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

[Tetsuo Handa]

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...