From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932398AbcEKQNY (ORCPT ); Wed, 11 May 2016 12:13:24 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:34043 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932165AbcEKQNW (ORCPT ); Wed, 11 May 2016 12:13:22 -0400 Date: Wed, 11 May 2016 18:13:18 +0200 From: Michal Hocko To: Arnd Bergmann Cc: Andrew Morton , Mel Gorman , Vlastimil Babka , David Rientjes , Joonsoo Kim , Taku Izumi , Johannes Weiner , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm, compaction: avoid uninitialized variable use Message-ID: <20160511161318.GA24901@dhcp22.suse.cz> References: <1462973126-1183468-1-git-send-email-arnd@arndb.de> <20160511144407.GA21503@dhcp22.suse.cz> <2695751.e2s15gCWav@wuerfel> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2695751.e2s15gCWav@wuerfel> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 11-05-16 16:52:41, Arnd Bergmann wrote: > On Wednesday 11 May 2016 16:44:07 Michal Hocko wrote: > > On Wed 11-05-16 15:24:44, Arnd Bergmann wrote: > > > A recent rework of the compaction code introduced a warning about > > > an uninitialized variable when CONFIG_COMPACTION is disabled and > > > __alloc_pages_direct_compact() does not set its 'compact_result' > > > output argument: > > > > > > mm/page_alloc.c: In function '__alloc_pages_nodemask': > > > mm/page_alloc.c:3651:6: error: 'compact_result' may be used uninitialized in this function [-Werror=maybe-uninitialized] > > > > > > This adds another check for CONFIG_COMPACTION to ensure we never > > > evaluate the uninitialized variable in this configuration, which > > > is probably the simplest way to avoid the warning. > > > > I think that hiding this into __alloc_pages_direct_compact is a better > > idea. See the diff below > > Ok, sounds good. > > > --- > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > index 4950d01ff935..14e3b4d93adc 100644 > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -3300,6 +3300,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, > > unsigned int alloc_flags, const struct alloc_context *ac, > > enum migrate_mode mode, enum compact_result *compact_result) > > { > > + *compact_result = COMPACT_DEFERRED; > > return NULL; > > } > > > > I thought about this but didn't know which COMPACT_* value was appropriate here. > > The behavior then changes a bit with your approach compared to mine, > because > > if (compact_result == COMPACT_DEFERRED) > goto nopage; > > is true now. I assume this is what we want though. No, we actually want to try to reclaim, that's why COMPACT_SKIPPED is more appropriate. COMPACT_SKIPPED is even correct semantically because the compaction wasn't invoked. -- Michal Hocko SUSE Labs