Re: [PATCH 2/2] mm, oom_reaper: do not mmput synchronously from the oom reaper context

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

Michal Hocko wrote:
> Tetsuo has properly noted that mmput slow path might get blocked waiting
> for another party (e.g. exit_aio waits for an IO). If that happens the
> oom_reaper would be put out of the way and will not be able to process
> next oom victim. We should strive for making this context as reliable
> and independent on other subsystems as much as possible.
> 
> Introduce mmput_async which will perform the slow path from an async
> (WQ) context. This will delay the operation but that shouldn't be a
> problem because the oom_reaper has reclaimed the victim's address space
> for most cases as much as possible and the remaining context shouldn't
> bind too much memory anymore. The only exception is when mmap_sem
> trylock has failed which shouldn't happen too often.
> 
> The issue is only theoretical but not impossible.

Just a random thought, but after this patch is applied, do we still need to use
a dedicated kernel thread for OOM-reap operation? If I recall correctly, the
reason we decided to use a dedicated kernel thread was that calling
down_read(&mm->mmap_sem) / mmput() from the OOM killer context is unsafe due to
dependency. By replacing mmput() with mmput_async(), since __oom_reap_task() will
no longer do operations that might block, can't we try OOM-reap operation from
current thread which called mark_oom_victim() or oom_scan_process_thread() ?
I want to start waking up the OOM reaper whenever TIF_MEMDIE is set or found.

Using a dedicated kernel thread is still better because memory allocation path
already consumed a lot of kernel stack? But we don't need to give up OOM-reaping
when kthread_run() failed.

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