Shakeel reported a crash in mem_cgroup_protected(), which
can be triggered by memcg reclaim if the legacy cgroup v1
use_hierarchy=0 mode is used:
[ 226.060572] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000120
[ 226.068310] PGD 8000001ff55da067 P4D 8000001ff55da067 PUD 1fdc7df067 PMD 0
[ 226.075191] Oops: 0000 [#4] SMP PTI
[ 226.078637] CPU: 0 PID: 15581 Comm: bash Tainted: G D
4.17.0-smp-clean #5
[ 226.086635] Hardware name: ...
[ 226.094546] RIP: 0010:mem_cgroup_protected+0x54/0x130
[ 226.099533] Code: 4c 8b 8e 00 01 00 00 4c 8b 86 08 01 00 00 48 8d
8a 08 ff ff ff 48 85 d2 ba 00 00 00 00 48 0f 44 ca 48 39 c8 0f 84 cf
00 00 00 <48> 8b 81 20 01 00 00 4d 89 ca 4c 39 c8 4c 0f 46 d0 4d 85 d2
74 05
[ 226.118194] RSP: 0000:ffffabe64dfafa58 EFLAGS: 00010286
[ 226.123358] RAX: ffff9fb6ff03d000 RBX: ffff9fb6f5b1b000 RCX: 0000000000000000
[ 226.130406] RDX: 0000000000000000 RSI: ffff9fb6f5b1b000 RDI: ffff9fb6f5b1b000
[ 226.137454] RBP: ffffabe64dfafb08 R08: 0000000000000000 R09: 0000000000000000
[ 226.144503] R10: 0000000000000000 R11: 000000000000c800 R12: ffffabe64dfafb88
[ 226.151551] R13: ffff9fb6f5b1b000 R14: ffffabe64dfafb88 R15: ffff9fb77fffe000
[ 226.158602] FS: 00007fed1f8ac700(0000) GS:ffff9fb6ff400000(0000)
knlGS:0000000000000000
[ 226.166594] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 226.172270] CR2: 0000000000000120 CR3: 0000001fdcf86003 CR4: 00000000001606f0
[ 226.179317] Call Trace:
[ 226.181732] ? shrink_node+0x194/0x510
[ 226.185435] do_try_to_free_pages+0xfd/0x390
[ 226.189653] try_to_free_mem_cgroup_pages+0x123/0x210
[ 226.194643] try_charge+0x19e/0x700
[ 226.198088] mem_cgroup_try_charge+0x10b/0x1a0
[ 226.202478] wp_page_copy+0x134/0x5b0
[ 226.206094] do_wp_page+0x90/0x460
[ 226.209453] __handle_mm_fault+0x8e3/0xf30
[ 226.213498] handle_mm_fault+0xfe/0x220
[ 226.217285] __do_page_fault+0x262/0x500
[ 226.221158] do_page_fault+0x28/0xd0
[ 226.224689] ? page_fault+0x8/0x30
[ 226.228048] page_fault+0x1e/0x30
[ 226.231323] RIP: 0033:0x485b72
The problem happens because parent_mem_cgroup() returns a NULL
pointer, which is dereferenced later without a check.
As cgroup v1 has no memory guarantee support, let's make
mem_cgroup_protected() immediately return MEMCG_PROT_NONE,
if the given cgroup has no parent (non-hierarchical mode is used).
Reported-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Tested-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Fixes: bf8d5d52ffe8 ("memcg: introduce memory.min")
---
mm/memcontrol.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c1e64d60ed02..5a3873e9d657 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5480,6 +5480,10 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
elow = memcg->memory.low;
parent = parent_mem_cgroup(memcg);
+ /* No parent means a non-hierarchical mode on v1 memcg */
+ if (!parent)
+ return MEMCG_PROT_NONE;
+
if (parent == root)
goto exit;
--
2.14.4
