Re: [PATCH v3] xfs: deprecate the V4 format

[Christoph Hellwig <hch@infradead.org>]

On Fri, Sep 11, 2020 at 09:43:11AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> The V4 filesystem format contains known weaknesses in the on-disk format
> that make metadata verification diffiult.  In addition, the format will
> does not support dates past 2038 and will not be upgraded to do so.
> Therefore, we should start the process of retiring the old format to
> close off attack surfaces and to encourage users to migrate onto V5.
> 
> Therefore, make XFS V4 support a configurable option.  For the first
> period it will be default Y in case some distributors want to withdraw
> support early; for the second period it will be default N so that anyone
> who wishes to continue support can do so; and after that, support will
> be removed from the kernel.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> v3: be a little more helpful about old xfsprogs and warn more loudly
> about deprecation
> v2: define what is a V4 filesystem, update the administrator guide

Whie this patch itself looks good, I think the ifdef as is is rather
silly as it just prevents mounting v4 file systems without reaping any
benefits from that.

So at very least we should add a little helper like this:

static inline bool xfs_sb_is_v4(truct xfs_sb *sbp)
{
	if (IS_ENABLED(CONFIG_XFS_SUPPORT_V4))
		return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4;
	return false;
}

and use it in all the feature test macros to let the compile eliminate
all the dead code.