From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752033Ab3K2XqW (ORCPT ); Fri, 29 Nov 2013 18:46:22 -0500 Received: from mail-yh0-f50.google.com ([209.85.213.50]:64840 "EHLO mail-yh0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751008Ab3K2XqT (ORCPT ); Fri, 29 Nov 2013 18:46:19 -0500 Date: Fri, 29 Nov 2013 15:46:16 -0800 (PST) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Michal Hocko cc: Johannes Weiner , Andrew Morton , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [patch] mm: memcg: do not declare OOM from __GFP_NOFAIL allocations In-Reply-To: <20131128102049.GF2761@dhcp22.suse.cz> Message-ID: References: <1385140676-5677-1-git-send-email-hannes@cmpxchg.org> <20131127163916.GB3556@cmpxchg.org> <20131127225340.GE3556@cmpxchg.org> <20131128102049.GF2761@dhcp22.suse.cz> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 28 Nov 2013, Michal Hocko wrote: > > Ok, so let's forget about GFP_KERNEL | __GFP_NOFAIL since anything doing > > __GFP_FS should not be holding such locks, we have some of those in the > > drivers code and that makes sense that they are doing GFP_KERNEL. > > > > Focusing on the GFP_NOFS | __GFP_NOFAIL allocations in the filesystem > > code, the kernel oom killer independent of memcg never gets called because > > !__GFP_FS and they'll simply loop around the page allocator forever. > > > > In the past, Andrew has expressed the desire to get rid of __GFP_NOFAIL > > entirely since it's flawed when combined with GFP_NOFS (and GFP_KERNEL | > > __GFP_NOFAIL could simply be reimplemented in the caller) because of the > > reason you point out in addition to making it very difficult in the page > > allocator to free memory independent of memcg. > > > > So I'm wondering if we should just disable the oom killer in memcg for > > __GFP_NOFAIL as you've done here, but not bypass to the root memcg and > > just allow them to spin? I think we should be focused on the fixing the > > callers rather than breaking memcg isolation. > > What if the callers simply cannot deal with the allocation failure? > 84235de394d97 (fs: buffer: move allocation failure loop into the > allocator) describes one such case when __getblk_slow tries desperately > to grow buffers relying on the reclaim to free something. As there might > be no reclaim going on we are screwed. > My suggestion is to spin, not return NULL. Bypassing to the root memcg can lead to a system oom condition whereas if memcg weren't involved at all the page allocator would just spin (because of !__GFP_FS). > That being said, while I do agree with you that we should strive for > isolation as much as possible there are certain cases when this is > impossible to achieve without seeing much worse consequences. For now, > we hope that __GFP_NOFAIL is used very scarcely. If that's true, why not bypass the per-zone min watermarks in the page allocator as well to allow these allocations to succeed?