Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752270AbdHHTNq (ORCPT ); Tue, 8 Aug 2017 15:13:46 -0400 Received: from mail-qt0-f181.google.com ([209.85.216.181]:38190 "EHLO mail-qt0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752030AbdHHTNo (ORCPT ); Tue, 8 Aug 2017 15:13:44 -0400 MIME-Version: 1.0 In-Reply-To: <20170808173704.GA22887@cmpxchg.org> References: <20170805155241.GA94821@jaegeuk-macbookpro.roam.corp.google.com> <20170808010150.4155-1-bradleybolen@gmail.com> <20170808162122.GA14689@cmpxchg.org> <20170808165601.GA7693@jaegeuk-macbookpro.roam.corp.google.com> <20170808173704.GA22887@cmpxchg.org> From: Brad Bolen Date: Tue, 8 Aug 2017 15:13:42 -0400 Message-ID: Subject: Re: kernel panic on null pointer on page->mem_cgroup To: Johannes Weiner Cc: Jaegeuk Kim , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6517 Lines: 130 On Tue, Aug 8, 2017 at 1:37 PM, Johannes Weiner wrote: > On Tue, Aug 08, 2017 at 09:56:01AM -0700, Jaegeuk Kim wrote: >> On 08/08, Johannes Weiner wrote: >> > Hi Jaegeuk and Bradley, >> > >> > On Mon, Aug 07, 2017 at 09:01:50PM -0400, Bradley Bolen wrote: >> > > I am getting a very similar error on v4.11 with an arm64 board. >> > > >> > > I, too, also see page->mem_cgroup checked to make sure that it is not >> > > NULL and then several instructions later it is NULL. It does appear >> > > that someone is changing that member without taking the lock. In my >> > > setup, I see >> > > >> > > crash> bt >> > > PID: 72 TASK: e1f48640 CPU: 0 COMMAND: "mmcqd/1" >> > > #0 [] (__crash_kexec) from [] >> > > #1 [] (panic) from [] >> > > #2 [] (svcerr_panic) from [] >> > > #3 [] (_SvcErr_) from [] >> > > #4 [] (die) from [] >> > > #5 [] (__do_kernel_fault) from [] >> > > #6 [] (do_page_fault) from [] >> > > #7 [] (do_DataAbort) from [] >> > > pc : [] lr : [] psr: a0000193 >> > > sp : c1a19cc8 ip : 00000000 fp : c1a19d04 >> > > r10: 0006ae29 r9 : 00000000 r8 : dfbf1800 >> > > r7 : dfbf1800 r6 : 00000001 r5 : f3c1107c r4 : e2fb6424 >> > > r3 : 00000000 r2 : 00040228 r1 : 221e3000 r0 : a0000113 >> > > Flags: NzCv IRQs off FIQs on Mode SVC_32 ISA ARM >> > > #8 [] (__dabt_svc) from [] >> > > #9 [] (test_clear_page_writeback) from [] >> > > #10 [] (end_page_writeback) from [] >> > > #11 [] (end_swap_bio_write) from [] >> > > #12 [] (bio_endio) from [] >> > > #13 [] (dec_pending) from [] >> > > #14 [] (clone_endio) from [] >> > > #15 [] (bio_endio) from [] >> > > #16 [] (crypt_dec_pending [dm_crypt]) from [] >> > > #17 [] (crypt_endio [dm_crypt]) from [] >> > > #18 [] (bio_endio) from [] >> > > #19 [] (blk_update_request) from [] >> > > #20 [] (blk_update_bidi_request) from [] >> > > #21 [] (blk_end_bidi_request) from [] >> > > #22 [] (blk_end_request) from [] >> > > #23 [] (mmc_blk_issue_rw_rq) from [] >> > > #24 [] (mmc_blk_issue_rq) from [] >> > > #25 [] (mmc_queue_thread) from [] >> > > #26 [] (kthread) from [] >> > > crash> sym c0112540 >> > > c0112540 (T) test_clear_page_writeback+512 >> > > /kernel-source/include/linux/memcontrol.h: 518 >> > > >> > > crash> bt 35 >> > > PID: 35 TASK: e1d45dc0 CPU: 1 COMMAND: "kswapd0" >> > > #0 [] (__schedule) from [] >> > > #1 [] (schedule) from [] >> > > #2 [] (schedule_timeout) from [] >> > > #3 [] (io_schedule_timeout) from [] >> > > #4 [] (mempool_alloc) from [] >> > > #5 [] (bio_alloc_bioset) from [] >> > > #6 [] (get_swap_bio) from [] >> > > #7 [] (__swap_writepage) from [] >> > > #8 [] (swap_writepage) from [] >> > > #9 [] (shmem_writepage) from [] >> > > #10 [] (shrink_page_list) from [] >> > > #11 [] (shrink_inactive_list) from [] >> > > #12 [] (shrink_node_memcg) from [] >> > > #13 [] (shrink_node) from [] >> > > #14 [] (kswapd) from [] >> > > #15 [] (kthread) from [] >> > > >> > > It appears that uncharge_list() in mm/memcontrol.c is not taking the >> > > page lock when it sets mem_cgroup to NULL. I am not familiar with the >> > > mm code so I do not know if this is on purpose or not. There is a >> > > comment in uncharge_list that makes me believe that the crashing code >> > > should not have been running: >> > > /* >> > > * Nobody should be changing or seriously looking at >> > > * page->mem_cgroup at this point, we have fully >> > > * exclusive access to the page. >> > > */ >> > > However, I am new to looking at this area of the kernel so I am not >> > > sure. >> > >> > The lock is for pages that are actively being used, whereas the free >> > path requires the page refcount to be 0; nobody else should be having >> > access to the page at that time. >> >> Given various trials for a while, using __mod_memcg_state() instead of >> mod_memcg_state() ssems somehow blowing the panic away. It might be caused >> by kernel preemption? > > That's puzzling. Is that reliably the case? Because on x86-64, > __this_cpu_add and this_cpu_add should result in the same code: > > #define raw_cpu_add_8(pcp, val) percpu_add_op((pcp), val) > #define this_cpu_add_8(pcp, val) percpu_add_op((pcp), val) > > which boils down to single instructions - incq, decq, addq - and so > never needs explicit disabling of scheduler or interrupt preemption. > >> > > I was able to create a reproducible scenario by using a udelay to >> > > increase the time between the if (page->mem_cgroup) check and the later >> > > dereference of it to increase the race window. I then mounted an empty >> > > ext4 partition and ran the following no more than twice before it >> > > crashed. >> > > dd if=/dev/zero of=/tmp/ext4disk/test bs=1M count=100 >> > >> > Thanks, that's useful. I'm going to try to reproduce this also. >> > >> > There is a >> > >> > VM_BUG_ON_PAGE(!PageHWPoison(page) && page_count(page), page); >> > >> > inside uncharge_list() that verifies that there shouldn't in fact be >> > any pages ending writeback when they get into that function. Can you >> > build your kernel with CONFIG_DEBUG_VM to enable that test? >> >> I'll test this as well. ;) > > Thanks. I'm trying various udelays in between the NULL-check and the > dereference, but I cannot seem to make it happen with the ext4-dd on > my local machine. I am using a fairly fat udelay of 100. I turned on CONFIG_DEBUG_VM but it didn't bug for me, although the original problem still reproduced. I can insert some tracing to try to get some more information, but unfortunately I am stuck with v4.11 at the moment and can't try v4.13-rc4 on this setup.