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? 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? -- 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>