Re: [PATCH] dm-raid: stack limits instead of overwriting them.

[Mikulas Patocka <mpatocka@redhat.com>]

On Thu, 24 Sep 2020, John Dorminy wrote:

> I don't understand how this works...
> 
> Can chunk_size_bytes be 0? If not, how is discard_granularity being set to 0?

Yes - chunk_size_bytes is 0 here.

> I think also limits is local to the ti in question here, initialized
> by blk_set_stacking_limits() via dm-table.c, and therefore has only
> default values and not anything to do with the underlying queue. So

In dm_calculate_queue_limits, there's
	blk_set_stacking_limits(limits);
- that will initialize the default limits (discard_granularity == 0)

Then, there's:
	ti->type->iterate_devices(ti, dm_set_device_limits, &ti_limits);
- that will unify limits for all RAID legs (in this case, it sets 
discard_granularity == 1024)

Then, there's
	if (ti->type->io_hints) ti->type->io_hints(ti, &ti_limits);
And that will incorrectly overwrite discard_granularity with zero.

> setting discard_granularity=max(discard_granularity, chunk_size_bytes)
> doesn't seem like it should be working, unless I'm not understanding
> what it's there for...
> 
> And shouldn't melding in the target's desired io_hints into the
> existing queue limits be happening in blk_stack_limits() instead?
> (Also, it does lcm_not_zero() for stacking granularity, instead of
> max()...)

Well, you can add blk_stack_limits to raid_io_hints and use it to stack 
the limits, but that would just complicate that function.

Mikulas