Hi Xiongfeng, thanks for taking the time to look at this.

On Wed, 2022-02-09 at 16:55 +0800, Xiongfeng Wang wrote:

Hi Nicolas，

When I applied the patchset on the following commit and tested on QEMU, I came
accross the following CallTrace.
  commit dd81e1c7d5fb126e5fbc5c9e334d7b3ec29a16a0

I wrote a userspace application to consume the memory. When the memory is used
out, the OOM killer is triggered and the following Calltrace is printed. I am
not sure if it is related to this patchset. But when I reverted this patchset,
the 'NULL pointer' Calltrace didn't show.

It's a silly mistake on my part, while cleaning up the code I messed up one of
the 'struct per_cpu_pages' accessors. This should fix it:

------------------------->8-------------------------

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0caa7155ca34..e65b991c3dc8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -3279,7 +3279,7 @@ static void __drain_all_pages(struct zone *zone, bool force_all_cpus)
                                has_pcps = true;
                } else {
                        for_each_populated_zone(z) {
-                               pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
+                               pcp = per_cpu_ptr(z->per_cpu_pageset, cpu);
                                lp = rcu_dereference_protected(pcp->lp,
                                                mutex_is_locked(&pcpu_drain_mutex));
                                if (lp->count) {
------------------------->8-------------------------

Thanks!

-- 
Nicolás Sáenz