From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932086AbaE0Txy (ORCPT ); Tue, 27 May 2014 15:53:54 -0400 Received: from zene.cmpxchg.org ([85.214.230.12]:33396 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753019AbaE0Txv (ORCPT ); Tue, 27 May 2014 15:53:51 -0400 Date: Tue, 27 May 2014 15:53:42 -0400 From: Johannes Weiner To: Michal Hocko Cc: linux-mm@kvack.org, Hugh Dickins , Tejun Heo , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [patch 7/9] mm: memcontrol: do not acquire page_cgroup lock for kmem pages Message-ID: <20140527195342.GD2878@cmpxchg.org> References: <1398889543-23671-1-git-send-email-hannes@cmpxchg.org> <1398889543-23671-8-git-send-email-hannes@cmpxchg.org> <20140523133938.GC22135@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140523133938.GC22135@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 23, 2014 at 03:39:38PM +0200, Michal Hocko wrote: > I am adding Vladimir to CC > > On Wed 30-04-14 16:25:41, Johannes Weiner wrote: > > Kmem page charging and uncharging is serialized by means of exclusive > > access to the page. Do not take the page_cgroup lock and don't set > > pc->flags atomically. > > > > Signed-off-by: Johannes Weiner > > The patch is correct I just have some comments below. > Anyway > Acked-by: Michal Hocko Thanks! > > mm/memcontrol.c | 16 +++------------- > > 1 file changed, 3 insertions(+), 13 deletions(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index c528ae9ac230..d3961fce1d54 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -3535,10 +3535,8 @@ void __memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg, > > } > > > > /* > * given page is newly allocated and invisible to everybody but > * the caller so there is no need to use page_cgroup lock nor > * SetPageCgroupUsed > */ > > would be helpful? That makes sense, I added the following: + /* + * The page is freshly allocated and not visible to any + * outside callers yet. Set up pc non-atomically. + */ > > pc = lookup_page_cgroup(page); > > - lock_page_cgroup(pc); > > pc->mem_cgroup = memcg; > > - SetPageCgroupUsed(pc); > > - unlock_page_cgroup(pc); > > + pc->flags = PCG_USED; > > } > > > > void __memcg_kmem_uncharge_pages(struct page *page, int order) > > @@ -3548,19 +3546,11 @@ void __memcg_kmem_uncharge_pages(struct page *page, int order) > > > > > > pc = lookup_page_cgroup(page); > > - /* > > - * Fast unlocked return. Theoretically might have changed, have to > > - * check again after locking. > > - */ > > This comment was there since the code has been merged. Maybe it was true > at the time but after "mm: get rid of __GFP_KMEMCG" it is definitely out > of date. > > /* > * the pages is going away and will be freed and nobody can see > * it anymore so no need to take page_cgroup lock. > */ > > if (!PageCgroupUsed(pc)) > > return; > > > > - lock_page_cgroup(pc); > > - if (PageCgroupUsed(pc)) { > > - memcg = pc->mem_cgroup; > > - ClearPageCgroupUsed(pc); > > - } > > - unlock_page_cgroup(pc); > > maybe add > WARN_ON_ONCE(pc->flags != PCG_USED); > > to check for an unexpected flags usage in the kmem path? There is no overlap between page types that use PCG_USED and those that don't. What would be the value of adding this?