From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755523AbbCBVmx (ORCPT ); Mon, 2 Mar 2015 16:42:53 -0500 Received: from mail-we0-f178.google.com ([74.125.82.178]:34202 "EHLO mail-we0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753775AbbCBVmu (ORCPT ); Mon, 2 Mar 2015 16:42:50 -0500 Date: Mon, 2 Mar 2015 22:42:47 +0100 From: Michal Hocko To: David Rientjes Cc: linux-mm@kvack.org, Andrew Morton , Johannes Weiner , Dave Chinner , "Theodore Ts'o" , Mel Gorman , Tetsuo Handa , "David S. Miller" , sparclinux@vger.kernel.org, Vipul Pandya , netdev@vger.kernel.org, LKML Subject: Re: [RFC 2/4] jbd2: revert must-not-fail allocation loops back to GFP_NOFAIL Message-ID: <20150302214247.GB31974@dhcp22.suse.cz> References: <1425304483-7987-1-git-send-email-mhocko@suse.cz> <1425304483-7987-3-git-send-email-mhocko@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Mon 02-03-15 12:33:21, David Rientjes wrote: > On Mon, 2 Mar 2015, Michal Hocko wrote: > > > This basically reverts 47def82672b3 (jbd2: Remove __GFP_NOFAIL from jbd2 > > layer). The deprecation of __GFP_NOFAIL was a bad choice because it led > > to open coding the endless loop around the allocator rather than > > removing the dependency on the non failing allocation. So the > > deprecation was a clear failure and the reality tells us that > > __GFP_NOFAIL is not even close to go away. > > > > It is still true that __GFP_NOFAIL allocations are generally discouraged > > and new uses should be evaluated and an alternative (pre-allocations or > > reservations) should be considered but it doesn't make any sense to lie > > the allocator about the requirements. Allocator can take steps to help > > making a progress if it knows the requirements. > > > > The changelog should state that this only changes the source code, there > is no functional change since alloc_buffer_head() and > kmem_cache_zalloc(transaction_cache) are already implicitly nofail due to > the allocation order. The failure code added by the commit you cite are > never executed. Well, even when those allocation would fail the resulting behavior is basically the same (modulo congestion_wait which imho doesn't make much difference). So I would prefer not getting that way and simply stay with the external loop vs. looping within the allocator. > I agree that if the implementation of the page allocator were to change > with respect to PAGE_ALLOC_COSTLY_ORDER that we'd need __GFP_NOFAIL and > that such an allocation is better handled in the page allocator. > > > Signed-off-by: Michal Hocko > > Acked-by: David Rientjes Thanks > GFP_NOFS|__GFP_NOFAIL is scary. Yes it is but as I've learned nothing unusual in the fs land and the situation should be improved a lot if we go reservation way suggested by David. Then __GFP_NOFAIL would consume the pre-reserved memory rather than trigger OOM killer. -- Michal Hocko SUSE Labs