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

[Brian Foster <bfoster@redhat.com>]

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
>  	 */
>