[PATCH v6 13/13] xfs: update atomic write max size

John Garry
2025-03-13T17:14:57Z

Now that CoW-based atomic writes are supported, update the max size of an
atomic write.

For simplicity, limit at the max of what the mounted bdev can support in
terms of atomic write limits. Maybe in future we will have a better way
to advertise this optimised limit.

In addition, the max atomic write size needs to be aligned to the agsize.
Limit the size of atomic writes to the greatest power-of-two factor of the
agsize so that allocations for an atomic write will always be aligned
compatibly with the alignment requirements of the storage.

rtvol is not commonly used, so it is not very important to support large
atomic writes there initially.

Furthermore, adding large atomic writes for rtvol would be complicated due
to alignment already offered by rtextsize and also the limitation of
reflink support only be possible for rtextsize is a power-of-2.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
fs/xfs/xfs_iops.c | 14 +++++++++++++-
fs/xfs/xfs_mount.c | 29 +++++++++++++++++++++++++++++
fs/xfs/xfs_mount.h | 1 +
3 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 64b1f8c73824..7c22eefd6b89 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -615,10 +615,22 @@ unsigned int
xfs_get_atomic_write_max_attr(
struct xfs_inode *ip)
{
+ struct xfs_buftarg *target = xfs_inode_buftarg(ip);
+ struct xfs_mount *mp = ip->i_mount;
+
if (!xfs_inode_can_atomicwrite(ip))
return 0;

- return ip->i_mount->m_sb.sb_blocksize;
+ /*
+ * rtvol is not commonly used and supporting large atomic writes
+ * would also be complicated to support there, so limit to a single
+ * block for now.
+ */
+ if (XFS_IS_REALTIME_INODE(ip))
+ return mp->m_sb.sb_blocksize;
+
+ return min_t(unsigned int, XFS_FSB_TO_B(mp, mp->m_awu_max),
+ target->bt_bdev_awu_max);
}

static void
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index e65a659901d5..fd89cb7a83fd 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -666,6 +666,33 @@ xfs_agbtree_compute_maxlevels(
mp->m_agbtree_maxlevels = max(levels, mp->m_refc_maxlevels);
}

+static inline void
+xfs_compute_awu_max(
+ struct xfs_mount *mp)
+{
+ xfs_agblock_t agsize = mp->m_sb.sb_agblocks;
+ xfs_agblock_t awu_max;
+
+ if (!xfs_has_reflink(mp)) {
+ mp->m_awu_max = 1;
+ return;
+ }
+
+ /*
+ * Find highest power-of-2 evenly divisible into agsize and which
+ * also fits into an unsigned int field.
+ */
+ awu_max = 1;
+ while (1) {
+ if (agsize % (awu_max * 2))
+ break;
+ if (XFS_FSB_TO_B(mp, awu_max * 2) > UINT_MAX)
+ break;
+ awu_max *= 2;
+ }
+ mp->m_awu_max = awu_max;
+}
+
/* Compute maximum possible height for realtime btree types for this fs. */
static inline void
xfs_rtbtree_compute_maxlevels(
@@ -751,6 +778,8 @@ xfs_mountfs(
xfs_agbtree_compute_maxlevels(mp);
xfs_rtbtree_compute_maxlevels(mp);

+ xfs_compute_awu_max(mp);
+
/*
* Check if sb_agblocks is aligned at stripe boundary. If sb_agblocks
* is NOT aligned turn off m_dalign since allocator alignment is within
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index 799b84220ebb..1b0136da2aec 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -229,6 +229,7 @@ typedef struct xfs_mount {
bool m_finobt_nores; /* no per-AG finobt resv. */
bool m_update_sb; /* sb needs update in mount */
unsigned int m_max_open_zones;
+ xfs_extlen_t m_awu_max; /* data device max atomic write */

/*
* Bitsets of per-fs metadata that have been checked and/or are sick.
--
2.31.1