[PATCH 25/26] xfs: don't let hidden_space go negative in xfs_metafile_resv_init

[Eduard Zateev <hackerowskiy@gmail.com>]

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

LOLLM points out that if the amount of fdblocks that we can reserve for
a metadata btree file goes below the space already used by that file,
then the hidden_space subtraction can underflow, causing
xfs_dec_fdblocks to subtract a huge amount of space.  We never want the
target to be less than the used sapce, so fix the logic that adjusts
dblocks_avail downwards.

Also fix an error in the adjacent comment.

Cc: stable@vger.kernel.org # v6.15
Fixes: 1df8d75030b787 ("xfs: make metabtree reservations global")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Assisted-by: LOLLM # finding obvious bugs
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
---
 fs/xfs/libxfs/xfs_metafile.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_metafile.c b/fs/xfs/libxfs/xfs_metafile.c
index 71f004e9dc64..1f54d39003c2 100644
--- a/fs/xfs/libxfs/xfs_metafile.c
+++ b/fs/xfs/libxfs/xfs_metafile.c
@@ -297,14 +297,14 @@ xfs_metafile_resv_init(
 		goto out_unlock;
 
 	/*
-	 * Space taken by the per-AG metadata btrees are accounted on-disk as
-	 * used space.  We therefore only hide the space that is reserved but
-	 * not used by the trees.
+	 * Space taken by metadata btrees are accounted on-disk as used space.
+	 * We therefore only hide the space that is reserved but not used by
+	 * the trees.
 	 */
 	if (used > target)
 		target = used;
 	else if (target > dblocks_avail)
-		target = dblocks_avail;
+		target = max(dblocks_avail, used);
 	hidden_space = target - used;
 
 	error = xfs_dec_fdblocks(mp, hidden_space, true);
-- 
2.55.0