linux-kernel - [PATCH v6 12/12] xfs: update atomic write limits

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Message-Id: <20250408104209.1852036-13-john.g.garry@oracle.com>
Date: Tue,  8 Apr 2025 10:42:09 +0000
From: John Garry <john.g.garry@...cle.com>
To: brauner@...nel.org, djwong@...nel.org, hch@....de, viro@...iv.linux.org.uk,
        jack@...e.cz, cem@...nel.org
Cc: linux-fsdevel@...r.kernel.org, dchinner@...hat.com,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        ojaswin@...ux.ibm.com, ritesh.list@...il.com,
        martin.petersen@...cle.com, linux-ext4@...r.kernel.org,
        linux-block@...r.kernel.org, catherine.hoang@...cle.com,
        John Garry <john.g.garry@...cle.com>
Subject: [PATCH v6 12/12] xfs: update atomic write limits

Update the limits returned from xfs_get_atomic_write_{min, max, max_opt)().

No reflink support always means no CoW-based atomic writes.

For updating xfs_get_atomic_write_min(), we support blocksize only and that
depends on HW or reflink support.

For updating xfs_get_atomic_write_max(), for rtvol or no reflink, we are
limited to blocksize but only if HW support. Otherwise we are limited to
combined limit in mp->m_atomic_write_unit_max.

For updating xfs_get_atomic_write_max_opt(), ultimately we are limited by
the bdev atomic write limit. If xfs_get_atomic_write_max() does not report
> 1x blocksize, then just continue to report 0 as before.

Signed-off-by: John Garry <john.g.garry@...cle.com>
---
 fs/xfs/xfs_file.c |  2 +-
 fs/xfs/xfs_iops.c | 37 +++++++++++++++++++++++++++++++------
 2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 81a377f65aa3..d1ddbc4a98c3 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -1557,7 +1557,7 @@ xfs_file_open(
 	if (xfs_is_shutdown(XFS_M(inode->i_sb)))
 		return -EIO;
 	file->f_mode |= FMODE_NOWAIT | FMODE_CAN_ODIRECT;
-	if (xfs_inode_can_hw_atomicwrite(XFS_I(inode)))
+	if (xfs_get_atomic_write_min(XFS_I(inode)))
 		file->f_mode |= FMODE_CAN_ATOMIC_WRITE;
 	return generic_file_open(inode, file);
 }
diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 3b5aa39dbfe9..894f56f1a830 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -605,27 +605,52 @@ unsigned int
 xfs_get_atomic_write_min(
 	struct xfs_inode	*ip)
 {
-	if (!xfs_inode_can_hw_atomicwrite(ip))
-		return 0;
+	if (xfs_inode_can_hw_atomicwrite(ip) || xfs_has_reflink(ip->i_mount))
+		return ip->i_mount->m_sb.sb_blocksize;
 
-	return ip->i_mount->m_sb.sb_blocksize;
+	return 0;
 }
 
 unsigned int
 xfs_get_atomic_write_max(
 	struct xfs_inode	*ip)
 {
-	if (!xfs_inode_can_hw_atomicwrite(ip))
+	struct xfs_mount	*mp = ip->i_mount;
+
+	/*
+	 * If no reflink, then best we can do is 1x block as no CoW fallback
+	 * for when HW offload not possible.
+	 *
+	 * 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_has_reflink(mp) || XFS_IS_REALTIME_INODE(ip)) {
+		if (xfs_inode_can_hw_atomicwrite(ip))
+			return ip->i_mount->m_sb.sb_blocksize;
 		return 0;
+	}
 
-	return ip->i_mount->m_sb.sb_blocksize;
+	/*
+	 * Even though HW support could be larger (than CoW), we rely on
+	 * CoW-based method as a fallback for when HW-based is not possible,
+	 * so always limit at m_atomic_write_unit_max (which is evaluated
+	 * according to CoW-based limit.
+	 */
+	return XFS_FSB_TO_B(mp, mp->m_atomic_write_unit_max);
 }
 
 unsigned int
 xfs_get_atomic_write_max_opt(
 	struct xfs_inode	*ip)
 {
-	return 0;
+	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
+
+	/* if the max is 1x block, then just keep behaviour that opt is 0 */
+	if (xfs_get_atomic_write_max(ip) <= ip->i_mount->m_sb.sb_blocksize)
+		return 0;
+
+	return min(xfs_get_atomic_write_max(ip), target->bt_bdev_awu_max);
 }
 
 static void
-- 
2.31.1