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

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

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?

--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
> 
>