On 2021/8/2 18:42, Michal Hocko wrote:

On Mon 02-08-21 18:00:10, Miaohe Lin wrote:

quoted

On 2021/8/2 14:43, Michal Hocko wrote:

quoted

On Sat 31-07-21 10:05:51, Miaohe Lin wrote:

quoted

On 2021/7/30 14:44, Michal Hocko wrote:

quoted

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

quoted

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

Agree with you. But IMO it may not be a good idea to leave the rtpn without NULL check. We should defend
it though it could hardly happen. But I'm not insist on this check. I will drop this patch if you insist.

It is not that I would insist. I just do not see any point in the code
churn. This check is not going to ever trigger and there is nothing you
can do to recover anyway so crashing the kernel is likely the only
choice left.

I hope I get the point now. What you mean is nothing we can do to recover and panic'ing on a
NULL-pointer dereference is a perfect choice ? Should we declare that we leave the rtpn without
NULL check on purpose like below ?

Many thanks.

@@ -7109,8 +7109,12 @@ 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;
+               /*
+                * If this allocation 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.
+                */

I am not really sure this is really worth it. Really we do not really
want to have similar comments all over the early init code, do we?

Maybe not. Will drop this patch.

Thanks.

quoted

                rtpn->rb_root = RB_ROOT;
                rtpn->rb_rightmost = NULL;
                spin_lock_init(&rtpn->lock);