Re: [Xen-devel] Is: x86: mm: Fix vmalloc_fault oops during lazy MMU updates Was: Re: [PATCH] mm/x86: Flush lazy MMU when DEBUG_PAGEALLOC is set

["H. Peter Anvin" <hpa@xxxxxxxxx>]

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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