Re: [patch 01/12] mm: oom_kill: remove unnecessary locking in oom_enable()

[David Rientjes]

On Wed, 25 Mar 2015, Johannes Weiner wrote:

> Setting oom_killer_disabled to false is atomic, there is no need for
> further synchronization with ongoing allocations trying to OOM-kill.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  mm/oom_kill.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 2b665da1b3c9..73763e489e86 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -488,9 +488,7 @@ bool oom_killer_disable(void)
>   */
>  void oom_killer_enable(void)
>  {
> -	down_write(&oom_sem);
>  	oom_killer_disabled = false;
> -	up_write(&oom_sem);
>  }
>  
>  #define K(x) ((x) << (PAGE_SHIFT-10))

I haven't looked through the new disable-oom-killer-for-pm patchset that 
was merged, but this oom_killer_disabled thing already looks improperly 
handled.  I think any correctness or cleanups in this area would be very 
helpful.

I think mark_tsk_oom_victim() in mem_cgroup_out_of_memory() is just 
luckily not racing with a call to oom_killer_enable() and triggering the 
WARN_ON(oom_killer_disabled) since there's no "oom_sem" held here, and 
it's an improper context based on the comment of mark_tsk_oom_victim().  
There might be something else that is intended but not implemented 
correctly that I'm unaware of, but I know of no reason why setting of 
oom_killer_disabled would need to take a semaphore?

I'm thinking it has something to do with the remainder of that comment, 
specifically the "never after oom has been disabled already."

Michal?
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