[PATCH 5.10 v2 2/7] xfs: always succeed at setting the reserve pool size

[Amir Goldstein <amir73il@gmail.com>]

From: "Darrick J. Wong" <djwong@kernel.org>

commit 0baa2657dc4d79202148be79a3dc36c35f425060 upstream.

Nowadays, xfs_mod_fdblocks will always choose to fill the reserve pool
with freed blocks before adding to fdblocks.  Therefore, we can change
the behavior of xfs_reserve_blocks slightly -- setting the target size
of the pool should always succeed, since a deficiency will eventually
be made up as blocks get freed.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/xfs/xfs_fsops.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
index 6d4f4271e7be..dacead0d0934 100644
--- a/fs/xfs/xfs_fsops.c
+++ b/fs/xfs/xfs_fsops.c
@@ -380,11 +380,14 @@ xfs_reserve_blocks(
 	 * The code below estimates how many blocks it can request from
 	 * fdblocks to stash in the reserve pool.  This is a classic TOCTOU
 	 * race since fdblocks updates are not always coordinated via
-	 * m_sb_lock.
+	 * m_sb_lock.  Set the reserve size even if there's not enough free
+	 * space to fill it because mod_fdblocks will refill an undersized
+	 * reserve when it can.
 	 */
 	free = percpu_counter_sum(&mp->m_fdblocks) -
 						xfs_fdblocks_unavailable(mp);
 	delta = request - mp->m_resblks;
+	mp->m_resblks = request;
 	if (delta > 0 && free > 0) {
 		/*
 		 * We'll either succeed in getting space from the free block
@@ -401,10 +404,8 @@ xfs_reserve_blocks(
 		 * Update the reserve counters if blocks have been successfully
 		 * allocated.
 		 */
-		if (!error) {
-			mp->m_resblks += fdblks_delta;
+		if (!error)
 			mp->m_resblks_avail += fdblks_delta;
-		}
 	}
 out:
 	if (outval) {
-- 
2.25.1