[PATCH v5 08/10] xfs: Update atomic write max size

[John Garry]

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.

For RT inode, just limit to 1x block, even though larger can be supported
in future.

Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
 fs/xfs/xfs_iops.c  | 14 +++++++++++++-
 fs/xfs/xfs_mount.c | 28 ++++++++++++++++++++++++++++
 fs/xfs/xfs_mount.h |  1 +
 3 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index de065cc2e7cf..16a1f9541690 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -607,12 +607,24 @@ 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;
+
+	if (XFS_IS_REALTIME_INODE(ip)) {
+		/* For now, set limit at 1x block */
+		*unit_max = ip->i_mount->m_sb.sb_blocksize;
+	} else {
+		*unit_max =  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..414adfb944b9 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -665,6 +665,32 @@ 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_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
@@ -751,6 +777,8 @@ 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