Messages in this thread | | | Date | Tue, 30 Oct 2012 11:35:59 +0100 | From | Michal Hocko <> | Subject | Re: [PATCH v3 3/6] memcg: Simplify mem_cgroup_force_empty_list error handling |
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On Mon 29-10-12 15:00:22, Andrew Morton wrote: > On Mon, 29 Oct 2012 17:58:45 +0400 > Glauber Costa <glommer@parallels.com> wrote: > > > > + * move charges to its parent or the root cgroup if the group has no > > > + * parent (aka use_hierarchy==0). > > > + * Although this might fail (get_page_unless_zero, isolate_lru_page or > > > + * mem_cgroup_move_account fails) the failure is always temporary and > > > + * it signals a race with a page removal/uncharge or migration. In the > > > + * first case the page is on the way out and it will vanish from the LRU > > > + * on the next attempt and the call should be retried later. > > > + * Isolation from the LRU fails only if page has been isolated from > > > + * the LRU since we looked at it and that usually means either global > > > + * reclaim or migration going on. The page will either get back to the > > > + * LRU or vanish. > > > > I just wonder for how long can it go in the worst case? > > If the kernel is uniprocessor and the caller is SCHED_FIFO: ad infinitum!
You are right, if the rmdir (resp. echo > force_empty) at SCHED_FIFO races with put_page (on a shared page) which gets preempted after put_page_testzero and before __page_cache_release then we are screwed:
put_page(page) put_page_testzero <preempted and page still on LRU> mem_cgroup_force_empty_list page = list_entry(list->prev, struct page, lru); mem_cgroup_move_parent(page) get_page_unless_zero <fails> cond_resched() <scheduled again>
The race window is really small but it is definitely possible. I am not happy about this state and it should be probably mentioned in the patch description but I do not see any way around (except for hacks like sched_setscheduler for the current which is, ehm...) and still keep do_not_fail contract here.
Can we consider this as a corner case (it is much easier to kill a machine with SCHED_FIFO than this anyway) or the concern is really strong and we should come with a solution before this can get merged?
-- Michal Hocko SUSE Labs
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