Re: [PATCH -mm 2/4] oom: select_bad_process: PF_EXITING check should take ->mm into account

[anfei <anfei.zhou@gmail.com>]

On Tue, Apr 06, 2010 at 02:18:11PM +0200, Oleg Nesterov wrote:
> On 04/06, anfei wrote:
> >
> > On Fri, Apr 02, 2010 at 08:32:16PM +0200, Oleg Nesterov wrote:
> > > select_bad_process() checks PF_EXITING to detect the task which
> > > is going to release its memory, but the logic is very wrong.
> > >
> > > 	- a single process P with the dead group leader disables
> > > 	  select_bad_process() completely, it will always return
> > > 	  ERR_PTR() while P can live forever
> > >
> > > 	- if the PF_EXITING task has already released its ->mm
> > > 	  it doesn't make sense to expect it is goiing to free
> > > 	  more memory (except task_struct/etc)
> > >
> > > Change the code to ignore the PF_EXITING tasks without ->mm.
> > >
> > > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> > > ---
> > >
> > >  mm/oom_kill.c |    2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > --- MM/mm/oom_kill.c~2_FIX_PF_EXITING	2010-04-02 18:51:05.000000000 +0200
> > > +++ MM/mm/oom_kill.c	2010-04-02 18:58:37.000000000 +0200
> > > @@ -322,7 +322,7 @@ static struct task_struct *select_bad_pr
> > >  		 * the process of exiting and releasing its resources.
> > >  		 * Otherwise we could get an easy OOM deadlock.
> > >  		 */
> > > -		if (p->flags & PF_EXITING) {
> > > +		if ((p->flags & PF_EXITING) && p->mm) {
> >
> > Even this check is satisfied, it still can't say p is a good victim or
> > it will release memory automatically if multi threaded, as the exiting
> > of p doesn't mean the other threads are going to exit, so the ->mm won't
> > be released.
> 
> Yes, completely agreed.
> 
> Unfortunately I forgot to copy this into the changelog, but when I
> discussed this change I mentioned "still not perfect, but much better".
> 
> I do not really know what is the "right" solution. Even if we fix this
> check for mt case, we also have CLONE_VM tasks.
> 
What about checking mm->mm_users too? If there are any other users,
just let badness judge.  CLONE_VM tasks but not mt seem rare, and
badness doesn't consider it too.

> So, this patch just tries to improve things, to avoid the easy-to-trigger
> false positives.
> 
Agreed.

Thanks,
Anfei.

> Oleg.
>