Re: [PATCH 07/11] xfs_repair: force not-so-bad bmbt blocks back through the verifier

Dave Chinner
2015-08-26T00:55:02Z

On Tue, Aug 25, 2015 at 05:33:05PM -0700, Darrick J. Wong wrote:
> If during prefetch we encounter a bmbt block that fails the CRC check
> due to corruption in the unused part of the block, force the buffer
> back through the non-prefetch verifiers later so that the CRC is
> updated. Otherwise, the bad checksum goes unfixed and the kernel will
> still flag the bmbt block as invalid.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> repair/prefetch.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
>
> diff --git a/repair/prefetch.c b/repair/prefetch.c
> index 1de3ec0..77d29c8 100644
> --- a/repair/prefetch.c
> +++ b/repair/prefetch.c
> @@ -276,6 +276,14 @@ pf_scan_lbtree(
>
> XFS_BUF_SET_PRIORITY(bp, isadir ? B_DIR_BMAP : B_BMAP);
>
> + /*
> + * Make this bmbt buffer go back through the verifiers later so that
> + * we correct checksum errors stemming from bitflips in the unused
> + * parts of the bmbt block.
> + */
> + if (bp->b_error == -EFSBADCRC)
> + bp->b_flags |= LIBXFS_B_UNCHECKED;

This is because the next read of the buffer clears bp->b_error,
right?

So, while I think this is necessary, I also think the prefetch on
this btree should stop as we can't trust the contents of the buffer
to be correct. Hence I'd suggest that:

/*
* If the verfier flagged a problem with the buffer, we
* can't trust it's contents for the purposes of readahead.
* Stop prefetching the tree, and mark this buffer as
* unchecked so that the next read of the buffer by the
* repair code will retain the error status and hence be
* acted on appropriately.
*/
if (bp->b_error) {
bp->b_flags |= LIBXFS_B_UNCHECKED;
libxfs_putbuf(bp);
return 0;
}

Cheers,

Dave.

--
Dave Chinner
david@fromorbit.com