Re: [PATCH v2 19/20] x86/mm: Add speculative pagefault handling

[Laurent Dufour]

On 21/08/2017 09:29, Anshuman Khandual wrote:
> On 08/18/2017 03:35 AM, Laurent Dufour wrote:
>> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>>
>> Try a speculative fault before acquiring mmap_sem, if it returns with
>> VM_FAULT_RETRY continue with the mmap_sem acquisition and do the
>> traditional fault.
>>
>> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>>
>> [Clearing of FAULT_FLAG_ALLOW_RETRY is now done in
>>  handle_speculative_fault()]
>> [Retry with usual fault path in the case VM_ERROR is returned by
>>  handle_speculative_fault(). This allows signal to be delivered]
>> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
>> ---
>>  arch/x86/include/asm/pgtable_types.h |  7 +++++++
>>  arch/x86/mm/fault.c                  | 19 +++++++++++++++++++
>>  2 files changed, 26 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
>> index bf9638e1ee42..4fd2693a037e 100644
>> --- a/arch/x86/include/asm/pgtable_types.h
>> +++ b/arch/x86/include/asm/pgtable_types.h
>> @@ -234,6 +234,13 @@ enum page_cache_mode {
>>  #define PGD_IDENT_ATTR	 0x001		/* PRESENT (no other attributes) */
>>  #endif
>>  
>> +/*
>> + * Advertise that we call the Speculative Page Fault handler.
>> + */
>> +#ifdef CONFIG_X86_64
>> +#define __HAVE_ARCH_CALL_SPF
>> +#endif
>> +
>>  #ifdef CONFIG_X86_32
>>  # include <asm/pgtable_32_types.h>
>>  #else
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index 2a1fa10c6a98..4c070b9a4362 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -1365,6 +1365,24 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>>  	if (error_code & PF_INSTR)
>>  		flags |= FAULT_FLAG_INSTRUCTION;
>>  
>> +#ifdef __HAVE_ARCH_CALL_SPF
>> +	if (error_code & PF_USER) {
>> +		fault = handle_speculative_fault(mm, address, flags);
>> +
>> +		/*
>> +		 * We also check against VM_FAULT_ERROR because we have to
>> +		 * raise a signal by calling later mm_fault_error() which
>> +		 * requires the vma pointer to be set. So in that case,
>> +		 * we fall through the normal path.
> 
> Cant mm_fault_error() be called inside handle_speculative_fault() ?
> Falling through the normal page fault path again just to raise a
> signal seems overkill. Looking into mm_fault_error(), it seems they
> are different for x86 and powerpc.
> 
> X86:
> 
> mm_fault_error(struct pt_regs *regs, unsigned long error_code,
>                unsigned long address, struct vm_area_struct *vma,
>                unsigned int fault)
> 
> powerpc:
> 
> mm_fault_error(struct pt_regs *regs, unsigned long addr, int fault)
> 
> Even in case of X86, I guess we would have reference to the faulting
> VMA (after the SRCU search) which can be used to call this function
> directly.

Yes I think this is doable in the case of x86.