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. > 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? Thanks -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
