From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751747Ab1K1KM3 (ORCPT ); Mon, 28 Nov 2011 05:12:29 -0500 Received: from cantor2.suse.de ([195.135.220.15]:45978 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750821Ab1K1KM1 (ORCPT ); Mon, 28 Nov 2011 05:12:27 -0500 Date: Mon, 28 Nov 2011 11:12:25 +0100 From: Michal Hocko To: Johannes Weiner Cc: Andrew Morton , KAMEZAWA Hiroyuki , Balbir Singh , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [patch 4/8] mm: memcg: lookup_page_cgroup (almost) never returns NULL Message-ID: <20111128101225.GC18337@tiehlicka.suse.cz> References: <1322062951-1756-1-git-send-email-hannes@cmpxchg.org> <1322062951-1756-5-git-send-email-hannes@cmpxchg.org> <20111124095251.GD26036@tiehlicka.suse.cz> <20111124100549.GH6843@cmpxchg.org> <20111124102606.GF26036@tiehlicka.suse.cz> <20111128091518.GA9356@cmpxchg.org> <20111128093435.GC9356@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111128093435.GC9356@cmpxchg.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 28-11-11 10:34:35, Johannes Weiner wrote: > On Mon, Nov 28, 2011 at 10:15:18AM +0100, Johannes Weiner wrote: > > On Thu, Nov 24, 2011 at 11:26:06AM +0100, Michal Hocko wrote: > > > On Thu 24-11-11 11:05:49, Johannes Weiner wrote: > > > > On Thu, Nov 24, 2011 at 10:52:51AM +0100, Michal Hocko wrote: > > > > > On Wed 23-11-11 16:42:27, Johannes Weiner wrote: [...] > > > > > > @@ -3326,6 +3321,7 @@ static struct page_cgroup *lookup_page_cgroup_used(struct page *page) > > > > > > struct page_cgroup *pc; > > > > > > > > > > > > pc = lookup_page_cgroup(page); > > > > > > + /* Can be NULL while bootstrapping the page allocator */ > > > > > > if (likely(pc) && PageCgroupUsed(pc)) > > > > > > return pc; > > > > > > return NULL; > > > > > > > > We could add a lookup_page_cgroup_safe() for this DEBUG_VM-only > > > > callsite as an optimization separately and remove the NULL check from > > > > lookup_page_cgroup() itself. But this patch was purely about removing > > > > the actively misleading checks. > > > > > > Yes, but I am not sure whether code duplication is worth it. Let's just > > > stick with current form. Maybe just move the comment when it can be NULL > > > to the lookup_page_cgroup directly? > > > > Don't underestimate it, this function is used quite heavily while the > > case of the array being NULL is a minor fraction of all calls. But > > it's for another patch, anyway. > > Hm, how about this? yes, makes sense. Thanks > > diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c > index a14655d..58405ca 100644 > --- a/mm/page_cgroup.c > +++ b/mm/page_cgroup.c > @@ -28,9 +28,16 @@ struct page_cgroup *lookup_page_cgroup(struct page *page) > struct page_cgroup *base; > > base = NODE_DATA(page_to_nid(page))->node_page_cgroup; > +#ifdef CONFIG_DEBUG_VM > + /* > + * The sanity checks the page allocator does upon freeing a > + * page can reach here before the page_cgroup arrays are > + * allocated when feeding a range of pages to the allocator > + * for the first time during bootup or memory hotplug. > + */ > if (unlikely(!base)) > return NULL; > - > +#endif > offset = pfn - NODE_DATA(page_to_nid(page))->node_start_pfn; > return base + offset; > } > @@ -87,9 +94,16 @@ struct page_cgroup *lookup_page_cgroup(struct page *page) > { > unsigned long pfn = page_to_pfn(page); > struct mem_section *section = __pfn_to_section(pfn); > - > +#ifdef CONFIG_DEBUG_VM > + /* > + * The sanity checks the page allocator does upon freeing a > + * page can reach here before the page_cgroup arrays are > + * allocated when feeding a range of pages to the allocator > + * for the first time during bootup or memory hotplug. > + */ > if (!section->page_cgroup) > return NULL; > +#endif > return section->page_cgroup + pfn; > } > -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic