From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] memcg: simplify lock of memcg page stat accounting Date: Tue, 29 Jan 2013 11:40:03 +0100 Message-ID: <20130129103940.GA29574@dhcp22.suse.cz> References: <1359198756-3752-1-git-send-email-handai.szj@taobao.com> <51071AA1.7000207@jp.fujitsu.com> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <51071AA1.7000207-+CUm20s59erQFUHtdCDX3A@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Kamezawa Hiroyuki Cc: Sha Zhengju , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, gthelen-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, hughd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, Sha Zhengju On Tue 29-01-13 09:41:05, KAMEZAWA Hiroyuki wrote: > (2013/01/26 20:12), Sha Zhengju wrote: [...] > > So in order to make the lock simpler and clearer and also avoid the 'nesting' > > problem, a choice may be: > > (CPU-A does "page stat accounting" and CPU-B does "move") > > > > CPU-A CPU-B > > > > move_lock_mem_cgroup() > > memcg = pc->mem_cgroup > > TestSetPageDirty(page) > > move_unlock_mem_cgroup() > > move_lock_mem_cgroup() > > if (PageDirty) { > > old_memcg->nr_dirty --; > > new_memcg->nr_dirty ++; > > } > > pc->mem_cgroup = new_memcg > > move_unlock_mem_cgroup() > > > > memcg->nr_dirty ++ > > > > Hmm. no race with file truncate ? Shouldn't pte lock protect us in page_{add_file,remove}_rmap? [...] > > diff --git a/mm/rmap.c b/mm/rmap.c > > index 59b0dca..0d74c48 100644 > > --- a/mm/rmap.c > > +++ b/mm/rmap.c > > @@ -1112,13 +1112,25 @@ void page_add_file_rmap(struct page *page) > > { > > bool locked; > > unsigned long flags; > > + bool ret; > > + struct mem_cgroup *memcg = NULL; > > + struct cgroup_subsys_state *css = NULL; > > > > mem_cgroup_begin_update_page_stat(page, &locked, &flags); > > - if (atomic_inc_and_test(&page->_mapcount)) { > > + memcg = try_get_mem_cgroup_from_page(page); > > Toooooo heavy ! I can say NACK to this patch only because of this try_get(). Agreed. > To hold memcg alive, rcu_read_lock() will work (as current code does). > > BTW, does this patch fixes the nested-lock problem ? Because set_page_drity is called outside of mem_cgroup_{begin,end}_update_page_stat. That confused me too. -- Michal Hocko SUSE Labs