[PATCH RFC 09/10] xfs: Update atomic write max size

John Garry
2025-02-04T12:02:07Z

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.
Currently when attempting to use HW offload, we just check that the
mapping startblock is aligned. However, that is just the startblock within
the AG, and the AG may not be properly aligned to the underlying block
device atomic write limits.

As such, limit atomic writes to the greatest power-of-2 which fits in an
AG, so that aligning to the startblock will be mean that we are also
aligned to the disk block.

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
fs/xfs/xfs_iops.c | 7 ++++++-
fs/xfs/xfs_mount.c | 28 ++++++++++++++++++++++++++++
fs/xfs/xfs_mount.h | 1 +
3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index ea79fb246e33..95681d6c2bcd 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -606,12 +606,17 @@ xfs_get_atomic_write_attr(
unsigned int *unit_min,
unsigned int *unit_max)
{
+ struct xfs_buftarg *target = xfs_inode_buftarg(ip);
+ struct xfs_mount *mp = ip->i_mount;
+
if (!xfs_inode_can_atomicwrite(ip)) {
*unit_min = *unit_max = 0;
return;
}

- *unit_min = *unit_max = ip->i_mount->m_sb.sb_blocksize;
+ *unit_min = ip->i_mount->m_sb.sb_blocksize;
+ *unit_max = min_t(unsigned int, XFS_FSB_TO_B(mp, mp->awu_max),
+ target->bt_bdev_awu_max);
}

static void
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 477c5262cf91..4e60347f6b7e 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -651,6 +651,32 @@ xfs_agbtree_compute_maxlevels(
levels = max(levels, mp->m_rmap_maxlevels);
mp->m_agbtree_maxlevels = max(levels, mp->m_refc_maxlevels);
}
+static inline void
+xfs_mp_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->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->awu_max = awu_max;
+}

/* Compute maximum possible height for realtime btree types for this fs. */
static inline void
@@ -736,6 +762,8 @@ xfs_mountfs(
xfs_agbtree_compute_maxlevels(mp);
xfs_rtbtree_compute_maxlevels(mp);

+ xfs_mp_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 fbed172d6770..34286c87ac4a 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -198,6 +198,7 @@ typedef struct xfs_mount {
bool m_fail_unmount;
bool m_finobt_nores; /* no per-AG finobt resv. */
bool m_update_sb; /* sb needs update in mount */
+ xfs_extlen_t awu_max; /* max atomic write */

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