linux-kernel - Re: Is: x86: mm: Fix vmalloc_fault oops during lazy MMU updates Was: Re: [PATCH] mm/x86: Flush lazy MMU when DEBUG

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Date:	Thu, 28 Feb 2013 13:14:11 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Greg KH <gregkh@...uxfoundation.org>, mingo@...hat.com,
	tglx@...utronix.de, xen-devel@...ts.xen.org,
	linux-kernel@...r.kernel.org, samu.kallio@...rdeencloud.com,
	kraman@...hat.com, jwboyer@...hat.com
Subject: Re: Is: x86: mm: Fix vmalloc_fault oops during lazy MMU updates Was:
 Re: [PATCH] mm/x86: Flush lazy MMU when DEBUG_PAGEALLOC is set

On Thu, Feb 28, 2013 at 11:27:23AM -0500, Boris Ostrovsky wrote:
> On 02/28/2013 11:22 AM, Borislav Petkov wrote:
> >On Thu, Feb 28, 2013 at 11:20:20AM -0500, Boris Ostrovsky wrote:
> >>On 02/28/2013 11:10 AM, Borislav Petkov wrote:
> >>>On Thu, Feb 28, 2013 at 07:53:44AM -0800, H. Peter Anvin wrote:
> >>>>At the very least we should have an early filter for the **COMMON!**
> >>>>case that we are not on a PV platform.
> >>>... or, patch it out with the alternatives on baremetal, as Steven
> >>>suggested.
> 
> What was the suggestion exactly? I don't remember seeing that message.

Yeah, Borislav talked to me privately on IRC about this and I pointed
him to the apply_paravirt() function in arch/x86/kernel/alternative.c
where things that apply only to paravirt get patched out on baremetal.

It may add complexity, but there's a method for doing it and I rather
not burden baremetal for pravirt nonsense. I know adding a simple call
to preempt_count() can show a noticable impact to function tracing. It
requires referencing the gs segment register and doing some offset
games (as it's stored as a per cpu pointer) to find the stack.

I was actually a bit amazed that it had as big of an impact as it did. I
can understand why Christoph Lameter tried hard not to add a
preempt_disable() in his code for just a tiny location.

-- Steve

> 
> -boris
> 
> >>>
> >>I think making a check for paravirt_enabled() is safe enough. I'll
> >>send a patch shortly.
> >Why not make it absolutely for free?
> >
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