Re: [PATCH 09/10] xfs: validate feature support when recovering rmap/refcount/bmap intents

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Dec 04, 2020 at 09:00:36AM -0500, Brian Foster wrote:
> On Thu, Dec 03, 2020 at 05:12:30PM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > The bmap, rmap, and refcount log intent items were added to support the
> > rmap and reflink features.  Because these features come with changes to
> > the ondisk format, the log items aren't tied to a log incompat flag.
> > 
> > However, the log recovery routines don't actually check for those
> > feature flags.  The kernel has no business replayng an intent item for a
> > feature that isn't enabled, so check that as part of recovered log item
> > validation.  (Note that kernels pre-dating rmap and reflink will fail
> > the mount on the unknown log item type code.)
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/xfs/xfs_bmap_item.c     |    4 ++++
> >  fs/xfs/xfs_refcount_item.c |    3 +++
> >  fs/xfs/xfs_rmap_item.c     |    3 +++
> >  3 files changed, 10 insertions(+)
> > 
> > 
> > diff --git a/fs/xfs/xfs_bmap_item.c b/fs/xfs/xfs_bmap_item.c
> > index 78346d47564b..4ea9132716c6 100644
> > --- a/fs/xfs/xfs_bmap_item.c
> > +++ b/fs/xfs/xfs_bmap_item.c
> > @@ -425,6 +425,10 @@ xfs_bui_validate(
> >  {
> >  	struct xfs_map_extent		*bmap;
> >  
> > +	if (!xfs_sb_version_hasrmapbt(&mp->m_sb) &&
> > +	    !xfs_sb_version_hasreflink(&mp->m_sb))
> > +		return false;
> > +
> 
> Took me a minute to realize we use the map/unmap for extent swap if rmap
> is enabled. That does make me wonder a bit.. had we made this kind of
> recovery feature validation change before that came around (such that we
> probably would have only checked _hasreflink() here), would we have
> created an unnecessary backwards incompatibility?

Yes.

I confess to cheating a little here -- technically the bmap intents were
introduced with reflink in 4.9, whereas rmap was introduced in 4.8.  The
proper solution is probably to introduce a new log incompat bit for bmap
intents when reflink isn't enabled, but TBH there were enough other rmap
bugs in 4.8 (not to mention the EXPERIMENTAL warning) that nobody should
be running that old of a kernel on a production system.

(Also we don't enable rmap by default yet whereas reflink has been
enabled by default since 4.18, so the number of people affected probably
isn't very high...)

Secondary question: should we patch 4.9 and 4.14 to disable rmap and
reflink support, since they both still have EXPERIMENTAL warnings?

--D

> Brian
> 
> >  	/* Only one mapping operation per BUI... */
> >  	if (buip->bui_format.bui_nextents != XFS_BUI_MAX_FAST_EXTENTS)
> >  		return false;
> > diff --git a/fs/xfs/xfs_refcount_item.c b/fs/xfs/xfs_refcount_item.c
> > index 8ad6c81f6d8f..2b28f5643c0b 100644
> > --- a/fs/xfs/xfs_refcount_item.c
> > +++ b/fs/xfs/xfs_refcount_item.c
> > @@ -423,6 +423,9 @@ xfs_cui_validate_phys(
> >  	struct xfs_mount		*mp,
> >  	struct xfs_phys_extent		*refc)
> >  {
> > +	if (!xfs_sb_version_hasreflink(&mp->m_sb))
> > +		return false;
> > +
> >  	if (refc->pe_flags & ~XFS_REFCOUNT_EXTENT_FLAGS)
> >  		return false;
> >  
> > diff --git a/fs/xfs/xfs_rmap_item.c b/fs/xfs/xfs_rmap_item.c
> > index f296ec349936..2628bc0080fe 100644
> > --- a/fs/xfs/xfs_rmap_item.c
> > +++ b/fs/xfs/xfs_rmap_item.c
> > @@ -466,6 +466,9 @@ xfs_rui_validate_map(
> >  	struct xfs_mount		*mp,
> >  	struct xfs_map_extent		*rmap)
> >  {
> > +	if (!xfs_sb_version_hasrmapbt(&mp->m_sb))
> > +		return false;
> > +
> >  	if (rmap->me_flags & ~XFS_RMAP_EXTENT_FLAGS)
> >  		return false;
> >  
> > 
>