On Thu 29-07-21 20:12:43, Roman Gushchin wrote:

On Thu, Jul 29, 2021 at 08:57:54PM +0800, Miaohe Lin wrote:

quoted

rtpn might be NULL in very rare case. We have better to check it before
dereferencing it. Since memcg can live with NULL rb_tree_per_node in
soft_limit_tree, warn this case and continue.

Signed-off-by: Miaohe Lin <redacted>
---
 mm/memcontrol.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5b4592d1e0f2..70a32174e7c4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -7109,6 +7109,8 @@ static int __init mem_cgroup_init(void)
 		rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL,
 				    node_online(node) ? node : NUMA_NO_NODE);
 
+		if (WARN_ON_ONCE(!rtpn))
+			continue;

I also really doubt that it makes any sense to continue in this case.
If this allocations fails (at the very beginning of the system's life, it's an __init function),
something is terribly wrong and panic'ing on a NULL-pointer dereference sounds like
a perfect choice.

Moreover this is 24B allocation during early boot. Kernel will OOM and
panic when not being able to find any victim. I do not think we need to
do any special handling here.
-- 
Michal Hocko
SUSE Labs