a crash when running strace from persistent memory

[Mikulas Patocka]

Hi

There's a bug when you run strace from dax-based filesystem.

-- create real or emulated persistent memory device (/dev/pmem0)
mkfs.ext2 /dev/pmem0
-- mount it
mount -t ext2 -o dax /dev/pmem0 /mnt/test
-- copy the system to it (well, you can copy just a few files that are 
   needed for running strace and ls)
cp -ax / /mnt/test
-- bind the system directories
mount --bind /dev /mnt/test/dev
mount --bind /proc /mnt/test/proc
mount --bind /sys /mnt/test/sys
-- run strace on the ls command
chroot /mnt/test/ strace /bin/ls

You get this warning and ls is killed with SIGSEGV.

I bisected the problem and it is caused by the commit 
17839856fd588f4ab6b789f482ed3ffd7c403e1f (gup: document and work around 
"COW can break either way" issue). When I revert the patch (on the kernel 
5.9-rc3), the bug goes away.

Mikulas


[   84.190961] ------------[ cut here ]------------
[   84.191504] WARNING: CPU: 6 PID: 1350 at mm/memory.c:2486 wp_page_copy.cold+0xdb/0xf6
[   84.192398] Modules linked in: ext2 uvesafb cfbfillrect cfbimgblt cn cfbcopyarea fb fbdev ipv6 tun autofs4 binfmt_misc configfs af_packet mousedev virtio_balloon virtio_rng evdev rng_core pcspkr button raid10 raid456 async_raid6_recov async_memcpy async_pq raid6_pq async_xor xor async_tx libcrc32c raid1 raid0 md_mod sd_mod t10_pi virtio_scsi virtio_net psmouse net_failover scsi_mod failover
[   84.196301] CPU: 6 PID: 1350 Comm: strace Not tainted 5.9.0-rc3 #6
[   84.197020] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   84.197685] RIP: 0010:wp_page_copy.cold+0xdb/0xf6
[   84.198231] Code: ff ff ff 0f 00 eb 8e 48 8b 3c 24 48 8b 74 24 08 ba 00 10 00 00 e8 33 87 1f 00 85 c0 74 1f 48 c7 c7 a7 2b ba 81 e8 cc b6 f0 ff <0f> 0b 48 8b 3c 24 e8 08 82 1f 00 41 be 01 00 00 00 eb ae 41 be 01
[   84.200410] RSP: 0018:ffff88940c1dba58 EFLAGS: 00010282
[   84.201035] RAX: 0000000000000006 RBX: ffff88940c1dbb00 RCX: 0000000000000000
[   84.201842] RDX: 0000000000000003 RSI: ffffffff81b9c0fa RDI: 00000000ffffffff
[   84.202650] RBP: ffffea004f0e4d80 R08: 0000000000000000 R09: 0000000000000000
[   84.203460] R10: 0000000000000046 R11: 0000000000000000 R12: 0000000000000000
[   84.204265] R13: ffff88940aa86318 R14: 00000000f7fac000 R15: ffff8893c8db3c40
[   84.205083] FS:  00007fd8a8320740(0000) GS:ffff88940fb80000(0000) knlGS:0000000000000000
[   84.206000] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   84.206664] CR2: 00000000f7fac000 CR3: 00000013c93a6000 CR4: 00000000000006a0
[   84.207481] Call Trace:
[   84.207883]  do_wp_page+0x172/0x6a0
[   84.208285]  handle_mm_fault+0xd0b/0x1540
[   84.208753]  __get_user_pages+0x21a/0x6c0
[   84.209213]  __get_user_pages_remote+0xc8/0x2a0
[   84.209735]  process_vm_rw_core.isra.0+0x1ac/0x440
[   84.210318]  ? __might_fault+0x26/0x40
[   84.210758]  ? _copy_from_user+0x6a/0xa0
[   84.211208]  ? __might_fault+0x26/0x40
[   84.211642]  ? _copy_from_user+0x6a/0xa0
[   84.212091]  process_vm_rw+0xd1/0x100
[   84.212511]  ? _copy_to_user+0x69/0x80
[   84.212946]  ? ptrace_get_syscall_info+0x9b/0x180
[   84.213484]  ? find_held_lock+0x2b/0x80
[   84.213926]  ? __x64_sys_ptrace+0x106/0x140
[   84.214405]  ? fpregs_assert_state_consistent+0x19/0x40
[   84.215002]  ? exit_to_user_mode_prepare+0x2d/0x120
[   84.215556]  __x64_sys_process_vm_readv+0x22/0x40
[   84.216103]  do_syscall_64+0x2d/0x80
[   84.216518]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   84.217098] RIP: 0033:0x7fd8a84896da
[   84.217512] Code: 48 8b 15 b9 f7 0b 00 f7 d8 64 89 02 b8 ff ff ff ff eb d2 e8 18 f0 00 00 0f 1f 84 00 00 00 00 00 49 89 ca b8 36 01 00 00 0f 05 <48> 3d 00 f0 ff ff 77 06 c3 0f 1f 44 00 00 48 8b 15 81 f7 0b 00 f7
[   84.219618] RSP: 002b:00007ffd08c3c678 EFLAGS: 00000246 ORIG_RAX: 0000000000000136
[   84.220563] RAX: ffffffffffffffda RBX: 00000000f7fac000 RCX: 00007fd8a84896da
[   84.221380] RDX: 0000000000000001 RSI: 00007ffd08c3c680 RDI: 0000000000000549
[   84.222194] RBP: 00007ffd08c3c760 R08: 0000000000000001 R09: 0000000000000000
[   84.222999] R10: 00007ffd08c3c690 R11: 0000000000000246 R12: 00000000f7faca80
[   84.223804] R13: 0000000000000580 R14: 0000000000000549 R15: 00005589a50eee80
[   84.223804] R13: 0000000000000580 R14: 0000000000000549 R15: 00005589a50eee80
[   84.224612] ---[ end trace d8dbf2da5dc1b7ca ]---