Re: [PATCH v4 02/14] xfs: always tail align maxlen allocations

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

On Thu, Aug 29, 2024 at 06:58:02PM +0100, John Garry wrote:
> On 23/08/2024 17:31, Darrick J. Wong wrote:
> 
> sorry for the slow reply...
> 
> > On Tue, Aug 13, 2024 at 04:36:26PM +0000, John Garry wrote:
> > > From: Dave Chinner <dchinner@redhat.com>
> > > > > When we do a large allocation, the core free space allocation code
> > > assumes that args->maxlen is aligned to args->prod/args->mod. hence
> > > if we get a maximum sized extent allocated, it does not do tail
> > > alignment of the extent.
> > > 
> > > However, this assumes that nothing modifies args->maxlen between the
> > > original allocation context setup and trimming the selected free
> > > space extent to size. This assumption has recently been found to be
> > > invalid - xfs_alloc_space_available() modifies args->maxlen in low
> > > space situations - and there may be more situations we haven't yet
> > > found like this.
> > > 
> > > Force aligned allocation introduces the requirement that extents are
> > > correctly tail aligned, resulting in this occasional latent
> > > alignment failure to be reclassified from an unimportant curiousity
> > > to a must-fix bug.
> > > 
> > > Removing the assumption about args->maxlen allocations always being
> > > tail aligned is trivial, and should not impact anything because
> > > args->maxlen for inodes with extent size hints configured are
> > > already aligned. Hence all this change does it avoid weird corner
> > > cases that would have resulted in unaligned extent sizes by always
> > > trimming the extent down to an aligned size.
> > > 
> > > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > > Reviewed-by: "Darrick J. Wong" <djwong@kernel.org> [provisional on v1 series comment]
> > 
> > Still provisional -- neither the original patch author nor the submitter
> > have answered my question from June:
> > 
> > IOWs, we always trim rlen, unless there is no alignment (prod==1) or
> > rlen is less than mod.  For a forcealign file, it should never be the
> > case that minlen < mod because we'll have returned ENOSPC, right?
> 
> For forcealign, mod == 0, so naturally that (minlen < mod) would not happen.
> We want to alloc a multiple of align only, which is in prod.
> 
> If we consider minlen < prod, then that should not happen either as we would
> have returned ENOSPC. In xfs_bmap_select_minlen() we rounddown blen by
> args->alignment, and if that is less than the ap->minlen (1), i.e. if after
> rounddown we have 0, then we return ENOSPC for forcealign. So then minlen
> would not be less than prod after selecting minlen in
> xfs_bmap_select_minlen().
> 
> I hope that I am answering the question asked...

Yep, that satisfies my curiosity!  Thanks for getting back to me,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> 
> Thanks,
> John
> 
> > 
> > --D
> > 
> > > Signed-off-by: John Garry <john.g.garry@oracle.com>
> > > ---
> > >   fs/xfs/libxfs/xfs_alloc.c | 12 +++++-------
> > >   1 file changed, 5 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > > index d559d992c6ef..bf08b9e9d9ac 100644
> > > --- a/fs/xfs/libxfs/xfs_alloc.c
> > > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > > @@ -433,20 +433,18 @@ xfs_alloc_compute_diff(
> > >    * Fix up the length, based on mod and prod.
> > >    * len should be k * prod + mod for some k.
> > >    * If len is too small it is returned unchanged.
> > > - * If len hits maxlen it is left alone.
> > >    */
> > > -STATIC void
> > > +static void
> > >   xfs_alloc_fix_len(
> > > -	xfs_alloc_arg_t	*args)		/* allocation argument structure */
> > > +	struct xfs_alloc_arg	*args)
> > >   {
> > > -	xfs_extlen_t	k;
> > > -	xfs_extlen_t	rlen;
> > > +	xfs_extlen_t		k;
> > > +	xfs_extlen_t		rlen = args->len;
> > >   	ASSERT(args->mod < args->prod);
> > > -	rlen = args->len;
> > >   	ASSERT(rlen >= args->minlen);
> > >   	ASSERT(rlen <= args->maxlen);
> > > -	if (args->prod <= 1 || rlen < args->mod || rlen == args->maxlen ||
> > > +	if (args->prod <= 1 || rlen < args->mod ||
> > >   	    (args->mod == 0 && rlen < args->prod))
> > >   		return;
> > >   	k = rlen % args->prod;
> > > -- 
> > > 2.31.1
> > > 
> > > 
> 
>