[XFS] In actual allocation of file system blocks and freeing extents, the

transaction within each such operation may involve multiple locking of AGF
buffer. While the freeing extent function has sorted the extents based on
AGF number before entering into transaction, however, when the file system
space is very limited, the allocation of space would try every AGF to get
space allocated, this could potentially cause out-of-order locking, thus
deadlock could happen. This fix mitigates the scarce space for allocation
by setting aside a few blocks without reservation, and avoid deadlock by
maintaining ascending order of AGF locking.

SGI-PV: 947395
SGI-Modid: xfs-linux-melb:xfs-kern:210801a

Signed-off-by: Yingping Lu <yingping@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>

1 files changed, 23 insertions, 6 deletions

diff --git a/fs/xfs/xfs_alloc.c b/fs/xfs/xfs_alloc.c
index 8558226281c4..22af489d3f34 100644
--- a/fs/xfs/xfs_alloc.c
+++ b/fs/xfs/xfs_alloc.c
@@ -1862,7 +1862,7 @@ xfs_alloc_fix_freelist(
 		(pag->pagf_longest - delta) :
 		(pag->pagf_flcount > 0 || pag->pagf_longest > 0);
 	if (args->minlen + args->alignment + args->minalignslop - 1 > longest ||
-	    (args->minleft &&
+	    (!(flags & XFS_ALLOC_FLAG_FREEING) &&
 	     (int)(pag->pagf_freeblks + pag->pagf_flcount -
 		   need - args->total) <
 	     (int)args->minleft)) {
@@ -1898,7 +1898,7 @@ xfs_alloc_fix_freelist(
 	longest = (longest > delta) ? (longest - delta) :
 		(be32_to_cpu(agf->agf_flcount) > 0 || longest > 0);
 	if (args->minlen + args->alignment + args->minalignslop - 1 > longest ||
-	     (args->minleft &&
+	     (!(flags & XFS_ALLOC_FLAG_FREEING) &&
 		(int)(be32_to_cpu(agf->agf_freeblks) +
 		   be32_to_cpu(agf->agf_flcount) - need - args->total) <
 	     (int)args->minleft)) {
@@ -1951,8 +1951,14 @@ xfs_alloc_fix_freelist(
 		 * the restrictions correctly.  Can happen for free calls
 		 * on a completely full ag.
 		 */
-		if (targs.agbno == NULLAGBLOCK)
+		if (targs.agbno == NULLAGBLOCK) {
+			if (!(flags & XFS_ALLOC_FLAG_FREEING)) {
+				xfs_trans_brelse(tp, agflbp);
+				args->agbp = NULL;
+				return 0;
+			}
 			break;
+		}
 		/*
 		 * Put each allocated block on the list.
 		 */
@@ -2360,8 +2366,19 @@ xfs_alloc_vextent(
 			if (args->agno == sagno &&
 			    type == XFS_ALLOCTYPE_START_BNO)
 				args->type = XFS_ALLOCTYPE_THIS_AG;
-			if (++(args->agno) == mp->m_sb.sb_agcount)
-				args->agno = 0;
+			/*
+			* For the first allocation, we can try any AG to get
+			* space.  However, if we already have allocated a
+			* block, we don't want to try AGs whose number is below
+			* sagno. Otherwise, we may end up with out-of-order
+			* locking of AGF, which might cause deadlock.
+			*/
+			if (++(args->agno) == mp->m_sb.sb_agcount) {
+				if (args->firstblock != NULLFSBLOCK)
+					args->agno = sagno;
+				else
+					args->agno = 0;
+			}
 			/*
 			 * Reached the starting a.g., must either be done
 			 * or switch to non-trylock mode.
@@ -2443,7 +2460,7 @@ xfs_free_extent(
 	args.minlen = args.minleft = args.minalignslop = 0;
 	down_read(&args.mp->m_peraglock);
 	args.pag = &args.mp->m_perag[args.agno];
-	if ((error = xfs_alloc_fix_freelist(&args, 0)))
+	if ((error = xfs_alloc_fix_freelist(&args, XFS_ALLOC_FLAG_FREEING)))
 		goto error0;
 #ifdef DEBUG
 	ASSERT(args.agbp != NULL);