From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755474Ab3JKUwD (ORCPT ); Fri, 11 Oct 2013 16:52:03 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:47197 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755307Ab3JKUv7 (ORCPT ); Fri, 11 Oct 2013 16:51:59 -0400 Date: Fri, 11 Oct 2013 13:51:57 -0700 From: Andrew Morton To: Johannes Weiner Cc: Michal Hocko , azurIt , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [patch 2/2] fs: buffer: move allocation failure loop into the allocator Message-Id: <20131011135157.cad19680e02cc4140ecdff0b@linux-foundation.org> In-Reply-To: <1381265890-11333-2-git-send-email-hannes@cmpxchg.org> References: <1381265890-11333-1-git-send-email-hannes@cmpxchg.org> <1381265890-11333-2-git-send-email-hannes@cmpxchg.org> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 8 Oct 2013 16:58:10 -0400 Johannes Weiner wrote: > Buffer allocation has a very crude indefinite loop around waking the > flusher threads and performing global NOFS direct reclaim because it > can not handle allocation failures. > > The most immediate problem with this is that the allocation may fail > due to a memory cgroup limit, where flushers + direct reclaim might > not make any progress towards resolving the situation at all. Because > unlike the global case, a memory cgroup may not have any cache at all, > only anonymous pages but no swap. This situation will lead to a > reclaim livelock with insane IO from waking the flushers and thrashing > unrelated filesystem cache in a tight loop. > > Use __GFP_NOFAIL allocations for buffers for now. This makes sure > that any looping happens in the page allocator, which knows how to > orchestrate kswapd, direct reclaim, and the flushers sensibly. It > also allows memory cgroups to detect allocations that can't handle > failure and will allow them to ultimately bypass the limit if reclaim > can not make progress. > > --- a/fs/buffer.c > +++ b/fs/buffer.c > @@ -1005,9 +1005,19 @@ grow_dev_page(struct block_device *bdev, sector_t block, > struct buffer_head *bh; > sector_t end_block; > int ret = 0; /* Will call free_more_memory() */ > + gfp_t gfp_mask; > > - page = find_or_create_page(inode->i_mapping, index, > - (mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS)|__GFP_MOVABLE); > + gfp_mask = mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS; > + gfp_mask |= __GFP_MOVABLE; > + /* > + * XXX: __getblk_slow() can not really deal with failure and > + * will endlessly loop on improvised global reclaim. Prefer > + * looping in the allocator rather than here, at least that > + * code knows what it's doing. > + */ > + gfp_mask |= __GFP_NOFAIL; Yup. When I added GFP_NOFAIL all those years ago there were numerous open-coded try-forever loops, and GFP_NOFAIL was more a cleanup than anything else - move the loop into the page allocator, leaving behind a sentinel which says "this code sucks and should be fixed". Of course, nothing has since been fixed :( So apart from fixing a bug, this patch continues this conversion. I can't think why I didn't do it a decade ago!