mm: workingset: fix NULL ptr in count_shadow_nodes
Commit 0a6b76dd23fa ("mm: workingset: make shadow node shrinker memcg
aware") has made the workingset shadow nodes shrinker memcg aware. The
implementation is not correct though because memcg_kmem_enabled() might
become true while we are doing a global reclaim when the sc->memcg might
be NULL which is exactly what Marek has seen:
BUG: unable to handle kernel NULL pointer dereference at 0000000000000400
IP: [<ffffffff8122d520>] mem_cgroup_node_nr_lru_pages+0x20/0x40
PGD 0
Oops: 0000 [#1] SMP
CPU: 0 PID: 60 Comm: kswapd0 Tainted: G O 4.8.10-12.pvops.qubes.x86_64 #1
task: ffff880011863b00 task.stack: ffff880011868000
RIP: mem_cgroup_node_nr_lru_pages+0x20/0x40
RSP: e02b:ffff88001186bc70 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff88001186bd20 RCX: 0000000000000002
RDX: 000000000000000c RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffff88001186bc70 R08: 28f5c28f5c28f5c3 R09: 0000000000000000
R10: 0000000000006c34 R11: 0000000000000333 R12: 00000000000001f6
R13: ffffffff81c6f6a0 R14: 0000000000000000 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff880013c00000(0000) knlGS:ffff880013d00000
CS: e033 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000400 CR3: 00000000122f2000 CR4: 0000000000042660
Call Trace:
count_shadow_nodes+0x9a/0xa0
shrink_slab.part.42+0x119/0x3e0
shrink_node+0x22c/0x320
kswapd+0x32c/0x700
kthread+0xd8/0xf0
ret_from_fork+0x1f/0x40
Code: 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 3b 35 dd eb b1 00 55 48 89 e5 73 2c 89 d2 31 c9 31 c0 4c 63 ce 48 0f a3 ca 73 13 <4a> 8b b4 cf 00 04 00 00 41 89 c8 4a 03 84 c6 80 00 00 00 83 c1
RIP mem_cgroup_node_nr_lru_pages+0x20/0x40
RSP <ffff88001186bc70>
CR2: 0000000000000400
---[ end trace 100494b9edbdfc4d ]---
This patch fixes the issue by checking sc->memcg rather than
memcg_kmem_enabled() which is sufficient because shrink_slab makes sure
that only memcg aware shrinkers will get non-NULL memcgs and only if
memcg_kmem_enabled is true.
Fixes: 0a6b76dd23fa ("mm: workingset: make shadow node shrinker memcg aware")
Link: http://lkml.kernel.org/r/20161201132156.21450-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Marek Marczykowski-Górecki <marmarek@mimuw.edu.pl>
Tested-by: Marek Marczykowski-Górecki <marmarek@mimuw.edu.pl>
Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Cc: <stable@vger.kernel.org> [4.6+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/workingset.c b/mm/workingset.c
index 617475f..fb1f918 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -348,7 +348,7 @@
shadow_nodes = list_lru_shrink_count(&workingset_shadow_nodes, sc);
local_irq_enable();
- if (memcg_kmem_enabled()) {
+ if (sc->memcg) {
pages = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
LRU_ALL_FILE);
} else {