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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
