[Intel-gfx] [PATCH] kernel.h: Add non_block_start/end()

[Michal Hocko]

On Tue 21-05-19 20:04:34, Tetsuo Handa wrote:
> On 2019/05/21 19:51, Michal Hocko wrote:
> > On Tue 21-05-19 19:44:01, Tetsuo Handa wrote:
> >> On 2019/05/21 19:06, Daniel Vetter wrote:
> >>> In some special cases we must not block, but there's not a
> >>> spinlock, preempt-off, irqs-off or similar critical section already
> >>> that arms the might_sleep() debug checks. Add a non_block_start/end()
> >>> pair to annotate these.
> >>>
> >>> This will be used in the oom paths of mmu-notifiers, where blocking is
> >>> not allowed to make sure there's forward progress. Quoting Michal:
> >>>
> >>> "The notifier is called from quite a restricted context - oom_reaper -
> >>> which shouldn't depend on any locks or sleepable conditionals. The code
> >>> should be swift as well but we mostly do care about it to make a forward
> >>> progress. Checking for sleepable context is the best thing we could come
> >>> up with that would describe these demands at least partially."
> >>>
> >>
> >> Can this be checked for OOM notifier as well?
> >>
> >>  	if (!is_memcg_oom(oc)) {
> >> +		non_block_start();
> >>  		blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
> >> +		non_block_end();
> >>  		if (freed > 0)
> >>  			/* Got some memory back in the last second. */
> >>  			return true;
> >>  	}
> >>
> >> It is not clear whether i915's oom_notifier function has such dependency.
> > 
> > It is not but then we should be using the non-blocking API if this is
> > a real problem. The above code just doesn't make any sense. We have a
> > blocking API called and wrapped by non-blocking one.
> 
> OOM notifiers should not depend on any locks or sleepable conditionals.
> If some lock directly or indirectly depended on __GFP_DIRECT_RECLAIM,
> it will deadlock. Thus, despite blocking API, this should effectively be
> non-blocking. All OOM notifier users except i915 seems to be atomic, but
> I can't evaluate i915 part...

Read again what I've written, please
-- 
Michal Hocko
SUSE Labs