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

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Date:   Wed, 09 Feb 2022 12:36:02 +0100
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     Xiongfeng Wang <wangxiongfeng2@...wei.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

On Wed, 2022-02-09 at 19:26 +0800, Xiongfeng Wang wrote:
> 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!

-- 
Nicolás Sáenz