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

Darrick J. Wong
2021-02-18T04:46:21Z

On Tue, Feb 16, 2021 at 06:55:34AM -0500, Brian Foster wrote:
> 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.

<nod> I'll remove that.

> > /*
> > * 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.

Yes, it does...

> I'm guessing the xfs_sb_version_needsrepair() check above cuts
> off any recursion issues,

...but as soon as we end up in the writeback hook, the b_ops and
XFS_SB_DADDR checks cause the hook to return without doing anything.

> but it might be cleaner to clear the callback pointer first.

We need to block all other writers until /after/ we've written the
primary super, which means that we can't clear the callback until some
time after the bwrite.

>
> > + 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.

Yep, will fix.

--D

> 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
> > */
> >
>