Re: [PATCH] mm,oom: use per signal_struct flag rather than clear TIF_MEMDIE

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Oleg Nesterov wrote:
> Since I mentioned TIF_MEMDIE in another thread, I simply can't resist.
> Sorry for grunting.
> 
> On 06/24, Tetsuo Handa wrote:
> >
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -801,6 +801,7 @@ struct signal_struct {
> >  	 * oom
> >  	 */
> >  	bool oom_flag_origin;
> > +	bool oom_ignore_victims;        /* Ignore oom_victims value */
> >  	short oom_score_adj;		/* OOM kill score adjustment */
> >  	short oom_score_adj_min;	/* OOM kill score adjustment min value.
> >  					 * Only settable by CAP_SYS_RESOURCE. */
> 
> Yet another kludge to fix yet another problem with TIF_MEMDIE. Not
> to mention that that wh
> 
> Can't we state the fact TIF_MEMDIE is just broken? The very idea imo.

Yes. TIF_MEMDIE is a trouble maker.

Setting TIF_MEMDIE is per task_struct operation.
Sending SIGKILL is per signal_struct operation.
OOM killer is per mm_struct operation.

> I am starting to seriously think we should kill this flag, fix the
> compilation errors, remove the dead code (including the oom_victims
> logic), and then try to add something else. Say, even MMF_MEMDIE looks
> better although I understand it is not that simple.

I wish that TIF_MEMDIE is per signal_struct flag. But since we allow
mm-less TIF_MEMDIE thread to use ALLOC_NO_WATERMARKS via TIF_MEMDIE
inside __mmput() from mmput() from exit_mm() from do_exit(), we can't
replace

  test_thread_flag(TIF_MEMDIE)

in gfp_to_alloc_flags() with

  current->signal->oom_killed

or

  current->mm && (current->mm->flags & MMF_MEMDIE)

. But

> 
> Just one question. Why do we need this bit outside of oom-kill.c? It
> affects page_alloc.c and this probably makes sense. But who get this
> flag when we decide to kill the memory hog? A random thread foung by
> find_lock_task_mm(), iow a random thread with ->mm != NULL, likely the
> group leader. This simply can not be right no matter what.

I agree that setting TIF_MEMDIE to only first ->mm != NULL thread
does not make sense.

I've proposed setting TIF_MEMDIE to all ->mm != NULL threads which are
killed by the OOM killer because doing so won't increase the risk of
depleting the memory reserves, for TIF_MEMDIE helps only if that thread is
doing memory allocation
( http://lkml.kernel.org/r/1458529634-5951-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp ),
but it did not happen.

> 
> And in any case I don't understand this patch but I have to admit that
> I failed to force myself to read the changelog and the actual change ;)
> In any case I agree that we should not set MMF_MEMDIE if ->mm == NULL,
> and if we ensure this then I do not understand why we can't rely on
> MMF_OOM_REAPED. Ignoring the obvious races, if ->oom_victims != 0 then
> find_lock_task_mm() should succed.

Since we are using

  mm = current->mm;
  current->mm = NULL;
  __mmput(mm); (may block for unbounded period waiting for somebody else's memory allocation)
  exit_oom_victim(current);

sequence, we won't be able to make find_lock_task_mm(tsk) != NULL when
tsk->signal->oom_victims != 0 unless we change this sequence.
My patch tries to rescue it using tsk->signal->oom_ignore_victims flag.

> 
> Oleg.
> 
> 

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