Re: [PATCH v5 4/7] xfs: Support FS_XFLAG_ATOMICWRITES for forcealign

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Aug 23, 2024 at 09:39:44AM +0100, John Garry wrote:
> On 22/08/2024 21:38, Darrick J. Wong wrote:
> > > > This (atomicwrites && !forcealign) ought to be checked in the superblock
> > > > verifier.
> > > You mean in xfs_fs_validate_params(), right?
> > xfs_validate_sb_common, where we do all the other ondisk superblock
> > validation.
> 
> I don't see any other xfs_has_XXX checks in xfs_validate_sb_common(), but
> this could be the first...

The superblock verifier runs at a lower level in the filesystem -- it
checks that the ondisk superblock doesn't contain any inconsistent
fields or impossible feature combinations, etc.  Once the ondisk
superblock is verified, the information there is used to set XFS_FEAT_*
bits in m_features, which is what the xfs_has_* predicates access.

Therefore, you have to look at the raw superblock fields, not the
xfs_has_ predicates:

	if ((sbp->sb_features_ro_compat & XFS_SB_FEAT_RO_COMPAT_ATOMICWRITES) &&
	    !(sbp->sb_features_ro_compat & XFS_SB_FEAT_RO_COMPAT_FORCEALIGN)) {
		xfs_warn(mp, "atomic writes feature requires force align feature.");
		return -EINVAL;
	}

The reason for checking this state here is that atomicwrites absolutely
requires forcealign and that dependency will always be true.

> The only other place in which I see a pattern of similar SB feature flag
> checks is in xfs_finish_flags() for checking xfs_has_crc() &&
> xfs_has_noattr2().
> 
> So if we go with xfs_validate_sb_common(), then should the check in
> xfs_fs_fill_super() for xfs_has_forcealign() && xfs_has_realtime()/reflink()
> be relocated to xfs_validate_sb_common() also:

No.  Contrast the above with (forcealign && !realtime), which at least
in theory is temporary, so that should live in xfs_fs_fill_super.  Or
put another way, xfs_fs_fill_super is where we screen out the kernel
being too stupid to support something it found on disk.

--D

> 
> https://lore.kernel.org/linux-xfs/20240813163638.3751939-8-john.g.garry@oracle.com/
> 
> Cheers,
> John
>