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Date:   Fri, 20 Jul 2018 21:43:16 +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>,
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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 Fri, Jul 20, 2018 at 12:27 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > 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.
> 
> If you commit this under this reasoning, then please at least make it say:
> 
> /* XXX: The vmalloc_fault() code is buggy on PTI+PAE systems, and this
> is a workaround. */
> 
> Let's not have code in the kernel that pretends to make sense but is
> actually voodoo magic that works around bugs elsewhere.  It's no fun
> to maintain down the road.

Fair enough. Lemme amend it. Joerg is looking into it, but I surely want to
get that stuff some exposure in next ASAP.

Thanks,

	tglx