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

[Tetsuo Handa]

On 2019/05/21 20:11, Michal Hocko wrote:
> 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
> 

Question to Daniel: Is i915's oom_notifier function atomic?