Re: [PATCH v3 01/14] xfs: only allow minlen allocations when near ENOSPC

[Dave Chinner <david@fromorbit.com>]

On Tue, Aug 06, 2024 at 11:51:38AM -0700, Darrick J. Wong wrote:
> On Thu, Aug 01, 2024 at 04:30:44PM +0000, John Garry wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > When we are near ENOSPC and don't have enough free
> > space for an args->maxlen allocation, xfs_alloc_space_available()
> > will trim args->maxlen to equal the available space. However, this
> > function has only checked that there is enough contiguous free space
> > for an aligned args->minlen allocation to succeed. Hence there is no
> > guarantee that an args->maxlen allocation will succeed, nor that the
> > available space will allow for correct alignment of an args->maxlen
> > allocation.
> > 
> > Further, by trimming args->maxlen arbitrarily, it breaks an
> > assumption made in xfs_alloc_fix_len() that if the caller wants
> > aligned allocation, then args->maxlen will be set to an aligned
> > value. It then skips the tail alignment and so we end up with
> > extents that aren't aligned to extent size hint boundaries as we
> > approach ENOSPC.
> > 
> > To avoid this problem, don't reduce args->maxlen by some random,
> > arbitrary amount. If args->maxlen is too large for the available
> > space, reduce the allocation to a minlen allocation as we know we
> > have contiguous free space available for this to succeed and always
> > be correctly aligned.
> > 
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > Signed-off-by: John Garry <john.g.garry@oracle.com>
> > ---
> >  fs/xfs/libxfs/xfs_alloc.c | 19 ++++++++++++++-----
> >  1 file changed, 14 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > index 59326f84f6a5..d559d992c6ef 100644
> > --- a/fs/xfs/libxfs/xfs_alloc.c
> > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > @@ -2524,14 +2524,23 @@ xfs_alloc_space_available(
> >  	if (available < (int)max(args->total, alloc_len))
> >  		return false;
> >  
> > +	if (flags & XFS_ALLOC_FLAG_CHECK)
> > +		return true;
> > +
> >  	/*
> > -	 * Clamp maxlen to the amount of free space available for the actual
> > -	 * extent allocation.
> > +	 * If we can't do a maxlen allocation, then we must reduce the size of
> > +	 * the allocation to match the available free space. We know how big
> > +	 * the largest contiguous free space we can allocate is, so that's our
> > +	 * upper bound. However, we don't exaclty know what alignment/size
> > +	 * constraints have been placed on the allocation, so we can't
> > +	 * arbitrarily select some new max size. Hence make this a minlen
> > +	 * allocation as we know that will definitely succeed and match the
> > +	 * callers alignment constraints.
> >  	 */
> > -	if (available < (int)args->maxlen && !(flags & XFS_ALLOC_FLAG_CHECK)) {
> > -		args->maxlen = available;
> > +	alloc_len = args->maxlen + (args->alignment - 1) + args->minalignslop;
> > +	if (longest < alloc_len) {
> > +		args->maxlen = args->minlen;
> 
> Same question as the June 21st posting:
> 
> Is it possible to reduce maxlen the largest multiple of the alignment
> that is still less than @longest?

Perhaps.

The comment does say "we don't exaclty know what alignment/size
constraints have been placed on the allocation, so we can't
arbitrarily select some new max size."

Given this unknown I simply punted the issue and went straight to
selecting a size the caller has guaranteed will be valid for their
constraints.

-Dave.

-- 
Dave Chinner
david@fromorbit.com