From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751111AbdAWMzx (ORCPT ); Mon, 23 Jan 2017 07:55:53 -0500 Received: from mx2.suse.de ([195.135.220.15]:41034 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751016AbdAWMzx (ORCPT ); Mon, 23 Jan 2017 07:55:53 -0500 Subject: Re: [PATCH] mm: ensure alloc_flags in slow path are initialized To: Arnd Bergmann , Andrew Morton References: <20170123121649.3180300-1-arnd@arndb.de> Cc: Mel Gorman , Michal Hocko , Johannes Weiner , Joonsoo Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org From: Vlastimil Babka Message-ID: Date: Mon, 23 Jan 2017 13:55:44 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0 MIME-Version: 1.0 In-Reply-To: <20170123121649.3180300-1-arnd@arndb.de> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/23/2017 01:16 PM, Arnd Bergmann wrote: > The __alloc_pages_slowpath() has gotten rather complex and gcc > is no longer able to follow the gotos and prove that the > alloc_flags variable is initialized at the time it is used: > > mm/page_alloc.c: In function '__alloc_pages_slowpath': > mm/page_alloc.c:3565:15: error: 'alloc_flags' may be used uninitialized in this function [-Werror=maybe-uninitialized] > > To be honest, I can't figure that out either, maybe it is or > maybe not, Seems the report is correct and not false positive, in scenario when we goto nopage before the assignment, and then goto retry because of __GFP_NOFAIL. > but moving the existing initialization up a little > higher looks safe and makes it obvious to both me and gcc that > the initialization comes before the first use. > > Fixes: 74eaa4a97e8e ("mm: consolidate GFP_NOFAIL checks in the allocator slowpath") That's a non-stable -next commit ID for mmotm patch: mm-consolidate-gfp_nofail-checks-in-the-allocator-slowpath.patch The patch itself was OK, the problem only comes from integration with another mmotm patch (also independently OK): mm-page_alloc-fix-premature-oom-when-racing-with-cpuset-mems-update.patch By their ordering in mmotm, it would work to treat this as a fix for the GFP_NOFAIL patch, possibly merged into it. > Signed-off-by: Arnd Bergmann Acked-by: Vlastimil Babka > --- > mm/page_alloc.c | 15 +++++++-------- > 1 file changed, 7 insertions(+), 8 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index cf641932c015..d9fa4564524f 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -3591,6 +3591,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > (__GFP_ATOMIC|__GFP_DIRECT_RECLAIM))) > gfp_mask &= ~__GFP_ATOMIC; > > + /* > + * The fast path uses conservative alloc_flags to succeed only until > + * kswapd needs to be woken up, and to avoid the cost of setting up > + * alloc_flags precisely. So we do that now. > + */ > + alloc_flags = gfp_to_alloc_flags(gfp_mask); > + > retry_cpuset: > compaction_retries = 0; > no_progress_loops = 0; > @@ -3607,14 +3614,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > if (!ac->preferred_zoneref->zone) > goto nopage; > > - > - /* > - * The fast path uses conservative alloc_flags to succeed only until > - * kswapd needs to be woken up, and to avoid the cost of setting up > - * alloc_flags precisely. So we do that now. > - */ > - alloc_flags = gfp_to_alloc_flags(gfp_mask); > - > if (gfp_mask & __GFP_KSWAPD_RECLAIM) > wake_all_kswapds(order, ac); > >