Re: [patch -mm] mm, oom: remove oom_lock from exit_mmap

[David Rientjes <rientjes@google.com>]

On Fri, 13 Jul 2018, Michal Hocko wrote:

> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 0fe4087d5151..e6328cef090f 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -488,9 +488,11 @@ void __oom_reap_task_mm(struct mm_struct *mm)
> >  	 * Tell all users of get_user/copy_from_user etc... that the content
> >  	 * is no longer stable. No barriers really needed because unmapping
> >  	 * should imply barriers already and the reader would hit a page fault
> > -	 * if it stumbled over a reaped memory.
> > +	 * if it stumbled over a reaped memory. If MMF_UNSTABLE is already set,
> > +	 * reaping as already occurred so nothing left to do.
> >  	 */
> > -	set_bit(MMF_UNSTABLE, &mm->flags);
> > +	if (test_and_set_bit(MMF_UNSTABLE, &mm->flags))
> > +		return;
> 
> This could lead to pre mature oom victim selection
> oom_reaper			exiting victim
> oom_reap_task			exit_mmap
>   __oom_reap_task_mm		  __oom_reap_task_mm
> 				    test_and_set_bit(MMF_UNSTABLE) # wins the race
>   test_and_set_bit(MMF_UNSTABLE)
> set_bit(MMF_OOM_SKIP) # new victim can be selected now.
> 

This is not the current state of the code in the -mm tree: MMF_OOM_SKIP 
only gets set by the oom reaper when the timeout has expired when the 
victim has failed to free memory in the exit path.

> Besides that, why should we back off in the first place. We can
> race the two without any problems AFAICS. We already do have proper
> synchronization between the two due to mmap_sem and MMF_OOM_SKIP.
> 

test_and_set_bit() here is not strictly required, I thought it was better 
since any unmapping done in this context is going to be handled by 
whichever thread set MMF_UNSTABLE.