linux-kernel - Re: [PATCH 0/2] mm/page_alloc: Remote per-cpu lists drain support

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Date:   Wed, 9 Feb 2022 19:26:29 +0800
From:   Xiongfeng Wang <wangxiongfeng2@...wei.com>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        <akpm@...ux-foundation.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <frederic@...nel.org>, <tglx@...utronix.de>, <mtosatti@...hat.com>,
        <mgorman@...e.de>, <linux-rt-users@...r.kernel.org>,
        <vbabka@...e.cz>, <cl@...ux.com>, <paulmck@...nel.org>,
        <willy@...radead.org>
Subject: Re: [PATCH 0/2] mm/page_alloc: Remote per-cpu lists drain support

Hi,

On 2022/2/9 17:45, Nicolas Saenz Julienne wrote:
> 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) {

I have tested it. It works well. No more 'NULL pointer' Calltrace.

Thanks,
Xiongfeng

> ------------------------->8-------------------------
> 
> Thanks!
>