On Mon 29-10-12 15:00:22, Andrew Morton wrote:

On Mon, 29 Oct 2012 17:58:45 +0400
Glauber Costa [off-list ref] wrote:

quoted

quoted

+ * 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