Re: [PATCH v5] xfs: validate log record version against superblock log version

[Christoph Hellwig <hch@infradead.org>]

> Hrmm maybe we ought to reserve XLOG_VERSION==0x3 so that whenever we do
> log v3 we don't accidentally write logs with bits that won't be
> validated quite right on old kernels?

Why do we need to reserve that?  The code checks for either 1 or 2
right now based on the log feature flag.  If we add a v3 log we'll
have to ammend this, but reservations won't help with that.

> > +	if (xfs_has_logv2(mp)) {
> > +		if (XFS_IS_CORRUPT(mp, h_version != XLOG_VERSION_2))
> 
> Being pedantic here, but the kernel cpu_to_be32 wrappers are magic in
> that they compile to byteswapped constants so you can avoid the runtime
> overhead of byteswapping rhead->h_version by doing:
> 
> 	if (XFS_IS_CORRUPT(mp,
> 	    rhead->h_version != cpu_to_be32(XLOG_VERSION_2)))
> 		return -EFSCORRUPTED;
> 
> But seeing as this is log validation for recovery, I think the
> performance implications are vanishingly small.

Yes.