[REGRESSION v5.13-rc1] NULL dereference in do_shrink_slab()

["NOMURA JUNICHI(野村 淳一)" <junichi.nomura@nec.com>]

v5.13-rc1 sometimes causes NULL pointer dereference during kdump, where
memcg is disabled with "cgroup_disable=memory" boot option.
I haven't seen this problem with v5.12, so it looks like regression.

[   73.199590] BUG: kernel NULL pointer dereference, address: 0000000000000000
[   73.206593] #PF: supervisor write access in kernel mode
[   73.211845] #PF: error_code(0x0002) - not-present page
[   73.217010] PGD 0 P4D 0
[   73.219556] Oops: 0002 [#1] SMP NOPTI
[   73.223236] CPU: 0 PID: 95 Comm: kswapd0 Tainted: G          I       5.13.0-rc1 #1
[   73.239418] RIP: 0010:do_shrink_slab+0x85/0x2d0
[   73.243977] Code: 49 63 44 24 04 be 00 00 00 00 49 8b 4c 24 18 f6 c2 02 48 0f 44 c6 48 85 c9 74 09 83 e2 04 0f 85 19 02 00 00 49 8b 4f 38 31 d2 <48> 87 14 c1 48 89 55 b8 41 8b 77 18 4c 89 f0 85 f6 0f 84 82 01 00
[   73.262856] RSP: 0018:ffffc900001abc18 EFLAGS: 00010246
[   73.268108] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[   73.275281] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000064
[   73.282454] RBP: ffffc900001abc70 R08: 28f5c28f5c28f5c3 R09: 0000000000000000
[   73.289628] R10: 0000000000000000 R11: 0000000000000004 R12: ffffc900001abca0
[   73.296800] R13: 0000000000000400 R14: 0000000000000002 R15: ffff88805344bc10
[   73.303972] FS:  0000000000000000(0000) GS:ffff888072c00000(0000) knlGS:0000000000000000
[   73.312108] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   73.317883] CR2: 0000000000000000 CR3: 000000005cf68004 CR4: 00000000007706b0
[   73.325055] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   73.332227] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   73.339400] PKRU: 55555554
[   73.342117] Call Trace:
[   73.344576]  shrink_slab+0xa9/0x2b0
[   73.348083]  ? __update_load_avg_se+0x298/0x320
[   73.352640]  shrink_node+0x248/0x6f0
[   73.356234]  balance_pgdat+0x303/0x5f0
[   73.360002]  kswapd+0x20b/0x390
[   73.363157]  ? finish_wait+0x80/0x80
[   73.366752]  ? balance_pgdat+0x5f0/0x5f0
[   73.370693]  kthread+0x124/0x140
[   73.373937]  ? kthread_park+0x90/0x90
[   73.377617]  ret_from_fork+0x1f/0x30
[   73.381215] Modules linked in: xfs libcrc32c sd_mod t10_pi sr_mod cdrom sg crc32c_intel ahci libahci libata smartpqi scsi_transport_sas overlay squashfs loop
[   73.395386] CR2: 0000000000000000
[   73.398716] ---[ end trace 9752d71309d33c00 ]---

The code around do_shrink_slab+0x85 is:
0xffffffff9d094925 <do_shrink_slab+0x65>:       mov    0x18(%r12),%rcx
0xffffffff9d09492a <do_shrink_slab+0x6a>:       test   $0x2,%dl
0xffffffff9d09492d <do_shrink_slab+0x6d>:       cmove  %rsi,%rax
0xffffffff9d094931 <do_shrink_slab+0x71>:       test   %rcx,%rcx
0xffffffff9d094934 <do_shrink_slab+0x74>:       je     0xffffffff9d09493f <do_shrink_slab+0x7f>
0xffffffff9d094936 <do_shrink_slab+0x76>:       and    $0x4,%edx
0xffffffff9d094939 <do_shrink_slab+0x79>:       jne    0xffffffff9d094b58 <do_shrink_slab+0x298>
0xffffffff9d09493f <do_shrink_slab+0x7f>:       mov    0x38(%r15),%rcx
0xffffffff9d094943 <do_shrink_slab+0x83>:       xor    %edx,%edx
0xffffffff9d094945 <do_shrink_slab+0x85>:       xchg   %rdx,(%rcx,%rax,8)

The NULL dereference occurred at here in in-lined xchg_nr_deferred():

         return atomic_long_xchg(&shrinker->nr_deferred[nid], 0);

that means "shrinker->nr_deferred" was NULL.

Though I haven't fully bisected between v5.12 and v5.13-rc1, I can reproduce
the problem with this commit:

   476b30a0949a mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers

but not with this previous commit:

   867508304685 mm: vmscan: use per memcg nr_deferred of shrinker

With the commit 476b30a0949a, if a memcg-aware shrinker is registered before
cgroup_init(), shrinker->nr_deferred is NULL.  However xchg_nr_deferred()
tries to use it as memcg is turned off via "cgroup_disable=memory".

Any thoughts?

-- 
Jun'ichi Nomura, NEC Corporation / NEC Solution Innovators, Ltd.