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

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

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
> 
>