[PATCH v2 1/7] xfs: set buffer ops when repair probes for btree type

[Brian Foster <bfoster@redhat.com>]

From: "Darrick J. Wong" <darrick.wong@oracle.com>

In xrep_findroot_block, we work out the btree type and correctness of a
given block by calling different btree verifiers on root block
candidates.  However, we leave the NULL b_ops while ->verify_read
validates the block, which means that if the verifier calls
xfs_buf_verifier_error it'll crash on the null b_ops.  Fix it to set
b_ops before calling the verifier and unsetting it if the verifier
fails.

Furthermore, improve the documentation around xfs_buf_ensure_ops, which
is the function that is responsible for cleaning up the b_ops state of
buffers that go through xrep_findroot_block but don't match anything.

[bfoster: Renamed xfs_buf_ensure_ops() and modified comment.]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/xfs/scrub/repair.c  | 11 ++++++++---
 fs/xfs/xfs_buf.c       | 22 ++++++++++++++++------
 fs/xfs/xfs_buf.h       |  2 +-
 fs/xfs/xfs_trans_buf.c |  2 +-
 4 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/fs/xfs/scrub/repair.c b/fs/xfs/scrub/repair.c
index 1c8eecfe52b8..6acf1bfa0bfe 100644
--- a/fs/xfs/scrub/repair.c
+++ b/fs/xfs/scrub/repair.c
@@ -768,18 +768,23 @@ xrep_findroot_block(
 		if (!uuid_equal(&btblock->bb_u.s.bb_uuid,
 				&mp->m_sb.sb_meta_uuid))
 			goto out;
+		/*
+		 * Read verifiers can reference b_ops, so we set the pointer
+		 * here.  If the verifier fails we'll reset the buffer state
+		 * to what it was before we touched the buffer.
+		 */
+		bp->b_ops = fab->buf_ops;
 		fab->buf_ops->verify_read(bp);
 		if (bp->b_error) {
+			bp->b_ops = NULL;
 			bp->b_error = 0;
 			goto out;
 		}
 
 		/*
 		 * Some read verifiers will (re)set b_ops, so we must be
-		 * careful not to blow away any such assignment.
+		 * careful not to change b_ops after running the verifier.
 		 */
-		if (!bp->b_ops)
-			bp->b_ops = fab->buf_ops;
 	}
 
 	/*
diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index eedc5e0156ff..222b5260ed35 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -776,13 +776,23 @@ _xfs_buf_read(
 }
 
 /*
- * If the caller passed in an ops structure and the buffer doesn't have ops
- * assigned, set the ops and use them to verify the contents.  If the contents
- * cannot be verified, we'll clear XBF_DONE.  We assume the buffer has no
- * recorded errors and is already in XBF_DONE state.
+ * Reverify a buffer found in cache without an attached ->b_ops.
+ *
+ * If the caller passed an ops structure and the buffer doesn't have ops
+ * assigned, set the ops and use it to verify the contents. If verification
+ * fails, clear XBF_DONE. We assume the buffer has no recorded errors and is
+ * already in XBF_DONE state on entry.
+ *
+ * Under normal operations, every in-core buffer is verified on read I/O
+ * completion. There are two scenarios that can lead to in-core buffers without
+ * an assigned ->b_ops. The first is during log recovery of buffers on a V4
+ * filesystem, though these buffers are purged at the end of recovery. The
+ * other is online repair, which intentionally reads with a NULL buffer ops to
+ * run several verifiers across an in-core buffer in order to establish buffer
+ * type.
  */
 int
-xfs_buf_ensure_ops(
+xfs_buf_reverify(
 	struct xfs_buf		*bp,
 	const struct xfs_buf_ops *ops)
 {
@@ -824,7 +834,7 @@ xfs_buf_read_map(
 		return bp;
 	}
 
-	xfs_buf_ensure_ops(bp, ops);
+	xfs_buf_reverify(bp, ops);
 
 	if (flags & XBF_ASYNC) {
 		/*
diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
index b9f5511ea998..1c436a85b71d 100644
--- a/fs/xfs/xfs_buf.h
+++ b/fs/xfs/xfs_buf.h
@@ -385,6 +385,6 @@ extern int xfs_setsize_buftarg(xfs_buftarg_t *, unsigned int);
 #define xfs_getsize_buftarg(buftarg)	block_size((buftarg)->bt_bdev)
 #define xfs_readonly_buftarg(buftarg)	bdev_read_only((buftarg)->bt_bdev)
 
-int xfs_buf_ensure_ops(struct xfs_buf *bp, const struct xfs_buf_ops *ops);
+int xfs_buf_reverify(struct xfs_buf *bp, const struct xfs_buf_ops *ops);
 
 #endif	/* __XFS_BUF_H__ */
diff --git a/fs/xfs/xfs_trans_buf.c b/fs/xfs/xfs_trans_buf.c
index 629f1479c9d2..7d65ebf1e847 100644
--- a/fs/xfs/xfs_trans_buf.c
+++ b/fs/xfs/xfs_trans_buf.c
@@ -277,7 +277,7 @@ xfs_trans_read_buf_map(
 		 * release this buffer when it kills the tranaction.
 		 */
 		ASSERT(bp->b_ops != NULL);
-		error = xfs_buf_ensure_ops(bp, ops);
+		error = xfs_buf_reverify(bp, ops);
 		if (error) {
 			xfs_buf_ioerror_alert(bp, __func__);
 
-- 
2.17.2