On Fri, Jun 21, 2024 at 10:05:29AM +0000, John Garry wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> 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 e reclassified from an unimportant curiousity
to be
> 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.
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?
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
If the answer is 'yes' and the typo gets fixed,
Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
--D
> ---
> 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 5855a21d4864..32f72217c126 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -432,20 +432,18 @@ 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
>
>
