xfs: set buffer ops when repair probes for btree type [Linux 5.0]

xfs: set buffer ops when repair probes for btree type [Linux 5.0]

This Linux kernel change "xfs: set buffer ops when repair probes for btree type" is included in the Linux 5.0 release. This change is authored by Darrick J. Wong <darrick.wong [at] oracle.com> on Sun Feb 3 14:03:59 2019 -0800. The commit for this change in Linux stable tree is add46b3 (patch).

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 <[email protected]>
Reviewed-by: Brian Foster <[email protected]>

There are 27 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 fs/xfs/scrub/repair.c | 11 ++++++++---
 fs/xfs/xfs_buf.c      | 16 ++++++++++++++++
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/scrub/repair.c b/fs/xfs/scrub/repair.c
index 1c8eecf..6acf1bf 100644
--- a/fs/xfs/scrub/repair.c
+++ b/fs/xfs/scrub/repair.c
@@ -768,18 +768,23 @@ struct xrep_findroot {
        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 1f9857e..4f5f2ff 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -776,10 +776,26 @@ struct xfs_buf *
 }

 /*
+ * 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(

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