xfs: buffer lru reference count error injection tag

XFS uses a fixed reference count for certain types of buffers in the
internal LRU cache. These reference counts dictate how aggressively
certain buffers are reclaimed vs. others. While the reference counts
implements priority across different buffer types, all buffers
(other than uncached buffers) are typically cached for at least one
reclaim cycle.

We've had at least one bug recently that has been hidden by a
released buffer sitting around in the LRU. Users hitting the problem
were able to reproduce under enough memory pressure to cause
aggressive reclaim in a particular window of time.

To support future xfstests cases, add an error injection tag to
hardcode the buffer reference count to zero. When enabled, this
bypasses caching of associated buffers and facilitates test cases
that depend on this behavior.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 2f97c12ca75e..d481dd2b29a6 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -42,6 +42,7 @@
 #include "xfs_mount.h"
 #include "xfs_trace.h"
 #include "xfs_log.h"
+#include "xfs_error.h"
 
 static kmem_zone_t *xfs_buf_zone;
 
@@ -2129,3 +2130,18 @@ xfs_buf_terminate(void)
 {
 	kmem_zone_destroy(xfs_buf_zone);
 }
+
+void xfs_buf_set_ref(struct xfs_buf *bp, int lru_ref)
+{
+	struct xfs_mount	*mp = bp->b_target->bt_mount;
+
+	/*
+	 * Set the lru reference count to 0 based on the error injection tag.
+	 * This allows userspace to disrupt buffer caching for debug/testing
+	 * purposes.
+	 */
+	if (XFS_TEST_ERROR(false, mp, XFS_ERRTAG_BUF_LRU_REF))
+		lru_ref = 0;
+
+	atomic_set(&bp->b_lru_ref, lru_ref);
+}