Re: [PATC] block: update queue limits atomically

[Ming Lei <ming.lei@redhat.com>]

On Wed, Mar 19, 2025 at 10:18:39PM +0100, Mikulas Patocka wrote:
> 
> 
> On Tue, 18 Mar 2025, Jens Axboe wrote:
> 
> > > Yeah, it looks fine, but I feel it is still fragile, and not sure it is one
> > > accepted solution.
> > 
> > Agree - it'd be much better to have the bio drivers provide the same
> > guarantees that we get on the request side, rather than play games with
> > this and pretend that concurrent update and usage is fine.
> > 
> > -- 
> > Jens Axboe
> 
> And what mechanism should they use to read the queue limits?
> * locking? (would degrade performance)
> * percpu-rwsem? (no overhead for readers, writers wait for the RCU 
>   synchronization)
> * RCU?
> * anything else?

1) queue usage counter is for covering fast IO code path

- in __submit_bio(), queue usage counter is grabbed when calling
  ->submit_bio()

- the only trouble should be from dm-crypt or thin-provision which offloads
bio submission to other context, so you can grab the usage counter by
percpu_ref_get(&q->q_usage_counter) until this bio submission or queue
limit consumption is done

2) slow path: dm_set_device_limits

which is done before DM disk is on, so it should be fine by holding limit lock.

3) changing queue limits from bio->end_io() or request completion handler

- this usage need fix



thanks,
Ming