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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Message-ID: <512F7D88.4090703@zytor.com>
Date:	Thu, 28 Feb 2013 07:53:44 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>, 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 02/28/2013 07:38 AM, Borislav Petkov wrote:
> On Thu, Feb 28, 2013 at 09:29:10AM -0500, Konrad Rzeszutek Wilk wrote:
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index fb674fd..4f7d793 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -378,10 +378,12 @@ static noinline __kprobes int vmalloc_fault(unsigned long address)
>>  	if (pgd_none(*pgd_ref))
>>  		return -1;
>>  
>> -	if (pgd_none(*pgd))
>> +	if (pgd_none(*pgd)) {
>>  		set_pgd(pgd, *pgd_ref);
>> -	else
>> +		arch_flush_lazy_mmu_mode();
> 
> Do I understand it correctly that this would cost us a
> "preempt_disable(); preempt_enable()" needlessly on baremetal when
> running with CONFIG_PARAVIRT enabled?
> 

OK, this is mind-boggingly crazy and these things that just drives me
batty about PV.

arch_flush_lazy_mmu_mode() involves a function call and preemption off
*before it even tests for being a noop*.  On bare metal this function
doesn't do anything, and cannot do anything.

At the very least we should have an early filter for the **COMMON!**
case that we are not on a PV platform.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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