On Mon, May 20, 2024 at 11:39:14AM -0400, Mike Snitzer wrote:
> That's fair. My criticism was more about having to fix up DM targets
> to cope with the new normal of max_discard_sectors being set as a
> function of max_hw_discard_sectors and max_user_discard_sectors.
>
> With stacked devices in particular it is _very_ hard for the user to
> know their exerting control over a max discard limit is correct.
The user forcing a limit is always very sketchy, which is why I'm
not a fan of it.
> Yeah, but my concern is that if a user sets a value that is too low
> it'll break targets like DM thinp (which Ted reported). So forcibly
> setting both to indirectly set the required max_discard_sectors seems
> necessary.
Dm-think requiring a minimum discard size is a rather odd requirement.
Is this just a debug asswert, or is there a real technical reason
for it? If so we can introduce a now to force a minimum size or
disable user setting the value entirely.
> diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
> index 4793ad2aa1f7..c196f39579af 100644
> --- a/drivers/md/dm-thin.c
> +++ b/drivers/md/dm-thin.c
> @@ -4497,7 +4499,8 @@ static void thin_io_hints(struct dm_target *ti, struct
> queue_limits *limits)
>
> if (pool->pf.discard_enabled) {
> limits->discard_granularity = pool->sectors_per_block <<
> SECTOR_SHIFT;
> - limits->max_discard_sectors = pool->sectors_per_block *
> BIO_PRISON_MAX_RANGE;
> + limits->max_hw_discard_sectors =
> limits->max_user_discard_sectors =
> + pool->sectors_per_block * BIO_PRISON_MAX_RANGE;
> }
Drivers really have no business setting max_user_discard_sector,
the whole point of the field is to separate device/driver capabilities
from user policy. So if dm-think really has no way of handling
smaller discards, we need to ensure they can't be set.
I'm also kinda curious what actually sets a user limit in Ted's case
as that feels weird.
