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?
--
Michal Hocko
SUSE Labs
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