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

Eric Sandeen
2014-02-20T19:09:23Z

On 2/19/14, 11:55 PM, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> 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.

Thanks;

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

> Reported-by: Eric Sandeen <esandeen@redhat.com>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> 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;
>