Re: [PATCH 6/7] memcg: add per cgroup writeback pages accounting

[Sha Zhengju]

On 07/09/2012 12:18 PM, Kamezawa Hiroyuki wrote:
> (2012/07/09 13:14), Fengguang Wu wrote:
> > On Mon, Jul 09, 2012 at 11:36:11AM +0800, Sha Zhengju wrote:
> > > On 07/08/2012 10:53 PM, Fengguang Wu wrote:
> > > > > @@ -2245,7 +2252,10 @@ int test_set_page_writeback(struct page *page)
> > > > >   {
> > > > >       struct address_space *mapping = page_mapping(page);
> > > > >       int ret;
> > > > > +    bool locked;
> > > > > +    unsigned long flags;
> > > > >
> > > > > +    mem_cgroup_begin_update_page_stat(page,&locked,&flags);
> > > > >       if (mapping) {
> > > > >           struct backing_dev_info *bdi = mapping->backing_dev_info;
> > > > >           unsigned long flags;
> > > > > @@ -2272,6 +2282,8 @@ int test_set_page_writeback(struct page *page)
> > > > >       }
> > > > >       if (!ret)
> > > > >           account_page_writeback(page);
> > > > > +
> > > > > +    mem_cgroup_end_update_page_stat(page,&locked,&flags);
> > > > >       return ret;
> > > > >
> > > > >   }
> > > > Where is the MEM_CGROUP_STAT_FILE_WRITEBACK increased?
> > >
> > > It's in account_page_writeback().
> > >
> > >   void account_page_writeback(struct page *page)
> > >   {
> > > +    mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_FILE_WRITEBACK);
> > >       inc_zone_page_state(page, NR_WRITEBACK);
> > >   }
> >
> > I didn't find that chunk, perhaps it's lost due to rebase..
> >
> > > There isn't a unified interface to dec/inc writeback accounting, so
> > > I just follow that.
> > > Maybe we can rework account_page_writeback() to also account
> > > dec in?
> >
> > The current seperate inc/dec paths are fine. It sounds like
> > over-engineering if going any further.
> >
> > I'm a bit worried about some 3rd party kernel module to call
> > account_page_writeback() without 
> > mem_cgroup_begin/end_update_page_stat().
> > Will that lead to serious locking issues, or merely inaccurate
> > accounting?
>
> Ah, Hm. Maybe it's better to add some debug check in
>  mem_cgroup_update_page_stat(). rcu_read_lock_held() or some.

This also apply to account_page_dirtied()... But as an "range" lock, I 
think it's common
in current kernel: just as set_page_dirty(), the caller should call it 
under the page lock
(in most cases) and it's his responsibility to guarantee correctness. I 
can add some
comments or debug check as reminding but I think i can only do so...


Thanks,
Sha


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