xfs: simulate per-AG reservations being critically low

Create an error injection point that enables us to simulate being
critically low on per-AG block reservations.  This should enable us to
simulate this specific ENOSPC condition so that we can test falling back
to a regular file copy.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/libxfs/xfs_ag_resv.c

@@ -109,7 +109,9 @@ xfs_ag_resv_critical(
 	trace_xfs_ag_resv_critical(pag, type, avail);
 
 	/* Critically low if less than 10% or max btree height remains. */
-	return avail < orig / 10 || avail < XFS_BTREE_MAXLEVELS;
+	return XFS_TEST_ERROR(avail < orig / 10 || avail < XFS_BTREE_MAXLEVELS,
+			pag->pag_mount, XFS_ERRTAG_AG_RESV_CRITICAL,
+			XFS_RANDOM_AG_RESV_CRITICAL);
 }
 
 /*

fs/xfs/xfs_error.h

@@ -95,7 +95,8 @@ extern void xfs_verifier_error(struct xfs_buf *bp);
 #define XFS_ERRTAG_REFCOUNT_CONTINUE_UPDATE		24
 #define XFS_ERRTAG_REFCOUNT_FINISH_ONE			25
 #define XFS_ERRTAG_BMAP_FINISH_ONE			26
-#define XFS_ERRTAG_MAX					27
+#define XFS_ERRTAG_AG_RESV_CRITICAL			27
+#define XFS_ERRTAG_MAX					28
 
 /*
  * Random factors for above tags, 1 means always, 2 means 1/2 time, etc.
@@ -127,6 +128,7 @@ extern void xfs_verifier_error(struct xfs_buf *bp);
 #define XFS_RANDOM_REFCOUNT_CONTINUE_UPDATE		1
 #define XFS_RANDOM_REFCOUNT_FINISH_ONE			1
 #define XFS_RANDOM_BMAP_FINISH_ONE			1
+#define XFS_RANDOM_AG_RESV_CRITICAL			4
 
 #ifdef DEBUG
 extern int xfs_error_test_active;