Re: [PATCH memcg v3 2/3] mm, oom: do not trigger out_of_memory from the #PF

[Vasily Averin]

On 23.10.2021 18:01, Tetsuo Handa wrote:
> On 2021/10/23 22:20, Vasily Averin wrote:
>>  /*
>> - * The pagefault handler calls here because it is out of memory, so kill a
>> - * memory-hogging task. If oom_lock is held by somebody else, a parallel oom
>> - * killing is already in progress so do nothing.
>> + * The pagefault handler calls here because some allocation has failed. We have
>> + * to take care of the memcg OOM here because this is the only safe context without
>> + * any locks held but let the oom killer triggered from the allocation context care
>> + * about the global OOM.
>>   */
> 
> Excuse me for a stupid question. I consider
> 
>   if (!mutex_trylock(&oom_lock))
>     return;
>   out_of_memory(&oc);
>   mutex_unlock(&oom_lock);
> 
> here as the last resort (safeguard) when neither __alloc_pages_may_oom()
> nor mem_cgroup_out_of_memory() can make progress. This patch says
> 
>   let the oom killer triggered from the allocation context care
>   about the global OOM.
> 
> but what if the OOM killer cannot be invoked from the allocation context?
> Is there a guarantee that all memory allocations which might result in
> VM_FAULT_OOM can invoke the OOM killer?

I don't think this question is stupid, since I asked it myself :)
You can find this discussion here:
https://lkml.org/lkml/2021/10/21/900

Let me quote it here too
:> 1) VM_FAULT_OOM may be triggered w/o execution of out_of_memory()
:> for exampel it can be caused by incorrect vm fault operations, 
:> (a) which can return this error without calling allocator at all.
:
:I would argue this to be a bug. How can that particular code tell
:whether the system is OOM and the oom killer is the a reasonable measure
:to take?
:
:> (b) or which can provide incorrect gfp flags and allocator can fail without execution of out_of_memory.
:
: I am not sure I can see any sensible scenario where pagefault oom killer
: would be an appropriate fix for that.
:
:> (c) This may happen on stable/LTS kernels when successful allocation was failed by hit into limit of legacy memcg-kmem contoller.
:> We'll drop it in upstream kernels, however how to handle it in old kenrels?
:
:Triggering the global oom killer for legacy kmem charge failure is
:clearly wrong. Removing oom killer from #PF would fix that problem.

I would note: (c) is not theoretical but real life problem, in this case allocation was failed without execution of OOM,
however, it is in this case that execution out_of_memory() from pagefault_out_of_memory() leads to a disaster.

Thank you,
	Vasily Averin