[PATCH v11 10/16] xfs: add xfs_atomic_write_cow_iomap_begin()

John Garry
2025-05-04T09:01:23Z

For CoW-based atomic writes, reuse the infrastructure for reflink CoW fork
support.

Add ->iomap_begin() callback xfs_atomic_write_cow_iomap_begin() to create
staging mappings in the CoW fork for atomic write updates.

The general steps in the function are as follows:
- find extent mapping in the CoW fork for the FS block range being written
- if part or full extent is found, proceed to process found extent
- if no extent found, map in new blocks to the CoW fork
- convert unwritten blocks in extent if required
- update iomap extent mapping and return

The bulk of this function is quite similar to the processing in
xfs_reflink_allocate_cow(), where we try to find an extent mapping; if
none exists, then allocate a new extent in the CoW fork, convert unwritten
blocks, and return a mapping.

Performance testing has shown the XFS_ILOCK_EXCL locking to be quite
a bottleneck, so this is an area which could be optimised in future.

Christoph Hellwig contributed almost all of the code in
xfs_atomic_write_cow_iomap_begin().

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: add a new xfs_can_sw_atomic_write to convey intent better]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
fs/xfs/xfs_iomap.c | 128 +++++++++++++++++++++++++++++++++++++++++++
fs/xfs/xfs_iomap.h | 1 +
fs/xfs/xfs_mount.h | 5 ++
fs/xfs/xfs_reflink.c | 2 +-
fs/xfs/xfs_reflink.h | 2 +
fs/xfs/xfs_trace.h | 22 ++++++++
6 files changed, 159 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index cb23c8871f81..166fba2ff1ef 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1022,6 +1022,134 @@ const struct iomap_ops xfs_zoned_direct_write_iomap_ops = {
};
#endif /* CONFIG_XFS_RT */

+static int
+xfs_atomic_write_cow_iomap_begin(
+ struct inode *inode,
+ loff_t offset,
+ loff_t length,
+ unsigned flags,
+ struct iomap *iomap,
+ struct iomap *srcmap)
+{
+ struct xfs_inode *ip = XFS_I(inode);
+ struct xfs_mount *mp = ip->i_mount;
+ const xfs_fileoff_t offset_fsb = XFS_B_TO_FSBT(mp, offset);
+ xfs_fileoff_t end_fsb = xfs_iomap_end_fsb(mp, offset, length);
+ xfs_filblks_t count_fsb = end_fsb - offset_fsb;
+ int nmaps = 1;
+ xfs_filblks_t resaligned;
+ struct xfs_bmbt_irec cmap;
+ struct xfs_iext_cursor icur;
+ struct xfs_trans *tp;
+ unsigned int dblocks = 0, rblocks = 0;
+ int error;
+ u64 seq;
+
+ ASSERT(flags & IOMAP_WRITE);
+ ASSERT(flags & IOMAP_DIRECT);
+
+ if (xfs_is_shutdown(mp))
+ return -EIO;
+
+ if (!xfs_can_sw_atomic_write(mp)) {
+ ASSERT(xfs_can_sw_atomic_write(mp));
+ return -EINVAL;
+ }
+
+ /* blocks are always allocated in this path */
+ if (flags & IOMAP_NOWAIT)
+ return -EAGAIN;
+
+ trace_xfs_iomap_atomic_write_cow(ip, offset, length);
+
+ xfs_ilock(ip, XFS_ILOCK_EXCL);
+
+ if (!ip->i_cowfp) {
+ ASSERT(!xfs_is_reflink_inode(ip));
+ xfs_ifork_init_cow(ip);
+ }
+
+ if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &cmap))
+ cmap.br_startoff = end_fsb;
+ if (cmap.br_startoff <= offset_fsb) {
+ xfs_trim_extent(&cmap, offset_fsb, count_fsb);
+ goto found;
+ }
+
+ end_fsb = cmap.br_startoff;
+ count_fsb = end_fsb - offset_fsb;
+
+ resaligned = xfs_aligned_fsb_count(offset_fsb, count_fsb,
+ xfs_get_cowextsz_hint(ip));
+ xfs_iunlock(ip, XFS_ILOCK_EXCL);
+
+ if (XFS_IS_REALTIME_INODE(ip)) {
+ dblocks = XFS_DIOSTRAT_SPACE_RES(mp, 0);
+ rblocks = resaligned;
+ } else {
+ dblocks = XFS_DIOSTRAT_SPACE_RES(mp, resaligned);
+ rblocks = 0;
+ }
+
+ error = xfs_trans_alloc_inode(ip, &M_RES(mp)->tr_write, dblocks,
+ rblocks, false, &tp);
+ if (error)
+ return error;
+
+ /* extent layout could have changed since the unlock, so check again */
+ if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &cmap))
+ cmap.br_startoff = end_fsb;
+ if (cmap.br_startoff <= offset_fsb) {
+ xfs_trim_extent(&cmap, offset_fsb, count_fsb);
+ xfs_trans_cancel(tp);
+ goto found;
+ }
+
+ /*
+ * Allocate the entire reservation as unwritten blocks.
+ *
+ * Use XFS_BMAPI_EXTSZALIGN to hint at aligning new extents according to
+ * extszhint, such that there will be a greater chance that future
+ * atomic writes to that same range will be aligned (and don't require
+ * this COW-based method).
+ */
+ error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
+ XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC |
+ XFS_BMAPI_EXTSZALIGN, 0, &cmap, &nmaps);
+ if (error) {
+ xfs_trans_cancel(tp);
+ goto out_unlock;
+ }
+
+ xfs_inode_set_cowblocks_tag(ip);
+ error = xfs_trans_commit(tp);
+ if (error)
+ goto out_unlock;
+
+found:
+ if (cmap.br_state != XFS_EXT_NORM) {
+ error = xfs_reflink_convert_cow_locked(ip, offset_fsb,
+ count_fsb);
+ if (error)
+ goto out_unlock;
+ cmap.br_state = XFS_EXT_NORM;
+ }
+
+ length = XFS_FSB_TO_B(mp, cmap.br_startoff + cmap.br_blockcount);
+ trace_xfs_iomap_found(ip, offset, length - offset, XFS_COW_FORK, &cmap);
+ seq = xfs_iomap_inode_sequence(ip, IOMAP_F_SHARED);
+ xfs_iunlock(ip, XFS_ILOCK_EXCL);
+ return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags, IOMAP_F_SHARED, seq);
+
+out_unlock:
+ xfs_iunlock(ip, XFS_ILOCK_EXCL);
+ return error;
+}
+
+const struct iomap_ops xfs_atomic_write_cow_iomap_ops = {
+ .iomap_begin = xfs_atomic_write_cow_iomap_begin,
+};
+
static int
xfs_dax_write_iomap_end(
struct inode *inode,
diff --git a/fs/xfs/xfs_iomap.h b/fs/xfs/xfs_iomap.h
index d330c4a581b1..674f8ac1b9bd 100644
--- a/fs/xfs/xfs_iomap.h
+++ b/fs/xfs/xfs_iomap.h
@@ -56,5 +56,6 @@ extern const struct iomap_ops xfs_read_iomap_ops;
extern const struct iomap_ops xfs_seek_iomap_ops;
extern const struct iomap_ops xfs_xattr_iomap_ops;
extern const struct iomap_ops xfs_dax_write_iomap_ops;
+extern const struct iomap_ops xfs_atomic_write_cow_iomap_ops;

#endif /* __XFS_IOMAP_H__*/
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index e5192c12e7ac..e67bc3e91f98 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -464,6 +464,11 @@ static inline bool xfs_has_nonzoned(const struct xfs_mount *mp)
return !xfs_has_zoned(mp);
}

+static inline bool xfs_can_sw_atomic_write(struct xfs_mount *mp)
+{
+ return xfs_has_reflink(mp);
+}
+
/*
* Some features are always on for v5 file systems, allow the compiler to
* eliminiate dead code when building without v4 support.
diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index bd711c5bb6bb..f5d338916098 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -293,7 +293,7 @@ xfs_bmap_trim_cow(
return xfs_reflink_trim_around_shared(ip, imap, shared);
}

-static int
+int
xfs_reflink_convert_cow_locked(
struct xfs_inode *ip,
xfs_fileoff_t offset_fsb,
diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
index cc4e92278279..379619f24247 100644
--- a/fs/xfs/xfs_reflink.h
+++ b/fs/xfs/xfs_reflink.h
@@ -35,6 +35,8 @@ int xfs_reflink_allocate_cow(struct xfs_inode *ip, struct xfs_bmbt_irec *imap,
bool convert_now);
extern int xfs_reflink_convert_cow(struct xfs_inode *ip, xfs_off_t offset,
xfs_off_t count);
+int xfs_reflink_convert_cow_locked(struct xfs_inode *ip,
+ xfs_fileoff_t offset_fsb, xfs_filblks_t count_fsb);

extern int xfs_reflink_cancel_cow_blocks(struct xfs_inode *ip,
struct xfs_trans **tpp, xfs_fileoff_t offset_fsb,
diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
index e56ba1963160..9554578c6da4 100644
--- a/fs/xfs/xfs_trace.h
+++ b/fs/xfs/xfs_trace.h
@@ -1657,6 +1657,28 @@ DEFINE_RW_EVENT(xfs_file_direct_write);
DEFINE_RW_EVENT(xfs_file_dax_write);
DEFINE_RW_EVENT(xfs_reflink_bounce_dio_write);

+TRACE_EVENT(xfs_iomap_atomic_write_cow,
+ TP_PROTO(struct xfs_inode *ip, xfs_off_t offset, ssize_t count),
+ TP_ARGS(ip, offset, count),
+ TP_STRUCT__entry(
+ __field(dev_t, dev)
+ __field(xfs_ino_t, ino)
+ __field(xfs_off_t, offset)
+ __field(ssize_t, count)
+ ),
+ TP_fast_assign(
+ __entry->dev = VFS_I(ip)->i_sb->s_dev;
+ __entry->ino = ip->i_ino;
+ __entry->offset = offset;
+ __entry->count = count;
+ ),
+ TP_printk("dev %d:%d ino 0x%llx pos 0x%llx bytecount 0x%zx",
+ MAJOR(__entry->dev), MINOR(__entry->dev),
+ __entry->ino,
+ __entry->offset,
+ __entry->count)
+)
+
DECLARE_EVENT_CLASS(xfs_imap_class,
TP_PROTO(struct xfs_inode *ip, xfs_off_t offset, ssize_t count,
int whichfork, struct xfs_bmbt_irec *irec),
--
2.31.1