Re: [PATCH 1/3] xfs_repair: set NEEDSREPAIR the first time we write to a filesystem

Brian Foster
2021-02-16T11:57:27Z

On Fri, Feb 12, 2021 at 09:46:50PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
>
> Add a hook to the buffer cache so that xfs_repair can intercept the
> first write to a V5 filesystem to set the NEEDSREPAIR flag. In the
> event that xfs_repair dirties the filesystem and goes down, this ensures
> that the sysadmin will have to re-start repair before mounting.
>
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
> include/xfs_mount.h | 4 ++
> libxfs/rdwr.c | 4 ++
> repair/xfs_repair.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 103 insertions(+)
>
>
...
> diff --git a/repair/xfs_repair.c b/repair/xfs_repair.c
> index 90d1a95a..12e319ae 100644
> --- a/repair/xfs_repair.c
> +++ b/repair/xfs_repair.c
> @@ -720,6 +720,8 @@ clear_needsrepair(
> struct xfs_buf *bp;
> int error;
>
> + mp->m_buf_writeback_fn = NULL;
> +

Slight impedence mismatch in that we set the callback unconditionally
(assuming crc=1) but we only get to this call if needsrepair is set.

> /*
> * If we're going to clear NEEDSREPAIR, we need to make absolutely sure
> * that everything is ok with the ondisk filesystem. Make sure any
> @@ -751,6 +753,95 @@ clear_needsrepair(
> libxfs_buf_relse(bp);
> }
>
> +static void
> +update_sb_crc_only(
> + struct xfs_buf *bp)
> +{
> + xfs_buf_update_cksum(bp, XFS_SB_CRC_OFF);
> +}
> +
> +/* Forcibly write the primary superblock with the NEEDSREPAIR flag set. */
> +static void
> +force_needsrepair(
> + struct xfs_mount *mp)
> +{
> + struct xfs_buf_ops fake_ops;
> + struct xfs_buf *bp;
> + int error;
> +
> + if (!xfs_sb_version_hascrc(&mp->m_sb) ||
> + xfs_sb_version_needsrepair(&mp->m_sb))
> + return;
> +
> + bp = libxfs_getsb(mp);
> + if (!bp || bp->b_error) {
> + do_log(
> + _("couldn't get superblock to set needsrepair, err=%d\n"),
> + bp ? bp->b_error : ENOMEM);
> + return;
> + } else {
> + /*
> + * It's possible that we need to set NEEDSREPAIR before we've
> + * had a chance to fix the summary counters in the primary sb.
> + * With the exception of those counters, phase 1 already
> + * ensured that the geometry makes sense.
> + *
> + * Bad summary counters in the primary super can cause the
> + * write verifier to fail, so substitute a dummy that only sets
> + * the CRC. In the event of a crash, NEEDSREPAIR will prevent
> + * the kernel from mounting our potentially damaged filesystem
> + * until repair is run again, so it's ok to bypass the usual
> + * verification in this one case.
> + */
> + fake_ops = xfs_sb_buf_ops; /* struct copy */
> + fake_ops.verify_write = update_sb_crc_only;
> +
> + mp->m_sb.sb_features_incompat |=
> + XFS_SB_FEAT_INCOMPAT_NEEDSREPAIR;
> + libxfs_sb_to_disk(bp->b_addr, &mp->m_sb);
> +
> + /* Force the primary super to disk immediately. */
> + bp->b_ops = &fake_ops;
> + error = -libxfs_bwrite(bp);

This looks like it calls back into the writeback hook before it's been
cleared. I'm guessing the xfs_sb_version_needsrepair() check above cuts
off any recursion issues, but it might be cleaner to clear the callback
pointer first.

> + bp->b_ops = &xfs_sb_buf_ops;
> + if (error)
> + do_log(_("couldn't force needsrepair, err=%d\n"), error);
> + }
> + if (bp)
> + libxfs_buf_relse(bp);

Doesn't appear we can get here with bp == NULL. Otherwise the rest looks
reasonable to me.

Brian

> +}
> +
> +/* Intercept the first write to the filesystem so we can set NEEDSREPAIR. */
> +static void
> +repair_capture_writeback(
> + struct xfs_buf *bp)
> +{
> + struct xfs_mount *mp = bp->b_mount;
> + static pthread_mutex_t wb_mutex = PTHREAD_MUTEX_INITIALIZER;
> +
> + /* Higher level code modifying a superblock must set NEEDSREPAIR. */
> + if (bp->b_ops == &xfs_sb_buf_ops || bp->b_bn == XFS_SB_DADDR)
> + return;
> +
> + pthread_mutex_lock(&wb_mutex);
> +
> + /*
> + * If someone else already dropped the hook, then needsrepair has
> + * already been set on the filesystem and we can unlock.
> + */
> + if (mp->m_buf_writeback_fn != repair_capture_writeback)
> + goto unlock;
> +
> + /*
> + * If we get here, the buffer being written is not a superblock, and
> + * needsrepair needs to be set.
> + */
> + force_needsrepair(mp);
> + mp->m_buf_writeback_fn = NULL;
> +unlock:
> + pthread_mutex_unlock(&wb_mutex);
> +}
> +
> int
> main(int argc, char **argv)
> {
> @@ -847,6 +938,10 @@ main(int argc, char **argv)
> if (verbose > 2)
> mp->m_flags |= LIBXFS_MOUNT_WANT_CORRUPTED;
>
> + /* Capture the first writeback so that we can set needsrepair. */
> + if (xfs_sb_version_hascrc(&mp->m_sb))
> + mp->m_buf_writeback_fn = repair_capture_writeback;
> +
> /*
> * set XFS-independent status vars from the mount/sb structure
> */
>