Messages in this thread | | | Date | Wed, 31 Mar 2010 19:58:36 +0200 | From | Oleg Nesterov <> | Subject | Re: [patch] oom: give current access to memory reserves if it has been killed |
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On 03/30, David Rientjes wrote: > > On Tue, 30 Mar 2010, Oleg Nesterov wrote: > > > Note that __oom_kill_task() does force_sig(SIGKILL) which assumes that > > ->sighand != NULL. This is not true if out_of_memory() is called after > > current has already passed exit_notify(). > > We have an even bigger problem if current is in the oom killer at > exit_notify() since it has already detached its ->mm in exit_mm() :)
Can't understand... I thought that in theory even kmalloc(1) can trigger oom.
Say, right after exit_mm() we are doing acct_process(), and f_op->write() needs a page. So, you are saying that in this case __page_cache_alloc() can never trigger out_of_memory() ?
> > IOW, unless I missed something, it is very easy to hide the process > > from oom-kill: > > > > int main() > > { > > pthread_create(memory_hog_func); > > syscall(__NR_exit); > > } > > > > The check for !p->mm was moved in the -mm tree (and the oom killer was > entirely rewritten in that tree, so I encourage you to work off of it > instead
OK, but I guess this !p->mm check is still wrong for the same reason. In fact I do not understand why it is needed in select_bad_process() right before oom_badness() which checks ->mm too (and this check is equally wrong).
> with > oom-avoid-race-for-oom-killed-tasks-detaching-mm-prior-to-exit.patch to > even after the check for PF_EXITING. This is set in the exit path before > the ->mm is detached
Yes. Then I do not understand "if (!p->mm)" completely.
> so if the oom killer finds an already exiting task, > it will become a no-op since it should eventually free memory and avoids a > needless oom kill.
No, afaics, And this reminds that I already complained about this PF_EXITING check.
Once again, p is the group leader. It can be dead (no ->mm, PF_EXITING is set) but it can have sub-threads. This means, unless I missed something, any user can trivially disable select_bad_process() forever.
Well. Looks like, -mm has a lot of changes in oom_kill.c. Perhaps it would be better to fix these mt bugs first...
Say, oom_forkbomb_penalty() does list_for_each_entry(tsk->children). Again, this is not right even if we forget about !child->mm check. This list_for_each_entry() can only see the processes forked by the main thread.
Likewise, oom_kill_process()->list_for_each_entry() is not right too.
Hmm. Why oom_forkbomb_penalty() does thread_group_cputime() under task_lock() ? It seems, ->alloc_lock() is only needed for get_mm_rss().
Oleg.
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