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

David Rientjes
2018-07-17T04:14:39Z

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.