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From: anfei on 6 Apr 2010 07:50
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(a)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.

> if (p != current)
> return ERR_PTR(-1UL);
>
>
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From: anfei on 6 Apr 2010 09:10
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(a)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.
>
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