[PATCH 6.12.y] xfs: reduce new btree slack factor if free space is negative

[Sasha Levin <sashal@kernel.org>]

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

[ Upstream commit 80f13dc7225eec318da7499c7a562d7a34d25e6a ]

LOLLM points out that if the incore fdblocks counter is negative, the
assignment to uint64_t will mask this possibly low space situation and
we don't pack the btree blocks as tightly as we probably ought to.  If
it's the case that there's some free space and fdblocks is too low due
to an error in the superblock field, then we can end up failing btree
rebuilds due to ENOSPC unnecessarily.  This is a corner case, but let's
fix it anyway.

Cc: stable@vger.kernel.org # v6.8
Fixes: be408417630427 ("xfs: implement block reservation accounting for btrees we're staging")
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>
[ sashal: Replaced upstream’s xfs_sum_freecounter() call with percpu_counter_sum_positive() on m_fdblocks to match the existing counter API. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/xfs/scrub/newbt.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/xfs/scrub/newbt.c b/fs/xfs/scrub/newbt.c
index b818d9b215788..80f9bf7e399b4 100644
--- a/fs/xfs/scrub/newbt.c
+++ b/fs/xfs/scrub/newbt.c
@@ -60,7 +60,7 @@ xrep_newbt_estimate_slack(
 		free = sc->sa.pag->pagf_freeblks;
 		sz = xfs_ag_block_count(sc->mp, sc->sa.pag->pag_agno);
 	} else {
-		free = percpu_counter_sum(&sc->mp->m_fdblocks);
+		free = percpu_counter_sum_positive(&sc->mp->m_fdblocks);
 		sz = sc->mp->m_sb.sb_dblocks;
 	}
 
-- 
2.53.0