Re: Re: [PATCH] mm,oom_kill: Close race window of needlessly selecting new victims.

From: David Rientjes
Date: Wed Jun 21 2017 - 16:31:10 EST

On Wed, 21 Jun 2017, Tetsuo Handa wrote:

> Umm... So, you are pointing out that select_bad_process() aborts based on
> TIF_MEMDIE or MMF_OOM_SKIP is broken because victim threads can be removed
> from global task list or cgroup's task list. Then, the OOM killer will have to
> wait until all mm_struct of interested OOM domain (system wide or some cgroup)
> is reaped by the OOM reaper. Simplest way is to wait until all mm_struct are
> reaped by the OOM reaper, for currently we are not tracking which memory cgroup
> each mm_struct belongs to, are we? But that can cause needless delay when
> multiple OOM events occurred in different OOM domains. Do we want to (and can we)
> make it possible to tell whether each mm_struct queued to the OOM reaper's list
> belongs to the thread calling out_of_memory() ?

I am saying that taking mmget() in mark_oom_victim() and then only
dropping it with mmput_async() after it can grab mm->mmap_sem, which the
exit path itself takes, or the oom reaper happens to schedule, causes
__mmput() to be called much later and thus we remove the process from the
tasklist or call cgroup_exit() earlier than the memory can be unmapped
with your patch. As a result, subsequent calls to the oom killer kills
everything before the original victim's mm can undergo __mmput() because
the oom reaper still holds the reference.