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.
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