Re: [PATCH v2 8/8] mm: Centralize & improve oom reporting in show_mem.c

Tetsuo Handa
2022-04-23T11:49:20Z

On 2022/04/23 10:25, Roman Gushchin wrote:
>>> I agree. However the OOM killer _has_ to make the progress even in such rare
>>> circumstances.
>>
>> Oh, and the concern is allocator recursion? Yeah, that's a good point.
>
> Yes, but not the only problem.
>
>>
>> Do you know if using memalloc_noreclaim_(save|restore) is sufficient for that,
>> or do we want GFP_ATOMIC? I'm already using GFP_ATOMIC for allocations when we
>> generate the report on slabs, since we're taking the slab mutex there.
>
> And this is another problem: grabbing _any_ locks from the oom context is asking
> for trouble: you can potentially enter the oom path doing any allocation, so
> now you have to check that no allocations are ever made holding this lock.
> And I'm not aware of any reasonable way to test it, so most likely it ends up
> introducing some very subtle bags, which will be triggered once a year.
>

You can't allocate memory nor hold locks from OOM context. Since oom_lock mutex
serializes OOM reporting, you could use statically pre-allocated buffer for holding
one line of output. Correlating whole report will be done by the userspace program
with the aid of CONFIG_PRINTK_CALLER=y.