From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f197.google.com (mail-pf0-f197.google.com [209.85.192.197]) by kanga.kvack.org (Postfix) with ESMTP id 567126B0033 for ; Mon, 23 Jan 2017 18:56:40 -0500 (EST) Received: by mail-pf0-f197.google.com with SMTP id f144so219933450pfa.3 for ; Mon, 23 Jan 2017 15:56:40 -0800 (PST) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id p86si17131728pfk.75.2017.01.23.15.56.39 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 23 Jan 2017 15:56:39 -0800 (PST) Date: Mon, 23 Jan 2017 15:56:38 -0800 From: Andrew Morton Subject: Re: [PATCH] mm: ensure alloc_flags in slow path are initialized Message-Id: <20170123155638.db6036219cb6ab2582be104e@linux-foundation.org> In-Reply-To: <20170123121649.3180300-1-arnd@arndb.de> References: <20170123121649.3180300-1-arnd@arndb.de> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Arnd Bergmann Cc: Vlastimil Babka , Mel Gorman , Michal Hocko , Johannes Weiner , Joonsoo Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Mon, 23 Jan 2017 13:16:12 +0100 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, 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. > > ... > > --- 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); hm. But we later do if (gfp_pfmemalloc_allowed(gfp_mask)) alloc_flags = ALLOC_NO_WATERMARKS; ... if (read_mems_allowed_retry(cpuset_mems_cookie)) goto retry_cpuset; so with your patch there's a path where we can rerun everything with alloc_flags == ALLOC_NO_WATERMARKS. That's changed behaviour. When I saw the test robot warning I did this, which I think preserves behaviour? --- a/mm/page_alloc.c~mm-consolidate-gfp_nofail-checks-in-the-allocator-slowpath-fix +++ a/mm/page_alloc.c @@ -3577,6 +3577,14 @@ retry_cpuset: no_progress_loops = 0; compact_priority = DEF_COMPACT_PRIORITY; cpuset_mems_cookie = read_mems_allowed_begin(); + + /* + * 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); + /* * We need to recalculate the starting point for the zonelist iterator * because we might have used different nodemask in the fast path, or @@ -3588,14 +3596,6 @@ retry_cpuset: 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); _ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org