From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753076AbaJTTNB (ORCPT ); Mon, 20 Oct 2014 15:13:01 -0400 Received: from mail-wi0-f171.google.com ([209.85.212.171]:53237 "EHLO mail-wi0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751885AbaJTTM7 (ORCPT ); Mon, 20 Oct 2014 15:12:59 -0400 Date: Mon, 20 Oct 2014 21:12:56 +0200 From: Michal Hocko To: Johannes Weiner Cc: Andrew Morton , Hugh Dickins , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [patch] mm: memcontrol: update mem_cgroup_page_lruvec() documentation Message-ID: <20141020191256.GD505@dhcp22.suse.cz> References: <1413732616-15962-1-git-send-email-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1413732616-15962-1-git-send-email-hannes@cmpxchg.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun 19-10-14 11:30:16, Johannes Weiner wrote: > 7512102cf64d ("memcg: fix GPF when cgroup removal races with last > exit") added a pc->mem_cgroup reset into mem_cgroup_page_lruvec() to > prevent a crash where an anon page gets uncharged on unmap, the memcg > is released, and then the final LRU isolation on free dereferences the > stale pc->mem_cgroup pointer. > > But since 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API"), pages > are only uncharged AFTER that final LRU isolation, which guarantees > the memcg's lifetime until then. pc->mem_cgroup now only needs to be > reset for swapcache readahead pages. Do we want VM_BUG_ON_PAGE(!PageSwapCache, page) into the fixup path? > Update the comment and callsite requirements accordingly. > > Signed-off-by: Johannes Weiner Acked-by: Michal Hocko > --- > mm/memcontrol.c | 16 ++++++++-------- > 1 file changed, 8 insertions(+), 8 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 3a203c7ec6c7..fc1d7ca96b9d 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1262,9 +1262,13 @@ struct lruvec *mem_cgroup_zone_lruvec(struct zone *zone, > } > > /** > - * mem_cgroup_page_lruvec - return lruvec for adding an lru page > + * mem_cgroup_page_lruvec - return lruvec for isolating/putting an LRU page > * @page: the page > * @zone: zone of the page > + * > + * This function is only safe when following the LRU page isolation > + * and putback protocol: the LRU lock must be held, and the page must > + * either be PageLRU() or the caller must have isolated/allocated it. > */ > struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct zone *zone) > { > @@ -1282,13 +1286,9 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct zone *zone) > memcg = pc->mem_cgroup; > > /* > - * Surreptitiously switch any uncharged offlist page to root: > - * an uncharged page off lru does nothing to secure > - * its former mem_cgroup from sudden removal. > - * > - * Our caller holds lru_lock, and PageCgroupUsed is updated > - * under page_cgroup lock: between them, they make all uses > - * of pc->mem_cgroup safe. > + * Swapcache readahead pages are added to the LRU - and > + * possibly migrated - before they are charged. Ensure > + * pc->mem_cgroup is sane. > */ > if (!PageLRU(page) && !PageCgroupUsed(pc) && memcg != root_mem_cgroup) > pc->mem_cgroup = memcg = root_mem_cgroup; > -- > 2.1.2 > -- Michal Hocko SUSE Labs