xfs: terminate perag iteration reliably on agcount

Source kernel commit: 8ed004eb9d07a5d6114db3e97a166707c186262d

The for_each_perag_from() iteration macro relies on sb_agcount to
process every perag currently within EOFS from a given starting
point. It's perfectly valid to have perag structures beyond
sb_agcount, however, such as if a growfs is in progress. If a perag
loop happens to race with growfs in this manner, it will actually
attempt to process the post-EOFS perag where ->pag_agno ==
sb_agcount. This is reproduced by xfs/104 and manifests as the
following assert failure in superblock write verifier context:

XFS: Assertion failed: agno < mp->m_sb.sb_agcount, file: fs/xfs/libxfs/xfs_types.c, line: 22

Update the corresponding macro to only process perags that are
within the current sb_agcount.

Fixes: 58d43a7e3263 ("xfs: pass perags around in fsmap data dev functions")
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>

diff --git a/libxfs/xfs_ag.h b/libxfs/xfs_ag.h
index 9cd0669436d5d4909f6b4076109b27739a3e7c75..fae2a38e7fdb8f218c7aa2938b5c9c97d8bcdee4 100644 (file)
--- a/libxfs/xfs_ag.h
+++ b/libxfs/xfs_ag.h
@@ -144,7 +144,7 @@ xfs_perag_next(
                (pag) = xfs_perag_next((pag), &(agno)))
 
 #define for_each_perag_from(mp, agno, pag) \
-       for_each_perag_range((mp), (agno), (mp)->m_sb.sb_agcount, (pag))
+       for_each_perag_range((mp), (agno), (mp)->m_sb.sb_agcount - 1, (pag))
 
 
 #define for_each_perag(mp, agno, pag) \