[PATCH 4/6] xfs: look up cow fork extent earlier for buffered iomap_begin

Brian Foster
2025-10-16T18:58:49Z

To further isolate the need for flushing for zero range, we need to
know whether a hole in the data fork is fronted by blocks in the COW
fork or not. COW fork lookup currently occurs further down in the
function, after the zero range case is handled.

As a preparation step, lift the COW fork extent lookup to earlier in
the function, at the same time as the data fork lookup. Only the
lookup logic is lifted. The COW fork branch/reporting logic remains
as is to avoid any observable behavior change from an iomap
reporting perspective.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
fs/xfs/xfs_iomap.c | 46 +++++++++++++++++++++++++---------------------
1 file changed, 25 insertions(+), 21 deletions(-)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index b84c94558cc9..ba5697d8b8fd 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1753,14 +1753,29 @@ xfs_buffered_write_iomap_begin(
goto out_unlock;

/*
- * Search the data fork first to look up our source mapping. We
- * always need the data fork map, as we have to return it to the
- * iomap code so that the higher level write code can read data in to
- * perform read-modify-write cycles for unaligned writes.
+ * Search the data fork first to look up our source mapping. We always
+ * need the data fork map, as we have to return it to the iomap code so
+ * that the higher level write code can read data in to perform
+ * read-modify-write cycles for unaligned writes.
+ *
+ * Then search the COW fork extent list even if we did not find a data
+ * fork extent. This serves two purposes: first this implements the
+ * speculative preallocation using cowextsize, so that we also unshare
+ * block adjacent to shared blocks instead of just the shared blocks
+ * themselves. Second the lookup in the extent list is generally faster
+ * than going out to the shared extent tree.
*/
eof = !xfs_iext_lookup_extent(ip, &ip->i_df, offset_fsb, &icur, &imap);
if (eof)
imap.br_startoff = end_fsb; /* fake hole until the end */
+ if (xfs_is_cow_inode(ip)) {
+ if (!ip->i_cowfp) {
+ ASSERT(!xfs_is_reflink_inode(ip));
+ xfs_ifork_init_cow(ip);
+ }
+ cow_eof = !xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb,
+ &ccur, &cmap);
+ }

/* We never need to allocate blocks for unsharing a hole. */
if ((flags & IOMAP_UNSHARE) && imap.br_startoff > offset_fsb) {
@@ -1827,24 +1842,13 @@ xfs_buffered_write_iomap_begin(
}

/*
- * Search the COW fork extent list even if we did not find a data fork
- * extent. This serves two purposes: first this implements the
- * speculative preallocation using cowextsize, so that we also unshare
- * block adjacent to shared blocks instead of just the shared blocks
- * themselves. Second the lookup in the extent list is generally faster
- * than going out to the shared extent tree.
+ * Now that we've handled any operation specific special cases, at this
+ * point we can report a COW mapping if found.
*/
- if (xfs_is_cow_inode(ip)) {
- if (!ip->i_cowfp) {
- ASSERT(!xfs_is_reflink_inode(ip));
- xfs_ifork_init_cow(ip);
- }
- cow_eof = !xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb,
- &ccur, &cmap);
- if (!cow_eof && cmap.br_startoff <= offset_fsb) {
- trace_xfs_reflink_cow_found(ip, &cmap);
- goto found_cow;
- }
+ if (xfs_is_cow_inode(ip) &&
+ !cow_eof && cmap.br_startoff <= offset_fsb) {
+ trace_xfs_reflink_cow_found(ip, &cmap);
+ goto found_cow;
}

if (imap.br_startoff <= offset_fsb) {
--
2.51.0