Re: fail_last_dev and FailFast/LastDev flag incompatibility

[Xiao Ni <xni@redhat.com>]

Hi Marisuz

We don't need to consider MD_FAILFAST for raid456. Because only raid1
and raid10 support it.
MD_FAILFAST_SUPPORTED is only set in raid1_run/raid10_run. So LastDev
only be useful for
raid1/raid10. It should be good to only check Faulty here.

Best Regards
Xiao

On Wed, Feb 9, 2022 at 5:40 PM Mariusz Tkaczyk
<mariusz.tkaczyk@linux.intel.com> wrote:
>
> Hi All,
> During my work under failed arrays handling[1] improvements, I
> discovered potential issue with "failfast" and metadata writes. In
> commit message[2] Neil mentioned that:
> "If we get a failure writing metadata but the device doesn't
> fail, it must be the last device so we re-write without
> FAILFAST".
>
> Obviously, this is not true for RAID456 (again)[1] but it is also not
> true for RAID1 and RAID10 with "fail_las_dev"[3] functionality enabled.
>
> I did a quick check and can see that setter for "LastDev" flag is
> called if "Faulty" on device is not set. I proposed some changes in the
> area in my patchset[4] but after discussion we decided to drop changes
> here. Current approach is not correct for all branches, so my proposal
> is to change:
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 7b024912f1eb..3daec14ef6b2 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -931,7 +931,7 @@ static void super_written(struct bio *bio)
>                 pr_err("md: %s gets error=%d\n", __func__,
>                        blk_status_to_errno(bio->bi_status));
>                 md_error(mddev, rdev);
> -               if (!test_bit(Faulty, &rdev->flags)
> +               if (test_bit(MD_BROKEN, mddev->flag)
>                     && (bio->bi_opf & MD_FAILFAST)) {
>                         set_bit(MD_SB_NEED_REWRITE, &mddev->sb_flags);
>                         set_bit(LastDev, &rdev->flags);
>
>
> It will force "LastDev" to be set on every metadata rewrite if mddevice
> is known to be failed.
> Do you have any other suggestions?
>
> + Guoqing - author of fail_last_dev.
> + Xiao - you are familiarized with FailFast so please take a look.
>
> [1]https://lore.kernel.org/linux-raid/CAPhsuW54_9CTR6sh7mnQ6O77F2HNArLHGWHYsUdbNGy7pXgipQ@mail.gmail.com/T/#m8cf7c57429b6fd332220157186151900ce23865d
> [2]https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit/?id=46533ff7fefb7e9e3539494f5873b00091caa8eb
> [3]https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit/?id=9a567843f7ce
> [4]https://lore.kernel.org/linux-raid/CAPhsuW5bV+Bz=Od9jomNHoedaEMFAXymN11J80G62GVPwSp41g@mail.gmail.com/
>
> Thanks,
> Mariusz
>