xfs: set buffer ops when repair probes for btree type

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.

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

diff --git a/fs/xfs/scrub/repair.c b/fs/xfs/scrub/repair.c
index 1c8eecfe52b85688b48fd2507ceeeee18fe64878..6acf1bfa0bfee57e6d5f105e47797c4126d9d88b 100644 (file)
--- 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 1f9857e3630a52bc3ce597dd3ee63f6d3df05186..4f5f2ff3f70f944130f94a674f09f464d2b6c970 100644 (file)
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -776,10 +776,26 @@ _xfs_buf_read(
 }
 
 /*
+ * Set buffer ops on an unchecked buffer and validate it, if possible.
+ *
  * 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.
+ *
+ * Under normal operations, every in-core buffer must have buffer ops assigned
+ * to them when the buffer is read in from disk so that we can validate the
+ * metadata.
+ *
+ * However, there are two scenarios where one can encounter in-core buffers
+ * that don't have buffer 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 tries to match arbitrary metadata blocks
+ * with btree types in order to find the root.  If online repair doesn't match
+ * the buffer with /any/ btree type, the buffer remains in memory in DONE state
+ * with no ops, and a subsequent read_buf call from elsewhere will not set the
+ * ops.  This function helps us fix this situation.
  */
 int
 xfs_buf_ensure_ops(