[dm-devel] fstrim on raid1 LV with writemostly PV leads to system freeze

[Kirill Kirilenko]

On 25.09.2023 05:58 +0300, Yu Kuai wrote:
> Roman and Kirill, can you test the following patch?
>
> Thanks,
> Kuai
>
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index 4b30a1742162..4963f864ef99 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))) {

Thank you. I can confirm, that your patch eliminates freezes during
'fstrim' execution. Tested on kernel 6.5.0.
Still 'fstrim' takes more than 2 minutes, but I believe it's normal to a
file system with 1M+ inodes.

Probably I'm wrong here, but to me this doesn't look like a solution,
more like a masking the real problem.
Even with TRIM operations split in 1MB pieces, I don't expect kernel to
freeze.