Re: [patch v2 1/2] mm, mmu_notifier: annotate mmu notifiers with blockable invalidate callbacks

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2017/12/16 1:25, Michal Hocko wrote:
>>  struct mmu_notifier_ops {
>> +	/*
>> +	 * Flags to specify behavior of callbacks for this MMU notifier.
>> +	 * Used to determine which context an operation may be called.
>> +	 *
>> +	 * MMU_INVALIDATE_DOES_NOT_BLOCK: invalidate_{start,end} does not
>> +	 *				  block
>> +	 */
>> +	int flags;
> 
> This should be more specific IMHO. What do you think about the following
> wording?
> 
> invalidate_{start,end,range} doesn't block on any locks which depend
> directly or indirectly (via lock chain or resources e.g. worker context)
> on a memory allocation.

I disagree. It needlessly complicates validating the correctness.

What if the invalidate_{start,end} calls schedule_timeout_idle(10 * HZ) ?
schedule_timeout_idle() will not block on any locks which depend directly or
indirectly on a memory allocation, but we are already blocking other memory
allocating threads at mutex_trylock(&oom_lock) in __alloc_pages_may_oom().

This is essentially same with "sleeping forever due to schedule_timeout_killable(1) by
SCHED_IDLE thread with oom_lock held" versus "looping due to mutex_trylock(&oom_lock)
by all other allocating threads" lockup problem. The OOM reaper does not want to get
blocked for so long.

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