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

Darrick J. Wong
2024-08-06T18:51:38Z

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?

--D

> ASSERT(args->maxlen > 0);
> - ASSERT(args->maxlen >= args->minlen);
> }
>
> return true;
> --
> 2.31.1
>
>