On Tue 13-11-12 16:10:41, Johannes Weiner wrote: > On Tue, Oct 30, 2012 at 11:35:59AM +0100, Michal Hocko wrote: > > On Mon 29-10-12 15:00:22, Andrew Morton wrote: > > > On Mon, 29 Oct 2012 17:58:45 +0400 > > > Glauber Costa <glommer@xxxxxxxxxxxxx> 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? > > Wouldn't the much bigger race window be reclaim having the page > isolated and SCHED_FIFO preventing it from putback? We wouldn't see the page on the LRU then, right? > I also don't think this is a new class of problem, though. > > Would it make sense to stick a wait_on_page_locked() in there just so > that we don't busy spin on a page under migration/reclaim? Hmm, this would also mean that get_page_unless_zero would fail as well and so we would schedule in mem_cgroup_force_empty_list. It is true that there might be no other runnable task so we can busy loop so yes this would help. Care to cook the patch? Thanks -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>