linux-ext4 - [PATCH v12 16/17] xfs: update atomic write limits

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Message-Id: <20250506090427.2549456-17-john.g.garry@oracle.com>
Date: Tue,  6 May 2025 09:04:26 +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,
        linux-api@...r.kernel.org, John Garry <john.g.garry@...cle.com>
Subject: [PATCH v12 16/17] 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 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.

Reviewed-by: Darrick J. Wong <djwong@...nel.org>
[djwong: update comments in the helper functions]
Signed-off-by: Darrick J. Wong <djwong@...nel.org>
Reviewed-by: Christoph Hellwig <hch@....de>
Signed-off-by: John Garry <john.g.garry@...cle.com>
---
 fs/xfs/xfs_file.c |  2 +-
 fs/xfs/xfs_iops.c | 52 +++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index f4a66ff85748..48254a72071b 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_atomic_write(XFS_I(inode)))
+	if (xfs_get_atomic_write_min(XFS_I(inode)) > 0)
 		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 77a0606e9dc9..8cddbb7c149b 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -605,27 +605,67 @@ unsigned int
 xfs_get_atomic_write_min(
 	struct xfs_inode	*ip)
 {
-	if (!xfs_inode_can_hw_atomic_write(ip))
-		return 0;
+	struct xfs_mount	*mp = ip->i_mount;
+
+	/*
+	 * If we can complete an atomic write via atomic out of place writes,
+	 * then advertise a minimum size of one fsblock.  Without this
+	 * mechanism, we can only guarantee atomic writes up to a single LBA.
+	 *
+	 * If out of place writes are not available, we can guarantee an atomic
+	 * write of exactly one single fsblock if the bdev will make that
+	 * guarantee for us.
+	 */
+	if (xfs_inode_can_hw_atomic_write(ip) || xfs_can_sw_atomic_write(mp))
+		return mp->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_atomic_write(ip))
+	struct xfs_mount	*mp = ip->i_mount;
+
+	/*
+	 * If out of place writes are not available, we can guarantee an atomic
+	 * write of exactly one single fsblock if the bdev will make that
+	 * guarantee for us.
+	 */
+	if (!xfs_can_sw_atomic_write(mp)) {
+		if (xfs_inode_can_hw_atomic_write(ip))
+			return mp->m_sb.sb_blocksize;
 		return 0;
+	}
 
-	return ip->i_mount->m_sb.sb_blocksize;
+	/*
+	 * If we can complete an atomic write via atomic out of place writes,
+	 * then advertise a maximum size of whatever we can complete through
+	 * that means.  Hardware support is reported via max_opt, not here.
+	 */
+	if (XFS_IS_REALTIME_INODE(ip))
+		return XFS_FSB_TO_B(mp, mp->m_groups[XG_TYPE_RTG].awu_max);
+	return XFS_FSB_TO_B(mp, mp->m_groups[XG_TYPE_AG].awu_max);
 }
 
 unsigned int
 xfs_get_atomic_write_max_opt(
 	struct xfs_inode	*ip)
 {
-	return 0;
+	unsigned int		awu_max = xfs_get_atomic_write_max(ip);
+
+	/* if the max is 1x block, then just keep behaviour that opt is 0 */
+	if (awu_max <= ip->i_mount->m_sb.sb_blocksize)
+		return 0;
+
+	/*
+	 * Advertise the maximum size of an atomic write that we can tell the
+	 * block device to perform for us.  In general the bdev limit will be
+	 * less than our out of place write limit, but we don't want to exceed
+	 * the awu_max.
+	 */
+	return min(awu_max, xfs_inode_buftarg(ip)->bt_bdev_awu_max);
 }
 
 static void
-- 
2.31.1