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xfs: don't let hidden_space go negative in xfs_metafile_resv_init

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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: 1df8d75030 ("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>
This commit is contained in:
Darrick J. Wong 2026-09-14 22:39:10 -07:00 committed by Carlos Maiolino
parent ffb48dccce
commit 476582d754
1 changed files with 4 additions and 4 deletions
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8
fs/xfs/libxfs/xfs_metafile.c
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@ -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);