From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754733Ab1K2LAl (ORCPT ); Tue, 29 Nov 2011 06:00:41 -0500 Received: from cantor2.suse.de ([195.135.220.15]:47635 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753200Ab1K2LAi (ORCPT ); Tue, 29 Nov 2011 06:00:38 -0500 Date: Tue, 29 Nov 2011 12:00:35 +0100 From: Michal Hocko To: Johannes Weiner Cc: Andrew Morton , KAMEZAWA Hiroyuki , Balbir Singh , David Rientjes , Hugh Dickins , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [patch 5/7] mm: page_cgroup: check page_cgroup arrays in lookup_page_cgroup() only when necessary Message-ID: <20111129110035.GA6898@tiehlicka.suse.cz> References: <1322563925-1667-1-git-send-email-hannes@cmpxchg.org> <1322563925-1667-6-git-send-email-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1322563925-1667-6-git-send-email-hannes@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 Tue 29-11-11 11:52:03, Johannes Weiner wrote: > lookup_page_cgroup() is usually used only against pages that are used > in userspace. > > The exception is the CONFIG_DEBUG_VM-only memcg check from the page > allocator: it can run on pages without page_cgroup descriptors > allocated when the pages are fed into the page allocator for the first > time during boot or memory hotplug. > > Include the array check only when CONFIG_DEBUG_VM is set and save the > unnecessary check in production kernels. > > Signed-off-by: Johannes Weiner I was thinking about adding BUG_ON before dereferencing but this is questionable because NULL ptr dereference will provide the same information except sec. guys might be alerted. I like the smaller code more of course. Acked-by: Michal Hocko > --- > mm/page_cgroup.c | 18 ++++++++++++++++-- > 1 files changed, 16 insertions(+), 2 deletions(-) > > 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; > } > > -- > 1.7.6.4 > -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic