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Date:   Fri, 20 Jul 2018 21:27:44 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@kernel.org>
cc:     Joerg Roedel <joro@8bytes.org>, Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
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        Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>,
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        Joerg Roedel <jroedel@suse.de>,
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        Namhyung Kim <namhyung@kernel.org>
Subject: Re: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in
 all page-tables
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On Fri, 20 Jul 2018, Andy Lutomirski wrote:
> > On Jul 20, 2018, at 6:22 AM, Joerg Roedel <joro@8bytes.org> wrote:
> >
> > From: Joerg Roedel <jroedel@suse.de>
> >
> > The ring-buffer is accessed in the NMI handler, so we better
> > avoid faulting on it. Sync the vmalloc range with all
> > page-tables in system to make sure everyone has it mapped.
> >
> > This fixes a WARN_ON_ONCE() that can be triggered with PTI
> > enabled on x86-32:
> >
> >    WARNING: CPU: 4 PID: 0 at arch/x86/mm/fault.c:320 vmalloc_fault+0x220/0x230
> >
> > This triggers because with PTI enabled on an PAE kernel the
> > PMDs are no longer shared between the page-tables, so the
> > vmalloc changes do not propagate automatically.
> 
> It seems like it would be much more robust to fix the vmalloc_fault()
> code instead.

Right, but now the obvious fix for the issue at hand is this. We surely
should revisit this.

Thanks,

	tglx