Messages in this thread | | | Date | Wed, 24 Sep 2008 20:58:42 +0400 | From | Oleg Nesterov <> | Subject | Re: + mm-oom-killer-kills-more-than-needed.patch added to -mm tree |
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On 09/23, Chad Zanonie wrote: > > On Tue, Sep 23, 2008 at 5:40 AM, Oleg Nesterov <oleg@tv-sign.ru> wrote: > > On 09/22, Andrew Morton wrote: > >> > >> ------------------------------------------------------ > >> Subject: mm: oom-killer kills more than needed > >> From: "Chad Zanonie" <chad.zanonie@gmail.com> > >> > >> Possibility exists for an exiting application to be in between marking its > >> mm NULL and calling mmput when out_of_memory is invoked. > >> select_bad_process() will continue past this process as opposed to > >> returning -1UL due to its mm being NULL. This causes the oom killer in > >> certain scenarios to not only kill the memory culprit, but also kill the > >> runner up. > >> > >> EXIT_DEAD seems to be the only flag that guarantees that mmput() has > >> finished. > > > > I don't think this is right. > > > > Let's suppose we have a single zombie. Now select_bad_process() always > > returns -1 ? IOW, doesn't this means that, say, > > > > $ perl -e 'fork && sleep' > > > > disables oom-kill completely and forever? > > Ugh, looks like my description was slightly incorrect. I don't mean > for it to return -1 upon noticing a null mm. I mean for > select_bad_process to not skip (continue) over processes that haven't > provably finished unmapping their memory
Yes I see. But if select_bad_process() sees the PF_EXITING task it returns -1 (unless the task == current). So, with this change select_bad_process() will return -1 more often, because it doesn't skip the tasks without ->mm. And of course, !mm implies PF_EXITING.
I don't claim this is wrong. Unless we use EXIT_DEAD as the patch did, in that case the 'fork && sleep' above really disables oom-kill.
I must admit I don't understand why this change is good but this does not matter, I don't understand oom-kill anyway (but I think it has numerous bugs ;).
> Before I propagate this blunder anymore, I've found the root of my mistake. > > I really mean TASK_DEAD, not EXIT_DEAD. > > (p->state & TASK_DEAD)
Yes, this should work. But I think this "defers" the decision too far.
You can check "p->exit_state != 0". But still this is a bit strange, and needs a comment. For example, you can check p->files == NULL with the same effect to verify that the task has already passed exit_mm()->mmput().
The task can spend a lot of time before it sets TASK_DEAD or ->exit_state, and again, during this time oom-kill will be "disabled". Contrary, the window between "tsk->mm = NULL;" and mmput() in exit_mm() is very small. Well, unless CONFIG_MM_OWNER.
In short, I can't judge this patch, but could you please improve the changelog?
Oleg.
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