Re: [PATCH V2] memcg: add mlock statistic in memory.stat

[Ying Han <yinghan@google.com>]

On Thu, Apr 19, 2012 at 6:12 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Thu, Apr 19, 2012 at 09:59:20AM +0900, KAMEZAWA Hiroyuki wrote:
>> (2012/04/19 8:33), Andrew Morton wrote:
>>
>> > On Wed, 18 Apr 2012 11:21:55 -0700
>> > Ying Han <yinghan@google.com> wrote:
>> >>  static void __free_pages_ok(struct page *page, unsigned int order)
>> >>  {
>> >>    unsigned long flags;
>> >> -  int wasMlocked = __TestClearPageMlocked(page);
>> >> +  bool locked;
>> >>
>> >>    if (!free_pages_prepare(page, order))
>> >>            return;
>> >>
>> >>    local_irq_save(flags);
>> >> -  if (unlikely(wasMlocked))
>> >> +  mem_cgroup_begin_update_page_stat(page, &locked, &flags);
>> >
>> > hm, what's going on here.  The page now has a zero refcount and is to
>> > be returned to the buddy.  But mem_cgroup_begin_update_page_stat()
>> > assumes that the page still belongs to a memcg.  I'd have thought that
>> > any page_cgroup backreferences would have been torn down by now?
>> >
>> >> +  if (unlikely(__TestClearPageMlocked(page)))
>> >>            free_page_mlock(page);
>> >
>>
>>
>> Ah, this is problem. Now, we have following code.
>> ==
>>
>> > struct lruvec *mem_cgroup_lru_add_list(struct zone *zone, struct page *page,
>> >                                        enum lru_list lru)
>> > {
>> >         struct mem_cgroup_per_zone *mz;
>> >         struct mem_cgroup *memcg;
>> >         struct page_cgroup *pc;
>> >
>> >         if (mem_cgroup_disabled())
>> >                 return &zone->lruvec;
>> >
>> >         pc = lookup_page_cgroup(page);
>> >         memcg = pc->mem_cgroup;
>> >
>> >         /*
>> >          * Surreptitiously switch any uncharged page to root:
>> >          * an uncharged page off lru does nothing to secure
>> >          * its former mem_cgroup from sudden removal.
>> >          *
>> >          * Our caller holds lru_lock, and PageCgroupUsed is updated
>> >          * under page_cgroup lock: between them, they make all uses
>> >          * of pc->mem_cgroup safe.
>> >          */
>> >         if (!PageCgroupUsed(pc) && memcg != root_mem_cgroup)
>> >                 pc->mem_cgroup = memcg = root_mem_cgroup;
>>
>> ==
>>
>> Then, accessing pc->mem_cgroup without checking PCG_USED bit is dangerous.
>> It may trigger #GP because of suddern removal of memcg or because of above
>> code, mis-accounting will happen... pc->mem_cgroup may be overwritten already.
>>
>> Proposal from me is calling TestClearPageMlocked(page) via mem_cgroup_uncharge().
>>
>> Like this.
>> ==
>>         mem_cgroup_charge_statistics(memcg, anon, -nr_pages);
>>
>>       /*
>>          * Pages reach here when it's fully unmapped or dropped from file cache.
>>        * we are under lock_page_cgroup() and have no race with memcg activities.
>>          */
>>       if (unlikely(PageMlocked(page))) {
>>               if (TestClearPageMlocked())
>>                       decrement counter.
>>       }
>>
>>         ClearPageCgroupUsed(pc);
>> ==
>> But please check performance impact...
>
> This makes the lifetime rules of mlocked anon really weird.
>
> Plus this code runs for ALL uncharges, the unlikely() and preliminary
> flag testing don't make it okay.  It's bad that we have this in the
> allocator, but at least it would be good to hook into that branch and
> not add another one.

Johannes,
Can you give a more details of your last sentence above? :)

>
> pc->mem_cgroup stays intact after the uncharge.  Could we make the
> memcg removal path wait on the mlock counter to drop to zero instead
> and otherwise keep Ying's version?

Will it delay the memcg predestroy ? I am wondering if we have page in
mmu gather or pagevec, and they won't be freed until we flush?

--Ying

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