces fragmentation by enabling the filesystem to ask
 * for bigger chunks less often, which is exactly what we want for CoW.
 *
 * The delalloc mechanism begins when the kernel wants to make a block
 * writable (write_begin or page_mkwrite).  If the offset is not mapped, we
 * create a delalloc mapping, which is a regular in-core extent, but without
 * a real startblock.  (For delalloc mappings, the startblock encodes both
 * a flag that this is a delalloc mapping, and a worst-case estimate of how
 * many blocks might be required to put the mapping into the BMBT.)  delalloc
 * mappings are a reservation against the free space in the filesystem;
 * adjacent mappings can also be combined into fewer larger mappings.
 *
 * As an optimization, the CoW extent size hint (cowextsz) creates
 * outsized aligned delalloc reservations in the hope of landing out of
 * order nearby CoW writes in a single extent on disk, thereby reducing
 * fragmentation and improving future performance.
 *
 * D: --RRRRRRSSSRRRRRRRR--- (data fork)
 * C: ------DDDDDDD--------- (CoW fork)
 *
 * When dirty pages are being written out (typically in writepage), the
 * delalloc reservations are converted into unwritten mappings by
 * allocating blocks and replacing the delalloc mapping with real ones.
 * A delalloc mapping can be replaced by several unwritten ones if the
 * free space is fragmented.
 *
 * D: --RRRRRRSSSRRRRRRRR---
 * C: ------UUUUUUU---------
 *
 * We want to adapt the delalloc mechanism for copy-on-write, since the
 * write paths are similar.  The first two steps (creating the reservation
 * and allocating the blocks) are exactly the same as delalloc except that
 * the mappings must be stored in a separate CoW fork because we do not want
 * to disturb the mapping in the data fork until we're sure that the write
 * succeeded.  IO completion in this case is the process of removing the old
 * mapping from the data fork and moving the new mapping from the CoW fork to
 * the data fork.  This will be discussed shortly.
 *
 * For now, unaligned directio writes will be bounced back to the page cache.
 * Block-aligned directio writes will use the same mechanism as buffered
 * writes.
 *
 * Just prior to submitting the actual disk write requests, we convert
 * the extents representing the range of the file actually being written
 * (as opposed to extra pieces created for the cowextsize hint) to real
 * extents.  This will become important in the next step:
 *
 * D: --RRRRRRSSSRRRRRRRR---
 * C: ------UUrrUUU---------
 *
 * CoW remapping must be done after the data block write completes,
 * because we don't want to destroy the old data fork map until we're sure
 * the new block has been written.  Since the new mappings are kept in a
 * separate fork, we can simply iterate these mappings to find the ones
 * that cover the file blocks that we just CoW'd.  For each extent, simply
 * unmap the corresponding range in the data fork, map the new range into
 * the data fork, and remove the extent from the CoW fork.  Because of
 * the presence of the cowextsize hint, however, we must be careful
 * only to remap the blocks that we've actually written out --  we must
 * never remap delalloc reservations nor CoW staging blocks that have
 * yet to be written.  This corresponds exactly to the real extents in
 * the CoW fork:
 *
 * D: --RRRRRRrrSRRRRRRRR---
 * C: ------UU--UUU---------
 *
 * Since the remapping operation can be applied to an arbitrary file
 * range, we record the need for the remap step as a flag in the ioend
 * instead of declaring a new IO type.  This is required for direct io
 * because we only have ioend for the whole dio, and we have to be able to
 * remember the presence of unwritten blocks and CoW blocks with a single
 * ioend structure.  Better yet, the more ground we can cover with one
 * ioend, the better.
 */

/*
 * Given an AG extent, find the lowest-numbered run of shared blocks
 * within that range and return the range in fbno/flen.  If
 * find_end_of_shared is true, return the longest contiguous extent of
 * shared blocks.  If there are no shared extents, fbno and flen will
 * be set to NULLAGBLOCK and 0, respectively.
 */
static int
xfs_reflink_find_shared(
        struct xfs_perag        *pag,
        struct xfs_trans        *tp,
        xfs_agblock_t           agbno,
        xfs_extlen_t            aglen,
        xfs_agblock_t           *fbno,
        xfs_extlen_t            *flen,
        bool                    find_end_of_shared)
{
        struct xfs_buf          *agbp;
        struct xfs_btree_cur    *cur;
        int                     error;

        error = xfs_alloc_read_agf(pag, tp, 0, &agbp);
        if (error)
                return error;

        cur = xfs_refcountbt_init_cursor(pag->pag_mount, tp, agbp, pag);

        error = xfs_refcount_find_shared(cur, agbno, aglen, fbno, flen,
                        find_end_of_shared);

        xfs_btree_del_cursor(cur, error);

        xfs_trans_brelse(tp, agbp);
        return error;
}

/*
 * Trim the mapping to the next block where there's a change in the
 * shared/unshared status.  More specifically, this means that we
 * find the lowest-numbered extent of shared blocks that coincides with
 * the given block mapping.  If the shared extent overlaps the start of
 * the mapping, trim the mapping to the end of the shared extent.  If
 * the shared region intersects the mapping, trim the mapping to the
 * start of the shared extent.  If there are no shared regions that
 * overlap, just return the original extent.
 */
int
xfs_reflink_trim_around_shared(
        struct xfs_inode        *ip,
        struct xfs_bmbt_irec    *irec,
        bool                    *shared)
{
        struct xfs_mount        *mp = ip->i_mount;
        struct xfs_perag        *pag;
        xfs_agblock_t           agbno;
        xfs_extlen_t            aglen;
        xfs_agblock_t           fbno;
        xfs_extlen_t            flen;
        int                     error = 0;

        /* Holes, unwritten, and delalloc extents cannot be shared */
        if (!xfs_is_cow_inode(ip) || !xfs_bmap_is_written_extent(irec)) {
                *shared = false;
                return 0;
        }

        trace_xfs_reflink_trim_around_shared(ip, irec);

        pag = xfs_perag_get(mp, XFS_FSB_TO_AGNO(mp, irec->br_startblock));
        agbno = XFS_FSB_TO_AGBNO(mp, irec->br_startblock);
        aglen = irec->br_blockcount;

        error = xfs_reflink_find_shared(pag, NULL, agbno, aglen, &fbno, &flen,
                        true);
        xfs_perag_put(pag);
        if (error)
                return error;

        *shared = false;
        if (fbno == NULLAGBLOCK) {
                /* No shared blocks at all. */
                return 0;
        }

        if (fbno == agbno) {
                /*
                 * The start of this extent is shared.  Truncate the
                 * mapping at the end of the shared region so that a
                 * subsequent iteration starts at the start of the
                 * unshared region.
                 */
                irec->br_blockcount = flen;
                *shared = true;
                return 0;
        }

        /*
         * There's a shared extent midway through this extent.
         * Truncate the mapping at the start of the shared
         * extent so that a subsequent iteration starts at the
         * start of the shared region.
         */
        irec->br_blockcount = fbno - agbno;
        return 0;
}

int
xfs_bmap_trim_cow(
        struct xfs_inode        *ip,
        struct xfs_bmbt_irec    *imap,
        bool                    *shared)
{
        /* We can't update any real extents in always COW mode. */
        if (xfs_is_always_cow_inode(ip) &&
            !isnullstartblock(imap->br_startblock)) {
                *shared = true;
                return 0;
        }

        /* Trim the mapping to the nearest shared extent boundary. */
        return xfs_reflink_trim_around_shared(ip, imap, shared);
}

static int
xfs_reflink_convert_cow_locked(
        struct xfs_inode        *ip,
        xfs_fileoff_t           offset_fsb,
        xfs_filblks_t           count_fsb)
{
        struct xfs_iext_cursor  icur;
        struct xfs_bmbt_irec    got;
        struct xfs_btree_cur    *dummy_cur = NULL;
        int                     dummy_logflags;
        int                     error = 0;

        if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &got))
                return 0;

        do {
                if (got.br_startoff >= offset_fsb + count_fsb)
                        break;
                if (got.br_state == XFS_EXT_NORM)
                        continue;
                if (WARN_ON_ONCE(isnullstartblock(got.br_startblock)))
                        return -EIO;

                xfs_trim_extent(&got, offset_fsb, count_fsb);
                if (!got.br_blockcount)
                        continue;

                got.br_state = XFS_EXT_NORM;
                error = xfs_bmap_add_extent_unwritten_real(NULL, ip,
                                XFS_COW_FORK, &icur, &dummy_cur, &got,
                                &dummy_logflags);
                if (error)
                        return error;
        } while (xfs_iext_next_extent(ip->i_cowfp, &icur, &got));

        return error;
}

/* Convert all of the unwritten CoW extents in a file's range to real ones. */
int
xfs_reflink_convert_cow(
        struct xfs_inode        *ip,
        xfs_off_t               offset,
        xfs_off_t               count)
{
        struct xfs_mount        *mp = ip->i_mount;
        xfs_fileoff_t           offset_fsb = XFS_B_TO_FSBT(mp, offset);
        xfs_fileoff_t           end_fsb = XFS_B_TO_FSB(mp, offset + count);
        xfs_filblks_t           count_fsb = end_fsb - offset_fsb;
        int                     error;

        ASSERT(count != 0);

        xfs_ilock(ip, XFS_ILOCK_EXCL);
        error = xfs_reflink_convert_cow_locked(ip, offset_fsb, count_fsb);
        xfs_iunlock(ip, XFS_ILOCK_EXCL);
        return error;
}

/*
 * Find the extent that maps the given range in the COW fork. Even if the extent
 * is not shared we might have a preallocation for it in the COW fork. If so we
 * use it that rather than trigger a new allocation.
 */
static int
xfs_find_trim_cow_extent(
        struct xfs_inode        *ip,
        struct xfs_bmbt_irec    *imap,
        struct xfs_bmbt_irec    *cmap,
        bool                    *shared,
        bool                    *found)
{
        xfs_fileoff_t           offset_fsb = imap->br_startoff;
        xfs_filblks_t           count_fsb = imap->br_blockcount;
        struct xfs_iext_cursor  icur;

        *found = false;

        /*
         * If we don't find an overlapping extent, trim the range we need to
         * allocate to fit the hole we found.
         */
        if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, cmap))
                cmap->br_startoff = offset_fsb + count_fsb;
        if (cmap->br_startoff > offset_fsb) {
                xfs_trim_extent(imap, imap->br_startoff,
                                cmap->br_startoff - imap->br_startoff);
                return xfs_bmap_trim_cow(ip, imap, shared);
        }

        *shared = true;
        if (isnullstartblock(cmap->br_startblock)) {
                xfs_trim_extent(imap, cmap->br_startoff, cmap->br_blockcount);
                return 0;
        }

        /* real extent found - no need to allocate */
        xfs_trim_extent(cmap, offset_fsb, count_fsb);
        *found = true;
        return 0;
}

static int
xfs_reflink_convert_unwritten(
        struct xfs_inode        *ip,
        struct xfs_bmbt_irec    *imap,
        struct xfs_bmbt_irec    *cmap,
        bool                    convert_now)
{
        xfs_fileoff_t           offset_fsb = imap->br_startoff;
        xfs_filblks_t           count_fsb = imap->br_blockcount;
        int                     error;

        /*
         * cmap might larger than imap due to cowextsize hint.
         */
        xfs_trim_extent(cmap, offset_fsb, count_fsb);

        /*
         * COW fork extents are supposed to remain unwritten until we're ready
         * to initiate a disk write.  For direct I/O we are going to write the
         * data and need the conversion, but for buffered writes we're done.
         */
        if (!convert_now || cmap->br_state == XFS_EXT_NORM)
                return 0;

        trace_xfs_reflink_convert_cow(ip, cmap);

        error = xfs_reflink_convert_cow_locked(ip, offset_fsb, count_fsb);
        if (!error)
                cmap->br_state = XFS_EXT_NORM;

        return error;
}

static int
xfs_reflink_fill_cow_hole(
        struct xfs_inode        *ip,
        struct xfs_bmbt_irec    *imap,
        struct xfs_bmbt_irec    *cmap,
        bool                    *shared,
        uint                    *lockmode,
        bool                    convert_now)
{
        struct xfs_mount        *mp = ip->i_mount;
        struct xfs_trans        *tp;
        xfs_filblks_t           resaligned;
        xfs_extlen_t            resblks;
        int                     nimaps;
        int                     error;
        bool                    found;

        resaligned = xfs_aligned_fsb_count(imap->br_startoff,
                imap->br_blockcount, xfs_get_cowextsz_hint(ip));
        resblks = XFS_DIOSTRAT_SPACE_RES(mp, resaligned);

        xfs_iunlock(ip, *lockmode);
        *lockmode = 0;

        error = xfs_trans_alloc_inode(ip, &M_RES(mp)->tr_write, resblks, 0,
                        false, &tp);
        if (error)
                return error;

        *lockmode = XFS_ILOCK_EXCL;

        error = xfs_find_trim_cow_extent(ip, imap, cmap, shared, &found);
        if (error || !*shared)
                goto out_trans_cancel;

        if (found) {
                xfs_trans_cancel(tp);
                goto convert;
        }

        /* Allocate the entire reservation as unwritten blocks. */
        nimaps = 1;
        error = xfs_bmapi_write(tp, ip, imap->br_startoff, imap->br_blockcount,
                        XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC, 0, cmap,
                        &nimaps);
        if (error)
                goto out_trans_cancel;

        xfs_inode_set_cowblocks_tag(ip);
        error = xfs_trans_commit(tp);
        if (error)
                return error;

        /*
         * Allocation succeeded but the requested range was not even partially
         * satisfied?  Bail out!
         */
        if (nimaps == 0)
                return -ENOSPC;

convert:
        return xfs_reflink_convert_unwritten(ip, imap, cmap, convert_now);

out_trans_cancel:
        xfs_trans_cancel(tp);
        return error;
}

static int
xfs_reflink_fill_delalloc(
        struct xfs_inode        *ip,
        struct xfs_bmbt_irec    *imap,
        struct xfs_bmbt_irec    *cmap,
        bool                    *shared,
        uint                    *lockmode,
        bool                    convert_now)
{
        struct xfs_mount        *mp = ip->i_mount;
        struct xfs_trans        *tp;
        int                     nimaps;
        int                     error;
        bool                    found;

        do {
                xfs_iunlock(ip, *lockmode);
                *lockmode = 0;

                error = xfs_trans_alloc_inode(ip, &M_RES(mp)->tr_write, 0, 0,
                                false, &tp);
                if (error)
                        return error;

                *lockmode = XFS_ILOCK_EXCL;

                error = xfs_find_trim_cow_extent(ip, imap, cmap, shared,
                                &found);
                if (error || !*shared)
                        goto out_trans_cancel;

                if (found) {
                        xfs_trans_cancel(tp);
                        break;
                }

                ASSERT(isnullstartblock(cmap->br_startblock) ||
                       cmap->br_startblock == DELAYSTARTBLOCK);

                /*
                 * Replace delalloc reservation with an unwritten extent.
                 */
                nimaps = 1;
                error = xfs_bmapi_write(tp, ip, cmap->br_startoff,
                                cmap->br_blockcount,
                                XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC, 0,
                                cmap, &nimaps);
                if (error)
                        goto out_trans_cancel;

                xfs_inode_set_cowblocks_tag(ip);
                error = xfs_trans_commit(tp);
                if (error)
                        return error;

                /*
                 * Allocation succeeded but the requested range was not even
                 * partially satisfied?  Bail out!
                 */
                if (nimaps == 0)
                        return -ENOSPC;
        } while (cmap->br_startoff + cmap->br_blockcount <= imap->br_startoff);

        return xfs_reflink_convert_unwritten(ip, imap, cmap, convert_now);

out_trans_cancel:
        xfs_trans_cancel(tp);
        return error;
}

/* Allocate all CoW reservations covering a range of blocks in a file. */
int
xfs_reflink_allocate_cow(
        struct xfs_inode        *ip,
        struct xfs_bmbt_irec    *imap,
        struct xfs_bmbt_irec    *cmap,
        bool                    *shared,
        uint                    *lockmode,
        bool                    convert_now)
{
        int                     error;
        bool                    found;

        ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
        if (!ip->i_cowfp) {
                ASSERT(!xfs_is_reflink_inode(ip));
                xfs_ifork_init_cow(ip);
        }

        error = xfs_find_trim_cow_extent(ip, imap, cmap, shared, &found);
        if (error || !*shared)
                return error;

        /* CoW fork has a real extent */
        if (found)
                return xfs_reflink_convert_unwritten(ip, imap, cmap,
                                convert_now);

        /*
         * CoW fork does not have an extent and data extent is shared.
         * Allocate a real extent in the CoW fork.
         */
        if (cmap->br_startoff > imap->br_startoff)
                return xfs_reflink_fill_cow_hole(ip, imap, cmap, shared,
                                lockmode, convert_now);

        /*
         * CoW fork has a delalloc reservation. Replace it with a real extent.
         * There may or may not be a data fork mapping.
         */
        if (isnullstartblock(cmap->br_startblock) ||
            cmap->br_startblock == DELAYSTARTBLOCK)
                return xfs_reflink_fill_delalloc(ip, imap, cmap, shared,
                                lockmode, convert_now);

        /* Shouldn't get here. */
        ASSERT(0);
        return -EFSCORRUPTED;
}

/*
 * Cancel CoW reservations for some block range of an inode.
 *
 * If cancel_real is true this function cancels all COW fork extents for the
 * inode; if cancel_real is false, real extents are not cleared.
 *
 * Caller must have already joined the inode to the current transaction. The
 * inode will be joined to the transaction returned to the caller.
 */
int
xfs_reflink_cancel_cow_blocks(
        struct xfs_inode                *ip,
        struct xfs_trans                **tpp,
        xfs_fileoff_t                   offset_fsb,
        xfs_fileoff_t                   end_fsb,
        bool                            cancel_real)
{
        struct xfs_ifork                *ifp = xfs_ifork_ptr(ip, XFS_COW_FORK);
        struct xfs_bmbt_irec            got, del;
        struct xfs_iext_cursor          icur;
        int                             error = 0;

        if (!xfs_inode_has_cow_data(ip))
                return 0;
        if (!xfs_iext_lookup_extent_before(ip, ifp, &end_fsb, &icur, &got))
                return 0;

        /* Walk backwards until we're out of the I/O range... */
        while (got.br_startoff + got.br_blockcount > offset_fsb) {
                del = got;
                xfs_trim_extent(&del, offset_fsb, end_fsb - offset_fsb);

                /* Extent delete may have bumped ext forward */
                if (!del.br_blockcount) {
                        xfs_iext_prev(ifp, &icur);
                        goto next_extent;
                }

                trace_xfs_reflink_cancel_cow(ip, &del);

                if (isnullstartblock(del.br_startblock)) {
                        error = xfs_bmap_del_extent_delay(ip, XFS_COW_FORK,
                                        &icur, &got, &del);
                        if (error)
                                break;
                } else if (del.br_state == XFS_EXT_UNWRITTEN || cancel_real) {
                        ASSERT((*tpp)->t_highest_agno == NULLAGNUMBER);

                        /* Free the CoW orphan record. */
                        xfs_refcount_free_cow_extent(*tpp, del.br_startblock,
                                        del.br_blockcount);

                        error = xfs_free_extent_later(*tpp, del.br_startblock,
                                        del.br_blockcount, NULL,
                                        XFS_AG_RESV_NONE, false);
                        if (error)
                                break;

                        /* Roll the transaction */
                        error = xfs_defer_finish(tpp);
                        if (error)
                                break;

                        /* Remove the mapping from the CoW fork. */
                        xfs_bmap_del_extent_cow(ip, &icur, &got, &del);

                        /* Remove the quota reservation */
                        error = xfs_quota_unreserve_blkres(ip,
                                        del.br_blockcount);
                        if (error)
                                break;
                } else {
                        /* Didn't do anything, push cursor back. */
                        xfs_iext_prev(ifp, &icur);
                }
next_extent:
                if (!xfs_iext_get_extent(ifp, &icur, &got))
                        break;
        }

        /* clear tag if cow fork is emptied */
        if (!ifp->if_bytes)
                xfs_inode_clear_cowblocks_tag(ip);
        return error;
}

/*
 * Cancel CoW reservations for some byte range of an inode.
 *
 * If cancel_real is true this function cancels all COW fork extents for the
 * inode; if cancel_real is false, real extents are not cleared.
 */
int
xfs_reflink_cancel_cow_range(
        struct xfs_inode        *ip,
        xfs_off_t               offset,
        xfs_off_t               count,
        bool                    cancel_real)
{
        struct xfs_trans        *tp;
        xfs_fileoff_t           offset_fsb;
        xfs_fileoff_t           end_fsb;
        int                     error;

        trace_xfs_reflink_cancel_cow_range(ip, offset, count);
        ASSERT(ip->i_cowfp);

        offset_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
        if (count == NULLFILEOFF)
                end_fsb = NULLFILEOFF;
        else
                end_fsb = XFS_B_TO_FSB(ip->i_mount, offset + count);

        /* Start a rolling transaction to remove the mappings */
        error = xfs_trans_alloc(ip->i_mount, &M_RES(ip->i_mount)->tr_write,
                        0, 0, 0, &tp);
        if (error)
                goto out;

        xfs_ilock(ip, XFS_ILOCK_EXCL);
        xfs_trans_ijoin(tp, ip, 0);

        /* Scrape out the old CoW reservations */
        error = xfs_reflink_cancel_cow_blocks(ip, &tp, offset_fsb, end_fsb,
                        cancel_real);
        if (error)
                goto out_cancel;

        error = xfs_trans_commit(tp);

        xfs_iunlock(ip, XFS_ILOCK_EXCL);
        return error;

out_cancel:
        xfs_trans_cancel(tp);
        xfs_iunlock(ip, XFS_ILOCK_EXCL);
out:
        trace_xfs_reflink_cancel_cow_range_error(ip, error, _RET_IP_);
        return error;
}

/*
 * Remap part of the CoW fork into the data fork.
 *
 * We aim to remap the range starting at @offset_fsb and ending at @end_fsb
 * into the data fork; this function will remap what it can (at the end of the
 * range) and update @end_fsb appropriately.  Each remap gets its own
 * transaction because we can end up merging and splitting bmbt blocks for
 * every remap operation and we'd like to keep the block reservation
 * requirements as low as possible.
 */
STATIC int
xfs_reflink_end_cow_extent(
        struct xfs_inode        *ip,
        xfs_fileoff_t           *offset_fsb,
        xfs_fileoff_t           end_fsb)
{
        struct xfs_iext_cursor  icur;
        struct xfs_bmbt_irec    got, del, data;
        struct xfs_mount        *mp = ip->i_mount;
        struct xfs_trans        *tp;
        struct xfs_ifork        *ifp = xfs_ifork_ptr(ip, XFS_COW_FORK);
        unsigned int            resblks;
        int                     nmaps;
        int                     error;

        /* No COW extents?  That's easy! */
        if (ifp->if_bytes == 0) {
                *offset_fsb = end_fsb;
                return 0;
        }

        resblks = XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK);
        error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0,
                        XFS_TRANS_RESERVE, &tp);
        if (error)
                return error;

        /*
         * Lock the inode.  We have to ijoin without automatic unlock because
         * the lead transaction is the refcountbt record deletion; the data
         * fork update follows as a deferred log item.
         */
        xfs_ilock(ip, XFS_ILOCK_EXCL);
        xfs_trans_ijoin(tp, ip, 0);

        error = xfs_iext_count_may_overflow(ip, XFS_DATA_FORK,
                        XFS_IEXT_REFLINK_END_COW_CNT);
        if (error == -EFBIG)
                error = xfs_iext_count_upgrade(tp, ip,
                                XFS_IEXT_REFLINK_END_COW_CNT);
        if (error)
                goto out_cancel;

        /*
         * In case of racing, overlapping AIO writes no COW extents might be
         * left by the time I/O completes for the loser of the race.  In that
         * case we are done.
         */
        if (!xfs_iext_lookup_extent(ip, ifp, *offset_fsb, &icur, &got) ||
            got.br_startoff >= end_fsb) {
                *offset_fsb = end_fsb;
                goto out_cancel;
        }

        /*
         * Only remap real extents that contain data.  With AIO, speculative
         * preallocations can leak into the range we are called upon, and we
         * need to skip them.  Preserve @got for the eventual CoW fork
         * deletion; from now on @del represents the mapping that we're
         * actually remapping.
         */
        while (!xfs_bmap_is_written_extent(&got)) {
                if (!xfs_iext_next_extent(ifp, &icur, &got) ||
                    got.br_startoff >= end_fsb) {
                        *offset_fsb = end_fsb;
                        goto out_cancel;
                }
        }
        del = got;
        xfs_trim_extent(&del, *offset_fsb, end_fsb - *offset_fsb);

        /* Grab the corresponding mapping in the data fork. */
        nmaps = 1;
        error = xfs_bmapi_read(ip, del.br_startoff, del.br_blockcount, &data,
                        &nmaps, 0);
        if (error)
                goto out_cancel;

        /* We can only remap the smaller of the two extent sizes. */
        data.br_blockcount = min(data.br_blockcount, del.br_blockcount);
        del.br_blockcount = data.br_blockcount;

        trace_xfs_reflink_cow_remap_from(ip, &del);
        trace_xfs_reflink_cow_remap_to(ip, &data);

        if (xfs_bmap_is_real_extent(&data)) {
                /*
                 * If the extent we're remapping is backed by storage (written
                 * or not), unmap the extent and drop its refcount.
                 */
                xfs_bmap_unmap_extent(tp, ip, &data);
                xfs_refcount_decrease_extent(tp, &data);
                xfs_trans_mod_dquot_byino(tp, ip, XFS_TRANS_DQ_BCOUNT,
                                -data.br_blockcount);
        } else if (data.br_startblock == DELAYSTARTBLOCK) {
                int             done;

                /*
                 * If the extent we're remapping is a delalloc reservation,
                 * we can use the regular bunmapi function to release the
                 * incore state.  Dropping the delalloc reservation takes care
                 * of the quota reservation for us.
                 */
                error = xfs_bunmapi(NULL, ip, data.br_startoff,
                                data.br_blockcount, 0, 1, &done);
                if (error)
                        goto out_cancel;
                ASSERT(done);
        }

        /* Free the CoW orphan record. */
        xfs_refcount_free_cow_extent(tp, del.br_startblock, del.br_blockcount);

        /* Map the new blocks into the data fork. */
        xfs_bmap_map_extent(tp, ip, &del);

        /* Charge this new data fork mapping to the on-disk quota. */
        xfs_trans_mod_dquot_byino(tp, ip, XFS_TRANS_DQ_DELBCOUNT,
                        (long)del.br_blockcount);

        /* Remove the mapping from the CoW fork. */
        xfs_bmap_del_extent_cow(ip, &icur, &got, &del);

        error = xfs_trans_commit(tp);
        xfs_iunlock(ip, XFS_ILOCK_EXCL);
        if (error)
                return error;

        /* Update the caller about how much progress we made. */
        *offset_fsb = del.br_startoff + del.br_blockcount;
        return 0;

out_cancel:
        xfs_trans_cancel(tp);
        xfs_iunlock(ip, XFS_ILOCK_EXCL);
        return error;
}

/*
 * Remap parts of a file's data fork after a successful CoW.
 */
int
xfs_reflink_end_cow(
        struct xfs_inode                *ip,
        xfs_off_t                       offset,
        xfs_off_t                       count)
{
        xfs_fileoff_t                   offset_fsb;
        xfs_fileoff_t                   end_fsb;
        int                             error = 0;

        trace_xfs_reflink_end_cow(ip, offset, count);

        offset_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
        end_fsb = XFS_B_TO_FSB(ip->i_mount, offset + count);

        /*
         * Walk forwards until we've remapped the I/O range.  The loop function
         * repeatedly cycles the ILOCK to allocate one transaction per remapped
         * extent.
         *
         * If we're being called by writeback then the pages will still
         * have PageWriteback set, which prevents races with reflink remapping
         * and truncate.  Reflink remapping prevents races with writeback by
         * taking the iolock and mmaplock before flushing the pages and
         * remapping, which means there won't be any further writeback or page
         * cache dirtying until the reflink completes.
         *
         * We should never have two threads issuing writeback for the same file
         * region.  There are also have post-eof checks in the writeback
         * preparation code so that we don't bother writing out pages that are
         * about to be truncated.
         *
         * If we're being called as part of directio write completion, the dio
         * count is still elevated, which reflink and truncate will wait for.
         * Reflink remapping takes the iolock and mmaplock and waits for
         * pending dio to finish, which should prevent any directio until the
         * remap completes.  Multiple concurrent directio writes to the same
         * region are handled by end_cow processing only occurring for the
         * threads which succeed; the outcome of multiple overlapping direct
         * writes is not well defined anyway.
         *
         * It's possible that a buffered write and a direct write could collide
         * here (the buffered write stumbles in after the dio flushes and
         * invalidates the page cache and immediately queues writeback), but we
         * have never supported this 100%.  If either disk write succeeds the
         * blocks will be remapped.
         */
        while (end_fsb > offset_fsb && !error)
                error = xfs_reflink_end_cow_extent(ip, &offset_fsb, end_fsb);

        if (error)
                trace_xfs_reflink_end_cow_error(ip, error, _RET_IP_);
        return error;
}

/*
 * Free all CoW staging blocks that are still referenced by the ondisk refcount
 * metadata.  The ondisk metadata does not track which inode created the
 * staging extent, so callers must ensure that there are no cached inodes with
 * live CoW staging extents.
 */
int
xfs_reflink_recover_cow(
        struct xfs_mount        *mp)
{
        struct xfs_perag        *pag;
        xfs_agnumber_t          agno;
        int                     error = 0;

        if (!xfs_has_reflink(mp))
                return 0;

        for_each_perag(mp, agno, pag) {
                error = xfs_refcount_recover_cow_leftovers(mp, pag);
                if (error) {
                        xfs_perag_rele(pag);
                        break;
                }
        }

        return error;
}

/*
 * Reflinking (Block) Ranges of Two Files Together
 *
 * First, ensure that the reflink flag is set on both inodes.  The flag is an
 * optimization to avoid unnecessary refcount btree lookups in the write path.
 *
 * Now we can iteratively remap the range of extents (and holes) in src to the
 * corresponding ranges in dest.  Let drange and srange denote the ranges of
 * logical blocks in dest and src touched by the reflink operation.
 *
 * While the length of drange is greater than zero,
 *    - Read src's bmbt at the start of srange ("imap")
 *    - If imap doesn't exist, make imap appear to start at the end of srange
 *      with zero length.
 *    - If imap starts before srange, advance imap to start at srange.
 *    - If imap goes beyond srange, truncate imap to end at the end of srange.
 *    - Punch (imap start - srange start + imap len) blocks from dest at
 *      offset (drange start).
 *    - If imap points to a real range of pblks,
 *         > Increase the refcount of the imap's pblks
 *         > Map imap's pblks into dest at the offset
 *           (drange start + imap start - srange start)
 *    - Advance drange and srange by (imap start - srange start + imap len)
 *
 * Finally, if the reflink made dest longer, update both the in-core and
 * on-disk file sizes.
 *
 * ASCII Art Demonstration:
 *
 * Let's say we want to reflink this source file:
 *
 * ----SSSSSSS-SSSSS----SSSSSS (src file)
 *   <-------------------->
 *
 * into this destination file:
 *
 * --DDDDDDDDDDDDDDDDDDD--DDD (dest file)
 *        <-------------------->
 * '-' means a hole, and 'S' and 'D' are written blocks in the src and dest.
 * Observe that the range has different logical offsets in either file.
 *
 * Consider that the first extent in the source file doesn't line up with our
 * reflink range.  Unmapping  and remapping are separate operations, so we can
 * unmap more blocks from the destination file than we remap.
 *
 * ----SSSSSSS-SSSSS----SSSSSS
 *   <------->
 * --DDDDD---------DDDDD--DDD
 *        <------->
 *
 * Now remap the source extent into the destination file:
 *
 * ----SSSSSSS-SSSSS----SSSSSS
 *   <------->
 * --DDDDD--SSSSSSSDDDDD--DDD
 *        <------->
 *
 * Do likewise with the second hole and extent in our range.  Holes in the
 * unmap range don't affect our operation.
 *
 * ----SSSSSSS-SSSSS----SSSSSS
 *            <---->
 * --DDDDD--SSSSSSS-SSSSS-DDD
 *                 <---->
 *
 * Finally, unmap and remap part of the third extent.  This will increase the
 * size of the destination file.
 *
 * ----SSSSSSS-SSSSS----SSSSSS
 *                  <----->
 * --DDDDD--SSSSSSS-SSSSS----SSS
 *                       <----->
 *
 * Once we update the destination file's i_size, we're done.
 */

/*
 * Ensure the reflink bit is set in both inodes.
 */
STATIC int
xfs_reflink_set_inode_flag(
        struct xfs_inode        *src,
        struct xfs_inode        *dest)
{
        struct xfs_mount        *mp = src->i_mount;
        int                     error;
        struct xfs_trans        *tp;

        if (xfs_is_reflink_inode(src) && xfs_is_reflink_inode(dest))
                return 0;

        error = xfs_trans_alloc(mp, &M_RES(mp)->tr_ichange, 0, 0, 0, &tp);
        if (error)
                goto out_error;

        /* Lock both files against IO */
        if (src->i_ino == dest->i_ino)
                xfs_ilock(src, XFS_ILOCK_EXCL);
        else
                xfs_lock_two_inodes(src, XFS_ILOCK_EXCL, dest, XFS_ILOCK_EXCL);

        if (!xfs_is_reflink_inode(src)) {
                trace_xfs_reflink_set_inode_flag(src);
                xfs_trans_ijoin(tp, src, XFS_ILOCK_EXCL);
                src->i_diflags2 |= XFS_DIFLAG2_REFLINK;
                xfs_trans_log_inode(tp, src, XFS_ILOG_CORE);
                xfs_ifork_init_cow(src);
        } else
                xfs_iunlock(src, XFS_ILOCK_EXCL);

        if (src->i_ino == dest->i_ino)
                goto commit_flags;

        if (!xfs_is_reflink_inode(dest)) {
                trace_xfs_reflink_set_inode_flag(dest);
                xfs_trans_ijoin(tp, dest, XFS_ILOCK_EXCL);
                dest->i_diflags2 |= XFS_DIFLAG2_REFLINK;
                xfs_trans_log_inode(tp, dest, XFS_ILOG_CORE);
                xfs_ifork_init_cow(dest);
        } else
                xfs_iunlock(dest, XFS_ILOCK_EXCL);

commit_flags:
        error = xfs_trans_commit(tp);
        if (error)
                goto out_error;
        return error;

out_error:
        trace_xfs_reflink_set_inode_flag_error(dest, error, _RET_IP_);
        return error;
}

/*
 * Update destination inode size & cowextsize hint, if necessary.
 */
int
xfs_reflink_update_dest(
        struct xfs_inode        *dest,
        xfs_off_t               newlen,
        xfs_extlen_t            cowextsize,
        unsigned int            remap_flags)
{
        struct xfs_mount        *mp = dest->i_mount;
        struct xfs_trans        *tp;
        int                     error;

        if (newlen <= i_size_read(VFS_I(dest)) && cowextsize == 0)
                return 0;

        error = xfs_trans_alloc(mp, &M_RES(mp)->tr_ichange, 0, 0, 0, &tp);
        if (error)
                goto out_error;

        xfs_ilock(dest, XFS_ILOCK_EXCL);
        xfs_trans_ijoin(tp, dest, XFS_ILOCK_EXCL);

        if (newlen > i_size_read(VFS_I(dest))) {
                trace_xfs_reflink_update_inode_size(dest, newlen);
                i_size_write(VFS_I(dest), newlen);
                dest->i_disk_size = newlen;
        }

        if (cowextsize) {
                dest->i_cowextsize = cowextsize;
                dest->i_diflags2 |= XFS_DIFLAG2_COWEXTSIZE;
        }

        xfs_trans_log_inode(tp, dest, XFS_ILOG_CORE);

        error = xfs_trans_commit(tp);
        if (error)
                goto out_error;
        return error;

out_error:
        trace_xfs_reflink_update_inode_size_error(dest, error, _RET_IP_);
        return error;
}

/*
 * Do we have enough reserve in this AG to handle a reflink?  The refcount
 * btree already reserved all the space it needs, but the rmap btree can grow
 * infinitely, so we won't allow more reflinks when the AG is down to the
 * btree reserves.
 */
static int
xfs_reflink_ag_has_free_space(
        struct xfs_mount        *mp,
        xfs_agnumber_t          agno)
{
        struct xfs_perag        *pag;
        int                     error = 0;

        if (!xfs_has_rmapbt(mp))
                return 0;

        pag = xfs_perag_get(mp, agno);
        if (xfs_ag_resv_critical(pag, XFS_AG_RESV_RMAPBT) ||
            xfs_ag_resv_critical(pag, XFS_AG_RESV_METADATA))
                error = -ENOSPC;
        xfs_perag_put(pag);
        return error;
}

/*
 * Remap the given extent into the file.  The dmap blockcount will be set to
 * the number of blocks that were actually remapped.
 */
STATIC int
xfs_reflink_remap_extent(
        struct xfs_inode        *ip,
        struct xfs_bmbt_irec    *dmap,
        xfs_off_t               new_isize)
{
        struct xfs_bmbt_irec    smap;
        struct xfs_mount        *mp = ip->i_mount;
        struct xfs_trans        *tp;
        xfs_off_t               newlen;
        int64_t                 qdelta = 0;
        unsigned int            resblks;
        bool                    quota_reserved = true;
        bool                    smap_real;
        bool                    dmap_written = xfs_bmap_is_written_extent(dmap);
        int                     iext_delta = 0;
        int                     nimaps;
        int                     error;

        /*
         * Start a rolling transaction to switch the mappings.
         *
         * Adding a written extent to the extent map can cause a bmbt split,
         * and removing a mapped extent from the extent can cause a bmbt split.
         * The two operations cannot both cause a split since they operate on
         * the same index in the bmap btree, so we only need a reservation for
         * one bmbt split if either thing is happening.  However, we haven't
         * locked the inode yet, so we reserve assuming this is the case.
         *
         * The first allocation call tries to reserve enough space to handle
         * mapping dmap into a sparse part of the file plus the bmbt split.  We
         * haven't locked the inode or read the existing mapping yet, so we do
         * not know for sure that we need the space.  This should succeed most
         * of the time.
         *
         * If the first attempt fails, try again but reserving only enough
         * space to handle a bmbt split.  This is the hard minimum requirement,
         * and we revisit quota reservations later when we know more about what
         * we're remapping.
         */
        resblks = XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK);
        error = xfs_trans_alloc_inode(ip, &M_RES(mp)->tr_write,
                        resblks + dmap->br_blockcount, 0, false, &tp);
        if (error == -EDQUOT || error == -ENOSPC) {
                quota_reserved = false;
                error = xfs_trans_alloc_inode(ip, &M_RES(mp)->tr_write,
                                resblks, 0, false, &tp);
        }
        if (error)
                goto out;

        /*
         * Read what's currently mapped in the destination file into smap.
         * If smap isn't a hole, we will have to remove it before we can add
         * dmap to the destination file.
         */
        nimaps = 1;
        error = xfs_bmapi_read(ip, dmap->br_startoff, dmap->br_blockcount,
                        &smap, &nimaps, 0);
        if (error)
                goto out_cancel;
        ASSERT(nimaps == 1 && smap.br_startoff == dmap->br_startoff);
        smap_real = xfs_bmap_is_real_extent(&smap);

        /*
         * We can only remap as many blocks as the smaller of the two extent
         * maps, because we can only remap one extent at a time.
         */
        dmap->br_blockcount = min(dmap->br_blockcount, smap.br_blockcount);
        ASSERT(dmap->br_blockcount == smap.br_blockcount);

        trace_xfs_reflink_remap_extent_dest(ip, &smap);

        /*
         * Two extents mapped to the same physical block must not have
         * different states; that's filesystem corruption.  Move on to the next
         * extent if they're both holes or both the same physical extent.
         */
        if (dmap->br_startblock == smap.br_startblock) {
                if (dmap->br_state != smap.br_state)
                        error = -EFSCORRUPTED;
                goto out_cancel;
        }

        /* If both extents are unwritten, leave them alone. */
        if (dmap->br_state == XFS_EXT_UNWRITTEN &&
            smap.br_state == XFS_EXT_UNWRITTEN)
                goto out_cancel;

        /* No reflinking if the AG of the dest mapping is low on space. */
        if (dmap_written) {
                error = xfs_reflink_ag_has_free_space(mp,
                                XFS_FSB_TO_AGNO(mp, dmap->br_startblock));
                if (error)
                        goto out_cancel;
        }

        /*
         * Increase quota reservation if we think the quota block counter for
         * this file could increase.
         *
         * If we are mapping a written extent into the file, we need to have
         * enough quota block count reservation to handle the blocks in that
         * extent.  We log only the delta to the quota block counts, so if the
         * extent we're unmapping also has blocks allocated to it, we don't
         * need a quota reservation for the extent itself.
         *
         * Note that if we're replacing a delalloc reservation with a written
         * extent, we have to take the full quota reservation because removing
         * the delalloc reservation gives the block count back to the quota
         * count.  This is suboptimal, but the VFS flushed the dest range
         * before we started.  That should have removed all the delalloc
         * reservations, but we code defensively.
         *
         * xfs_trans_alloc_inode above already tried to grab an even larger
         * quota reservation, and kicked off a blockgc scan if it couldn't.
         * If we can't get a potentially smaller quota reservation now, we're
         * done.
         */
        if (!quota_reserved && !smap_real && dmap_written) {
                error = xfs_trans_reserve_quota_nblks(tp, ip,
                                dmap->br_blockcount, 0, false);
                if (error)
                        goto out_cancel;
        }

        if (smap_real)
                ++iext_delta;

        if (dmap_written)
                ++iext_delta;

        error = xfs_iext_count_may_overflow(ip, XFS_DATA_FORK, iext_delta);
        if (error == -EFBIG)
                error = xfs_iext_count_upgrade(tp, ip, iext_delta);
        if (error)
                goto out_cancel;

        if (smap_real) {
                /*
                 * If the extent we're unmapping is backed by storage (written
                 * or not), unmap the extent and drop its refcount.
                 */
                xfs_bmap_unmap_extent(tp, ip, &smap);
                xfs_refcount_decrease_extent(tp, &smap);
                qdelta -= smap.br_blockcount;
        } else if (smap.br_startblock == DELAYSTARTBLOCK) {
                int             done;

                /*
                 * If the extent we're unmapping is a delalloc reservation,
                 * we can use the regular bunmapi function to release the
                 * incore state.  Dropping the delalloc reservation takes care
                 * of the quota reservation for us.
                 */
                error = xfs_bunmapi(NULL, ip, smap.br_startoff,
                                smap.br_blockcount, 0, 1, &done);
                if (error)
                        goto out_cancel;
                ASSERT(done);
        }

        /*
         * If the extent we're sharing is backed by written storage, increase
         * its refcount and map it into the file.
         */
        if (dmap_written) {
                xfs_refcount_increase_extent(tp, dmap);
                xfs_bmap_map_extent(tp, ip, dmap);
                qdelta += dmap->br_blockcount;
        }

        xfs_trans_mod_dquot_byino(tp, ip, XFS_TRANS_DQ_BCOUNT, qdelta);

        /* Update dest isize if needed. */
        newlen = XFS_FSB_TO_B(mp, dmap->br_startoff + dmap->br_blockcount);
        newlen = min_t(xfs_off_t, newlen, new_isize);
        if (newlen > i_size_read(VFS_I(ip))) {
                trace_xfs_reflink_update_inode_size(ip, newlen);
                i_size_write(VFS_I(ip), newlen);
                ip->i_disk_size = newlen;
                xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
        }

        /* Commit everything and unlock. */
        error = xfs_trans_commit(tp);
        goto out_unlock;

out_cancel:
        xfs_trans_cancel(tp);
out_unlock:
        xfs_iunlock(ip, XFS_ILOCK_EXCL);
out:
        if (error)
                trace_xfs_reflink_remap_extent_error(ip, error, _RET_IP_);
        return error;
}

/* Remap a range of one file to the other. */
int
xfs_reflink_remap_blocks(
        struct xfs_inode        *src,
        loff_t                  pos_in,
        struct xfs_inode        *dest,
        loff_t                  pos_out,
        loff_t                  remap_len,
        loff_t                  *remapped)
{
        struct xfs_bmbt_irec    imap;
        struct xfs_mount        *mp = src->i_mount;
        xfs_fileoff_t           srcoff = XFS_B_TO_FSBT(mp, pos_in);
        xfs_fileoff_t           destoff = XFS_B_TO_FSBT(mp, pos_out);
        xfs_filblks_t           len;
        xfs_filblks_t           remapped_len = 0;
        xfs_off_t               new_isize = pos_out + remap_len;
        int                     nimaps;
        int                     error = 0;

        len = min_t(xfs_filblks_t, XFS_B_TO_FSB(mp, remap_len),
                        XFS_MAX_FILEOFF);

        trace_xfs_reflink_remap_blocks(src, srcoff, len, dest, destoff);

        while (len > 0) {
                unsigned int    lock_mode;

                /* Read extent from the source file */
                nimaps = 1;
                lock_mode = xfs_ilock_data_map_shared(src);
                error = xfs_bmapi_read(src, srcoff, len, &imap, &nimaps, 0);
                xfs_iunlock(src, lock_mode);
                if (error)
                        break;
                /*
                 * The caller supposedly flushed all dirty pages in the source
                 * file range, which means that writeback should have allocated
                 * or deleted all delalloc reservations in that range.  If we
                 * find one, that's a good sign that something is seriously
                 * wrong here.
                 */
                ASSERT(nimaps == 1 && imap.br_startoff == srcoff);
                if (imap.br_startblock == DELAYSTARTBLOCK) {
                        ASSERT(imap.br_startblock != DELAYSTARTBLOCK);
                        error = -EFSCORRUPTED;
                        break;
                }

                trace_xfs_reflink_remap_extent_src(src, &imap);

                /* Remap into the destination file at the given offset. */
                imap.br_startoff = destoff;
                error = xfs_reflink_remap_extent(dest, &imap, new_isize);
                if (error)
                        break;

                if (fatal_signal_pending(current)) {
                        error = -EINTR;
                        break;
                }

                /* Advance drange/srange */
                srcoff += imap.br_blockcount;
                destoff += imap.br_blockcount;
                len -= imap.br_blockcount;
                remapped_len += imap.br_blockcount;
        }

        if (error)
                trace_xfs_reflink_remap_blocks_error(dest, error, _RET_IP_);
        *remapped = min_t(loff_t, remap_len,
                          XFS_FSB_TO_B(src->i_mount, remapped_len));
        return error;
}

/*
 * If we're reflinking to a point past the destination file's EOF, we must
 * zero any speculative post-EOF preallocations that sit between the old EOF
 * and the destination file offset.
 */
static int
xfs_reflink_zero_posteof(
        struct xfs_inode        *ip,
        loff_t                  pos)
{
        loff_t                  isize = i_size_read(VFS_I(ip));

        if (pos <= isize)
                return 0;

        trace_xfs_zero_eof(ip, isize, pos - isize);
        return xfs_zero_range(ip, isize, pos - isize, NULL);
}

/*
 * Prepare two files for range cloning.  Upon a successful return both inodes
 * will have the iolock and mmaplock held, the page cache of the out file will
 * be truncated, and any leases on the out file will have been broken.  This
 * function borrows heavily from xfs_file_aio_write_checks.
 *
 * The VFS allows partial EOF blocks to "match" for dedupe even though it hasn't
 * checked that the bytes beyond EOF physically match. Hence we cannot use the
 * EOF block in the source dedupe range because it's not a complete block match,
 * hence can introduce a corruption into the file that has it's block replaced.
 *
 * In similar fashion, the VFS file cloning also allows partial EOF blocks to be
 * "block aligned" for the purposes of cloning entire files.  However, if the
 * source file range includes the EOF block and it lands within the existing EOF
 * of the destination file, then we can expose stale data from beyond the source
 * file EOF in the destination file.
 *
 * XFS doesn't support partial block sharing, so in both cases we have check
 * these cases ourselves. For dedupe, we can simply round the length to dedupe
 * down to the previous whole block and ignore the partial EOF block. While this
 * means we can't dedupe the last block of a file, this is an acceptible
 * tradeoff for simplicity on implementation.
 *
 * For cloning, we want to share the partial EOF block if it is also the new EOF
 * block of the destination file. If the partial EOF block lies inside the
 * existing destination EOF, then we have to abort the clone to avoid exposing
 * stale data in the destination file. Hence we reject these clone attempts with
 * -EINVAL in this case.
 */
int
xfs_reflink_remap_prep(
        struct file             *file_in,
        loff_t                  pos_in,
        struct file             *file_out,
        loff_t                  pos_out,
        loff_t                  *len,
        unsigned int            remap_flags)
{
        struct inode            *inode_in = file_inode(file_in);
        struct xfs_inode        *src = XFS_I(inode_in);
        struct inode            *inode_out = file_inode(file_out);
        struct xfs_inode        *dest = XFS_I(inode_out);
        int                     ret;

        /* Lock both files against IO */
        ret = xfs_ilock2_io_mmap(src, dest);
        if (ret)
                return ret;

        /* Check file eligibility and prepare for block sharing. */
        ret = -EINVAL;
        /* Don't reflink realtime inodes */
        if (XFS_IS_REALTIME_INODE(src) || XFS_IS_REALTIME_INODE(dest))
                goto out_unlock;

        /* Don't share DAX file data with non-DAX file. */
        if (IS_DAX(inode_in) != IS_DAX(inode_out))
                goto out_unlock;

        if (!IS_DAX(inode_in))
                ret = generic_remap_file_range_prep(file_in, pos_in, file_out,
                                pos_out, len, remap_flags);
        else
                ret = dax_remap_file_range_prep(file_in, pos_in, file_out,
                                pos_out, len, remap_flags, &xfs_read_iomap_ops);
        if (ret || *len == 0)
                goto out_unlock;

        /* Attach dquots to dest inode before changing block map */
        ret = xfs_qm_dqattach(dest);
        if (ret)
                goto out_unlock;

        /*
         * Zero existing post-eof speculative preallocations in the destination
         * file.
         */
        ret = xfs_reflink_zero_posteof(dest, pos_out);
        if (ret)
                goto out_unlock;

        /* Set flags and remap blocks. */
        ret = xfs_reflink_set_inode_flag(src, dest);
        if (ret)
                goto out_unlock;

        /*
         * If pos_out > EOF, we may have dirtied blocks between EOF and
         * pos_out. In that case, we need to extend the flush and unmap to cover
         * from EOF to the end of the copy length.
         */
        if (pos_out > XFS_ISIZE(dest)) {
                loff_t  flen = *len + (pos_out - XFS_ISIZE(dest));
                ret = xfs_flush_unmap_range(dest, XFS_ISIZE(dest), flen);
        } else {
                ret = xfs_flush_unmap_range(dest, pos_out, *len);
        }
        if (ret)
                goto out_unlock;

        xfs_iflags_set(src, XFS_IREMAPPING);
        if (inode_in != inode_out)
                xfs_ilock_demote(src, XFS_IOLOCK_EXCL | XFS_MMAPLOCK_EXCL);

        return 0;
out_unlock:
        xfs_iunlock2_io_mmap(src, dest);
        return ret;
}

/* Does this inode need the reflink flag? */
int
xfs_reflink_inode_has_shared_extents(
        struct xfs_trans                *tp,
        struct xfs_inode                *ip,
        bool                            *has_shared)
{
        struct xfs_bmbt_irec            got;
        struct xfs_mount                *mp = ip->i_mount;
        struct xfs_ifork                *ifp;
        struct xfs_iext_cursor          icur;
        bool                            found;
        int                             error;

        ifp = xfs_ifork_ptr(ip, XFS_DATA_FORK);
        error = xfs_iread_extents(tp, ip, XFS_DATA_FORK);
        if (error)
                return error;

        *has_shared = false;
        found = xfs_iext_lookup_extent(ip, ifp, 0, &icur, &got);
        while (found) {
                struct xfs_perag        *pag;
                xfs_agblock_t           agbno;
                xfs_extlen_t            aglen;
                xfs_agblock_t           rbno;
                xfs_extlen_t            rlen;

                if (isnullstartblock(got.br_startblock) ||
                    got.br_state != XFS_EXT_NORM)
                        goto next;

                pag = xfs_perag_get(mp, XFS_FSB_TO_AGNO(mp, got.br_startblock));
                agbno = XFS_FSB_TO_AGBNO(mp, got.br_startblock);
                aglen = got.br_blockcount;
                error = xfs_reflink_find_shared(pag, tp, agbno, aglen,
                                &rbno, &rlen, false);
                xfs_perag_put(pag);
                if (error)
                        return error;

                /* Is there still a shared block here? */
                if (rbno != NULLAGBLOCK) {
                        *has_shared = true;
                        return 0;
                }
next:
                found = xfs_iext_next_extent(ifp, &icur, &got);
        }

        return 0;
}

/*
 * Clear the inode reflink flag if there are no shared extents.
 *
 * The caller is responsible for joining the inode to the transaction passed in.
 * The inode will be joined to the transaction that is returned to the caller.
 */
int
xfs_reflink_clear_inode_flag(
        struct xfs_inode        *ip,
        struct xfs_trans        **tpp)
{
        bool                    needs_flag;
        int                     error = 0;

        ASSERT(xfs_is_reflink_inode(ip));

        error = xfs_reflink_inode_has_shared_extents(*tpp, ip, &needs_flag);
        if (error || needs_flag)
                return error;

        /*
         * We didn't find any shared blocks so turn off the reflink flag.
         * First, get rid of any leftover CoW mappings.
         */
        error = xfs_reflink_cancel_cow_blocks(ip, tpp, 0, XFS_MAX_FILEOFF,
                        true);
        if (error)
                return error;

        /* Clear the inode flag. */
        trace_xfs_reflink_unset_inode_flag(ip);
        ip->i_diflags2 &= ~XFS_DIFLAG2_REFLINK;
        xfs_inode_clear_cowblocks_tag(ip);
        xfs_trans_log_inode(*tpp, ip, XFS_ILOG_CORE);

        return error;
}

/*
 * Clear the inode reflink flag if there are no shared extents and the size
 * hasn't changed.
 */
STATIC int
xfs_reflink_try_clear_inode_flag(
        struct xfs_inode        *ip)
{
        struct xfs_mount        *mp = ip->i_mount;
        struct xfs_trans        *tp;
        int                     error = 0;

        /* Start a rolling transaction to remove the mappings */
        error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, 0, 0, 0, &tp);
        if (error)
                return error;

        xfs_ilock(ip, XFS_ILOCK_EXCL);
        xfs_trans_ijoin(tp, ip, 0);

        error = xfs_reflink_clear_inode_flag(ip, &tp);
        if (error)
                goto cancel;

        error = xfs_trans_commit(tp);
        if (error)
                goto out;

        xfs_iunlock(ip, XFS_ILOCK_EXCL);
        return 0;
cancel:
        xfs_trans_cancel(tp);
out:
        xfs_iunlock(ip, XFS_ILOCK_EXCL);
        return error;
}

/*
 * Pre-COW all shared blocks within a given byte range of a file and turn off
 * the reflink flag if we unshare all of the file's blocks.
 */
int
xfs_reflink_unshare(
        struct xfs_inode        *ip,
        xfs_off_t               offset,
        xfs_off_t               len)
{
        struct inode            *inode = VFS_I(ip);
        int                     error;

        if (!xfs_is_reflink_inode(ip))
                return 0;

        trace_xfs_reflink_unshare(ip, offset, len);

        inode_dio_wait(inode);

        if (IS_DAX(inode))
                error = dax_file_unshare(inode, offset, len,
                                &xfs_dax_write_iomap_ops);
        else
                error = iomap_file_unshare(inode, offset, len,
                                &xfs_buffered_write_iomap_ops);
        if (error)
                goto out;

        error = filemap_write_and_wait_range(inode->i_mapping, offset,
                        offset + len - 1);
        if (error)
                goto out;

        /* Turn off the reflink flag if possible. */
        error = xfs_reflink_try_clear_inode_flag(ip);
        if (error)
                goto out;
        return 0;

out:
        trace_xfs_reflink_unshare_error(ip, error, _RET_IP_);
        return error;
}