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From:   Pingfan Liu <kernelfans@gmail.com>
Date:   Tue, 9 Jul 2019 15:24:30 +0800
Message-ID: <CAFgQCTui7D6_FQ_v_ijj6k_=+TQzQ3PaGvzxd6p+XEGjQ2S6jw@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/numa: instance all parsed numa node
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     x86@kernel.org, Michal Hocko <mhocko@suse.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Tony Luck <tony.luck@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Oscar Salvador <osalvador@suse.de>,
        Pavel Tatashin <pavel.tatashin@microsoft.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Stephen Rothwell <sfr@canb.auug.org.au>, Qian Cai <cai@lca.pw>,
        Barret Rhoden <brho@google.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        David Rientjes <rientjes@google.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Jul 9, 2019 at 2:12 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Tue, 9 Jul 2019, Pingfan Liu wrote:
> > On Mon, Jul 8, 2019 at 5:35 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> > > It can and it does.
> > >
> > > That's the whole point why we bring up all CPUs in the 'nosmt' case and
> > > shut the siblings down again after setting CR4.MCE. Actually that's in fact
> > > a 'let's hope no MCE hits before that happened' approach, but that's all we
> > > can do.
> > >
> > > If we don't do that then the MCE broadcast can hit a CPU which has some
> > > firmware initialized state. The result can be a full system lockup, triple
> > > fault etc.
> > >
> > > So when the MCE hits a CPU which is still in the crashed kernel lala state,
> > > then all hell breaks lose.
> > Thank you for the comprehensive explain. With your guide, now, I have
> > a full understanding of the issue.
> >
> > But when I tried to add something to enable CR4.MCE in
> > crash_nmi_callback(), I realized that it is undo-able in some case (if
> > crashed, we will not ask an offline smt cpu to online), also it is
> > needless. "kexec -l/-p" takes the advantage of the cpu state in the
> > first kernel, where all logical cpu has CR4.MCE=1.
> >
> > So kexec is exempt from this bug if the first kernel already do it.
>
> No. If the MCE broadcast is handled by a CPU which is stuck in the old
> kernel stop loop, then it will execute on the old kernel and eventually run
> into the memory corruption which crashed the old one.
>
Yes, you are right. Stuck cpu may execute the old do_machine_check()
code. But I just found out that we have
do_machine_check()->__mc_check_crashing_cpu() to against this case.

And I think the MCE issue with nr_cpus is not closely related with
this series, can
be a separated issue. I had question whether Andy will take it, if
not, I am glad to do it.

Thanks and regards,
  Pingfan