[PATCH v8 10/15] xfs: add large atomic writes checks in xfs_direct_write_iomap_begin()

John Garry
2025-04-22T12:29:48Z

For when large atomic writes (> 1x FS block) are supported, there will be
various occasions when HW offload may not be possible.

Such instances include:
- unaligned extent mapping wrt write length
- extent mappings which do not cover the full write, e.g. the write spans
sparse or mixed-mapping extents
- the write length is greater than HW offload can support

In those cases, we need to fallback to the CoW-based atomic write mode. For
this, report special code -ENOPROTOOPT to inform the caller that HW
offload-based method is not possible.

In addition to the occasions mentioned, if the write covers an unallocated
range, we again judge that we need to rely on the CoW-based method when we
would need to allocate anything more than 1x block. This is because if we
allocate less blocks that is required for the write, then again HW
offload-based method would not be possible. So we are taking a pessimistic
approach to writes covering unallocated space.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
[djwong: don't allow too-small hw atomic writes either]
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
---
fs/xfs/xfs_iomap.c | 65 ++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 63 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 049655ebc3f7..150354e5b436 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -798,6 +798,41 @@ imap_spans_range(
return true;
}

+static bool
+xfs_bmap_hw_atomic_write_possible(
+ struct xfs_inode *ip,
+ struct xfs_bmbt_irec *imap,
+ xfs_fileoff_t offset_fsb,
+ xfs_fileoff_t end_fsb)
+{
+ struct xfs_mount *mp = ip->i_mount;
+ xfs_fsize_t len = XFS_FSB_TO_B(mp, end_fsb - offset_fsb);
+
+ /*
+ * atomic writes are required to be naturally aligned for disk blocks,
+ * which ensures that we adhere to block layer rules that we won't
+ * straddle any boundary or violate write alignment requirement.
+ */
+ if (!IS_ALIGNED(imap->br_startblock, imap->br_blockcount))
+ return false;
+
+ /*
+ * Spanning multiple extents would mean that multiple BIOs would be
+ * issued, and so would lose atomicity required for REQ_ATOMIC-based
+ * atomics.
+ */
+ if (!imap_spans_range(imap, offset_fsb, end_fsb))
+ return false;
+
+ /*
+ * The ->iomap_begin caller should ensure this, but check anyway.
+ */
+ if (len > xfs_inode_hw_atomicwrite_max(ip))
+ return false;
+
+ return true;
+}
+
static int
xfs_direct_write_iomap_begin(
struct inode *inode,
@@ -812,9 +847,11 @@ xfs_direct_write_iomap_begin(
struct xfs_bmbt_irec imap, cmap;
xfs_fileoff_t offset_fsb = XFS_B_TO_FSBT(mp, offset);
xfs_fileoff_t end_fsb = xfs_iomap_end_fsb(mp, offset, length);
+ xfs_fileoff_t orig_end_fsb = end_fsb;
int nimaps = 1, error = 0;
bool shared = false;
u16 iomap_flags = 0;
+ bool needs_alloc;
unsigned int lockmode;
u64 seq;

@@ -875,13 +912,37 @@ xfs_direct_write_iomap_begin(
(flags & IOMAP_DIRECT) || IS_DAX(inode));
if (error)
goto out_unlock;
- if (shared)
+ if (shared) {
+ if ((flags & IOMAP_ATOMIC) &&
+ !xfs_bmap_hw_atomic_write_possible(ip, &cmap,
+ offset_fsb, end_fsb)) {
+ error = -ENOPROTOOPT;
+ goto out_unlock;
+ }
goto out_found_cow;
+ }
end_fsb = imap.br_startoff + imap.br_blockcount;
length = XFS_FSB_TO_B(mp, end_fsb) - offset;
}

- if (imap_needs_alloc(inode, flags, &imap, nimaps))
+ needs_alloc = imap_needs_alloc(inode, flags, &imap, nimaps);
+
+ if (flags & IOMAP_ATOMIC) {
+ error = -ENOPROTOOPT;
+ /*
+ * If we allocate less than what is required for the write
+ * then we may end up with multiple extents, which means that
+ * REQ_ATOMIC-based cannot be used, so avoid this possibility.
+ */
+ if (needs_alloc && orig_end_fsb - offset_fsb > 1)
+ goto out_unlock;
+
+ if (!xfs_bmap_hw_atomic_write_possible(ip, &imap, offset_fsb,
+ orig_end_fsb))
+ goto out_unlock;
+ }
+
+ if (needs_alloc)
goto allocate_blocks;

/*
--
2.31.1