From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f72.google.com (mail-oi0-f72.google.com [209.85.218.72]) by kanga.kvack.org (Postfix) with ESMTP id 647CE6B0038 for ; Mon, 3 Apr 2017 09:13:31 -0400 (EDT) Received: by mail-oi0-f72.google.com with SMTP id n4so103006050oia.5 for ; Mon, 03 Apr 2017 06:13:31 -0700 (PDT) Received: from EUR03-VE1-obe.outbound.protection.outlook.com (mail-eopbgr50130.outbound.protection.outlook.com. [40.107.5.130]) by mx.google.com with ESMTPS id z2si6556754otc.177.2017.04.03.06.13.29 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Mon, 03 Apr 2017 06:13:30 -0700 (PDT) Subject: Re: [PATCH] mm/zswap: fix potential deadlock in zswap_frontswap_store() References: <20170331153009.11397-1-aryabinin@virtuozzo.com> <20170403084729.GG24661@dhcp22.suse.cz> <20170403124544.GN24661@dhcp22.suse.cz> From: Andrey Ryabinin Message-ID: <0908e647-d60b-4340-e6d2-4f6023663401@virtuozzo.com> Date: Mon, 3 Apr 2017 16:14:51 +0300 MIME-Version: 1.0 In-Reply-To: <20170403124544.GN24661@dhcp22.suse.cz> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Shakeel Butt , Seth Jennings , Dan Streetman , Linux MM , LKML , Andrew Morton 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: /* Avoid recursion of direct reclaim */ if (current->flags & PF_MEMALLOC) goto nopage; /* Try direct reclaim and then allocating */ page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac, So, recursion might look like this: alloc_pages() __perform_reclaim() current->flags |= PF_MEMALLOC; try_to_free_pages() alloc_pages(__GFP_NONMEMALLOC): __alloc_pages_direct_compact(): current->flags &= ~PF_MEMALLOC; if (current->flags & PF_MEMALLOC) //now it's false goto nopage; __alloc_pages_direct_reclaim() __perform_reclaim() -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org