On Tue, Aug 8, 2017 at 1:37 PM, Johannes Weiner <hannes@xxxxxxxxxxx> 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 [<c00ad35c>] (__crash_kexec) from [<c0101080>] >> > > #1 [<c0101080>] (panic) from [<c028cd6c>] >> > > #2 [<c028cd6c>] (svcerr_panic) from [<c028cdc4>] >> > > #3 [<c028cdc4>] (_SvcErr_) from [<c001474c>] >> > > #4 [<c001474c>] (die) from [<c00241f8>] >> > > #5 [<c00241f8>] (__do_kernel_fault) from [<c0560600>] >> > > #6 [<c0560600>] (do_page_fault) from [<c00092e8>] >> > > #7 [<c00092e8>] (do_DataAbort) from [<c055f9f0>] >> > > pc : [<c0112540>] lr : [<c0112518>] 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 [<c055f9f0>] (__dabt_svc) from [<c0112518>] >> > > #9 [<c0112540>] (test_clear_page_writeback) from [<c01046d4>] >> > > #10 [<c01046d4>] (end_page_writeback) from [<c0149bcc>] >> > > #11 [<c0149bcc>] (end_swap_bio_write) from [<c0261460>] >> > > #12 [<c0261460>] (bio_endio) from [<c042c800>] >> > > #13 [<c042c800>] (dec_pending) from [<c042e648>] >> > > #14 [<c042e648>] (clone_endio) from [<c0261460>] >> > > #15 [<c0261460>] (bio_endio) from [<bf60aa00>] >> > > #16 [<bf60aa00>] (crypt_dec_pending [dm_crypt]) from [<bf60c1e8>] >> > > #17 [<bf60c1e8>] (crypt_endio [dm_crypt]) from [<c0261460>] >> > > #18 [<c0261460>] (bio_endio) from [<c0269e34>] >> > > #19 [<c0269e34>] (blk_update_request) from [<c026a058>] >> > > #20 [<c026a058>] (blk_update_bidi_request) from [<c026a444>] >> > > #21 [<c026a444>] (blk_end_bidi_request) from [<c026a494>] >> > > #22 [<c026a494>] (blk_end_request) from [<c0458dbc>] >> > > #23 [<c0458dbc>] (mmc_blk_issue_rw_rq) from [<c0459e24>] >> > > #24 [<c0459e24>] (mmc_blk_issue_rq) from [<c045a018>] >> > > #25 [<c045a018>] (mmc_queue_thread) from [<c0048890>] >> > > #26 [<c0048890>] (kthread) from [<c0010388>] >> > > 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 [<c0559ab8>] (__schedule) from [<c0559edc>] >> > > #1 [<c0559edc>] (schedule) from [<c055e54c>] >> > > #2 [<c055e54c>] (schedule_timeout) from [<c055a3a4>] >> > > #3 [<c055a3a4>] (io_schedule_timeout) from [<c0106cb0>] >> > > #4 [<c0106cb0>] (mempool_alloc) from [<c0261668>] >> > > #5 [<c0261668>] (bio_alloc_bioset) from [<c0149d68>] >> > > #6 [<c0149d68>] (get_swap_bio) from [<c014a280>] >> > > #7 [<c014a280>] (__swap_writepage) from [<c014a3bc>] >> > > #8 [<c014a3bc>] (swap_writepage) from [<c011e5c8>] >> > > #9 [<c011e5c8>] (shmem_writepage) from [<c011a9b8>] >> > > #10 [<c011a9b8>] (shrink_page_list) from [<c011b528>] >> > > #11 [<c011b528>] (shrink_inactive_list) from [<c011c160>] >> > > #12 [<c011c160>] (shrink_node_memcg) from [<c011c400>] >> > > #13 [<c011c400>] (shrink_node) from [<c011d7dc>] >> > > #14 [<c011d7dc>] (kswapd) from [<c0048890>] >> > > #15 [<c0048890>] (kthread) from [<c0010388>] >> > > >> > > 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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