From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f197.google.com (mail-pg1-f197.google.com [209.85.215.197]) by kanga.kvack.org (Postfix) with ESMTP id C60068E0001 for ; Sun, 16 Dec 2018 22:52:08 -0500 (EST) Received: by mail-pg1-f197.google.com with SMTP id a2so9429378pgt.11 for ; Sun, 16 Dec 2018 19:52:08 -0800 (PST) Received: from bombadil.infradead.org (bombadil.infradead.org. [2607:7c80:54:e::133]) by mx.google.com with ESMTPS id e63si9735870pgc.239.2018.12.16.19.52.07 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sun, 16 Dec 2018 19:52:07 -0800 (PST) Date: Sun, 16 Dec 2018 19:51:57 -0800 From: Matthew Wilcox Subject: Re: [PATCH] squashfs: enable __GFP_FS in ->readpage to prevent hang in mem alloc Message-ID: <20181217035157.GK10600@bombadil.infradead.org> References: <20181204020840.49576-1-houtao1@huawei.com> <20181215143824.GJ10600@bombadil.infradead.org> <69457a5a-79c9-4950-37ae-eff7fa4f949a@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <69457a5a-79c9-4950-37ae-eff7fa4f949a@huawei.com> Sender: owner-linux-mm@kvack.org List-ID: To: Hou Tao Cc: phillip@squashfs.org.uk, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Michal Hocko On Sun, Dec 16, 2018 at 05:38:13PM +0800, Hou Tao wrote: > Hi, > > On 2018/12/15 22:38, Matthew Wilcox wrote: > > On Tue, Dec 04, 2018 at 10:08:40AM +0800, Hou Tao wrote: > >> There is no need to disable __GFP_FS in ->readpage: > >> * It's a read-only fs, so there will be no dirty/writeback page and > >> there will be no deadlock against the caller's locked page > >> * It just allocates one page, so compaction will not be invoked > >> * It doesn't take any inode lock, so the reclamation of inode will be fine > >> > >> And no __GFP_FS may lead to hang in __alloc_pages_slowpath() if a > >> squashfs page fault occurs in the context of a memory hogger, because > >> the hogger will not be killed due to the logic in __alloc_pages_may_oom(). > > > > I don't understand your argument here. There's a comment in > > __alloc_pages_may_oom() saying that we _should_ treat GFP_NOFS > > specially, but we currently don't. > I am trying to say that if __GFP_FS is used in pagecache_get_page() when it tries > to allocate a new page for squashfs, that will be no possibility of dead-lock for > squashfs. > > We do treat GFP_NOFS specially in out_of_memory(): > > /* > * The OOM killer does not compensate for IO-less reclaim. > * pagefault_out_of_memory lost its gfp context so we have to > * make sure exclude 0 mask - all other users should have at least > * ___GFP_DIRECT_RECLAIM to get here. > */ > if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS)) > return true; > > So if GFP_FS is used, no task will be killed because we will return from > out_of_memory() prematurely. And that will lead to an infinite loop in > __alloc_pages_slowpath() as we have observed: > > * a squashfs page fault occurred in the context of a memory hogger > * the page used for page fault allocated successfully > * in squashfs_readpage() squashfs will try to allocate other pages > in the same 128KB block, and __GFP_NOFS is used (actually GFP_HIGHUSER_MOVABLE & ~__GFP_FS) > * in __alloc_pages_slowpath() we can not get any pages through reclamation > (because most of memory is used by the current task) and we also can not kill > the current task (due to __GFP_NOFS), and it will loop forever until it's killed. Ah, yes, that makes perfect sense. Thank you for the explanation. I wonder if the correct fix, however, is not to move the check for GFP_NOFS in out_of_memory() down to below the check whether to kill the current task. That would solve your problem, and I don't _think_ it would cause any new ones. Michal, you touched this code last, what do you think?