From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753194AbdDCNX7 (ORCPT ); Mon, 3 Apr 2017 09:23:59 -0400 Received: from mx2.suse.de ([195.135.220.15]:33377 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752228AbdDCNX6 (ORCPT ); Mon, 3 Apr 2017 09:23:58 -0400 Date: Mon, 3 Apr 2017 15:23:53 +0200 From: Michal Hocko To: Andrey Ryabinin Cc: Shakeel Butt , Seth Jennings , Dan Streetman , Linux MM , LKML , Andrew Morton Subject: Re: [PATCH] mm/zswap: fix potential deadlock in zswap_frontswap_store() Message-ID: <20170403132353.GO24661@dhcp22.suse.cz> References: <20170331153009.11397-1-aryabinin@virtuozzo.com> <20170403084729.GG24661@dhcp22.suse.cz> <20170403124544.GN24661@dhcp22.suse.cz> <0908e647-d60b-4340-e6d2-4f6023663401@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0908e647-d60b-4340-e6d2-4f6023663401@virtuozzo.com> 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 03-04-17 16:14:51, Andrey Ryabinin wrote: > > > On 04/03/2017 03:45 PM, Michal Hocko wrote: > > On Mon 03-04-17 15:37:07, Andrey Ryabinin wrote: > >> > >> > >> On 04/03/2017 11:47 AM, Michal Hocko wrote: > >>> On Fri 31-03-17 10:00:30, Shakeel Butt wrote: > >>>> On Fri, Mar 31, 2017 at 8:30 AM, Andrey Ryabinin > >>>> wrote: > >>>>> zswap_frontswap_store() is called during memory reclaim from > >>>>> __frontswap_store() from swap_writepage() from shrink_page_list(). > >>>>> This may happen in NOFS context, thus zswap shouldn't use __GFP_FS, > >>>>> otherwise we may renter into fs code and deadlock. > >>>>> zswap_frontswap_store() also shouldn't use __GFP_IO to avoid recursion > >>>>> into itself. > >>>>> > >>>> > >>>> Is it possible to enter fs code (or IO) from zswap_frontswap_store() > >>>> other than recursive memory reclaim? However recursive memory reclaim > >>>> is protected through PF_MEMALLOC task flag. The change seems fine but > >>>> IMHO reasoning needs an update. Adding Michal for expert opinion. > >>> > >>> Yes this is true. > >> > >> Actually, no. I think we have a bug in allocator which may lead to > >> recursive direct reclaim. > >> > >> E.g. for costly order allocations (or order > 0 && > >> ac->migratetype != MIGRATE_MOVABLE) with __GFP_NOMEMALLOC > >> (gfp_pfmemalloc_allowed() returns false) __alloc_pages_slowpath() > >> may call __alloc_pages_direct_compact() and unconditionally clear > >> PF_MEMALLOC: > > > > Not sure what is the bug here. __GFP_NOMEMALLOC is supposed to inhibit > > PF_MEMALLOC. And we do not recurse to the reclaim path. We only do the > > compaction. Or what am I missing? > > > > The bug here is that __alloc_pages_direct_compact() will > *unconditionally* clear PF_MEMALLOC. So if we already > under direct reclaim (so PF_MEMALLOC was already set) > __alloc_pages_direct_compact() will clear that PF_MEMALLOC. If > compaction failed we may go into direct reclaim again because the > following following if in __alloc_pages_slowpath() is false: Ohh, I see what you mean. Yes this is true but I guess we do not have any real costly order __GFP_NOMEMALLOC users (not sure about MIGRATE_MOVABLE branch) so nobody has noticed this. Still worth fixing I guess. I already have a plan to change direct PF_MEMALLOC to use memalloc_noreclaim_{save,restore} API on my todo list. Just didn't get to it yet. Care to send a patch? -- Michal Hocko SUSE Labs