[PATCH 2/2] libxfs: clear stale buffer errors on write

[Dave Chinner <david@xxxxxxxxxxxxx>]

From: Dave Chinner <dchinner@xxxxxxxxxx>

If we've read a buffer and it's had an error (e.g a bad CRC) and the
caller corrects the problem with the buffer and writes it via
libxfs_writebuf() without clearing the error on the buffer,
subsequent reads of the buffer while it is still in cache can see
that error and fail inappropriately.

xfs/033 demonstrates this error, where phase 3 detects the corrupted
root inode and clears, but doesn't clear the b_error field. Later in
phase 6, the code that rebuilds the root directory tries to read the
root inode and sees a buffer with an error on it, thereby triggering
a fatal repair failure:

Phase 3 - for each AG...
        - scan and clear agi unlinked lists...
        - process known inodes and perform inode discovery...
        - agno = 0
xfs_inode_buf_verify: XFS_CORRUPTION_ERROR
bad magic number 0x0 on inode 64
....
cleared root inode 64
....
Phase 6 - check inode connectivity...
reinitializing root directory
xfs_imap_to_bp: xfs_trans_read_buf() returned error 117.

fatal error -- could not iget root inode -- error - 117
#

Fix this by assuming buffers that are written are clean and correct
and hence we can zero the b_error field before retiring the buffer
to the cache.

Reported-by: Eric Sandeen <esandeen@xxxxxxxxxx>
Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 libxfs/rdwr.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/libxfs/rdwr.c b/libxfs/rdwr.c
index 78a9b37..d0ff15b 100644
--- a/libxfs/rdwr.c
+++ b/libxfs/rdwr.c
@@ -890,6 +890,11 @@ libxfs_writebufr(xfs_buf_t *bp)
 int
 libxfs_writebuf_int(xfs_buf_t *bp, int flags)
 {
+       /*
+        * Clear any error hanging over from reading the buffer. This prevents
+        * subsequent reads after this write from seeing stale errors.
+        */
+       bp->b_error = 0;
        bp->b_flags |= (LIBXFS_B_DIRTY | flags);
        return 0;
 }
@@ -903,6 +908,11 @@ libxfs_writebuf(xfs_buf_t *bp, int flags)
                        (long long)LIBXFS_BBTOOFF64(bp->b_bn),
                        (long long)bp->b_bn);
 #endif
+       /*
+        * Clear any error hanging over from reading the buffer. This prevents
+        * subsequent reads after this write from seeing stale errors.
+        */
+       bp->b_error = 0;
        bp->b_flags |= (LIBXFS_B_DIRTY | flags);
        libxfs_putbuf(bp);
        return 0;
-- 
1.8.4.rc3