Re: [PATCH -next] md/raid1: don't split discard io for write behind

[Song Liu]

On Fri, Oct 6, 2023 at 8:24 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Currently, discad io is treated the same as normal write io, and for
> write behind case, io size is limited to:
>
> BIO_MAX_VECS * (PAGE_SIZE >> 9)
>
> For 0.5KB sector size and 4KB PAGE_SIZE, this is just 1MB. For
> consequence, if 'WriteMostly' is set to one of the underlying disks,
> then diskcard io will be splited into 1MB and it will take a long time
> for the diskcard to finish.
>
> Fix this problem by disable write behind for discard io.
>
> Reported-by: Roman Mamedov <rm@xxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/6a1165f7-c792-c054-b8f0-1ad4f7b8ae01@xxxxxxxxxxxxxx/
> Reported-and-tested-by: Kirill Kirilenko <kirill@xxxxxxxxxxxxxx>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>

Applied to md-next. Thanks!

Song

> ---
>  drivers/md/raid1.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index 3a78f79ee6d5..35d12948e0a9 100644
> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -1345,6 +1345,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
>         int first_clone;
>         int max_sectors;
>         bool write_behind = false;
> +       bool is_discard = (bio_op(bio) == REQ_OP_DISCARD);
>
>         if (mddev_is_clustered(mddev) &&
>              md_cluster_ops->area_resyncing(mddev, WRITE,
> @@ -1405,7 +1406,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
>                  * write-mostly, which means we could allocate write behind
>                  * bio later.
>                  */
> -               if (rdev && test_bit(WriteMostly, &rdev->flags))
> +               if (!is_discard && rdev && test_bit(WriteMostly, &rdev->flags))
>                         write_behind = true;
>
>                 if (rdev && unlikely(test_bit(Blocked, &rdev->flags))) {
> --
> 2.39.2
>