Re: [PATCH v3 02/14] xfs: always tail align maxlen allocations

John Garry
2024-08-13T15:01:50Z

On 01/08/2024 17:30, 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]

Note that I marked this as provisional, as not all questions were
answered from v1:
https://lore.kernel.org/linux-xfs/20240621195058.GS3058325@frogsfrogsfrogs/

I plan on sending v4 today like this, and hope to clear up any
unanswered questions then.

> 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;