Re: dm: use queue_limits_set

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On Mon, May 20, 2024 at 05:44:25PM +0200, Christoph Hellwig wrote:
> 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. 

thinp's discard implementation is constrained by the dm-bio-prison's
constraints.  One of the requirements of dm-bio-prison is that a
discard not exceed BIO_PRISON_MAX_RANGE.

My previous reply is a reasonible way to ensure best effort to respect
a users request but that takes into account the driver provided
discard_granularity.  It'll force suboptimal (too small) discards be
issued but at least they'll cover a full thinp block.
> > 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.

It can handle smaller so long as they respect discard_granularity.

> I'm also kinda curious what actually sets a user limit in Ted's case
> as that feels weird.

I agree, not sure... maybe the fstests is using the knob?

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