Re: [PATCH] mm, oom: remove oom_lock from oom_reaper

[David Rientjes <rientjes@google.com>]

On Thu, 19 Jul 2018, Michal Hocko wrote:

> From: Michal Hocko <mhocko@suse.com>
> 
> oom_reaper used to rely on the oom_lock since e2fe14564d33 ("oom_reaper:
> close race with exiting task"). We do not really need the lock anymore
> though. 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run
> concurrently") has removed serialization with the exit path based on the
> mm reference count and so we do not really rely on the oom_lock anymore.
> 
> Tetsuo was arguing that at least MMF_OOM_SKIP should be set under the
> lock to prevent from races when the page allocator didn't manage to get
> the freed (reaped) memory in __alloc_pages_may_oom but it sees the flag
> later on and move on to another victim. Although this is possible in
> principle let's wait for it to actually happen in real life before we
> make the locking more complex again.
> 
> Therefore remove the oom_lock for oom_reaper paths (both exit_mmap and
> oom_reap_task_mm). The reaper serializes with exit_mmap by mmap_sem +
> MMF_OOM_SKIP flag. There is no synchronization with out_of_memory path
> now.
> 
> Suggested-by: David Rientjes <rientjes@google.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

Acked-by: David Rientjes <rientjes@google.com>