Re: [PATCH v2 20/20] powerpc/mm: Add speculative page fault

[Laurent Dufour <ldufour@linux.vnet.ibm.com>]

On 21/08/2017 08:58, Anshuman Khandual wrote:
> On 08/18/2017 03:35 AM, Laurent Dufour wrote:
>> This patch enable the speculative page fault on the PowerPC
>> architecture.
>>
>> This will try a speculative page fault without holding the mmap_sem,
>> if it returns with WM_FAULT_RETRY, the mmap_sem is acquired and the
> 
> s/WM_FAULT_RETRY/VM_FAULT_RETRY/

Good catch ;)

>> traditional page fault processing is done.
>>
>> Support is only provide for BOOK3S_64 currently because:
>> - require CONFIG_PPC_STD_MMU because checks done in
>>   set_access_flags_filter()
> 
> What checks are done in set_access_flags_filter() ? We are just
> adding the code block in do_page_fault().

set_access_flags_filter() is checking for vm_flags & VM_EXEC which may be
changed in our back, leading to a spurious WARN displayed.
This being said, I focused on the BOOK3S as this meaningful for large
system, and I didn't get time to check for embedded systems.

> 
>> - require BOOK3S because we can't support for book3e_hugetlb_preload()
>>   called by update_mmu_cache()
>>
>> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
>> ---
>>  arch/powerpc/include/asm/book3s/64/pgtable.h |  5 +++++
>>  arch/powerpc/mm/fault.c                      | 30 +++++++++++++++++++++++++++-
>>  2 files changed, 34 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index 818a58fc3f4f..897f8b9f67e6 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -313,6 +313,11 @@ extern unsigned long pci_io_base;
>>  /* Advertise support for _PAGE_SPECIAL */
>>  #define __HAVE_ARCH_PTE_SPECIAL
>>  
>> +/* Advertise that we call the Speculative Page Fault handler */
>> +#if defined(CONFIG_PPC_BOOK3S_64)
>> +#define __HAVE_ARCH_CALL_SPF
>> +#endif
>> +
>>  #ifndef __ASSEMBLY__
>>  
>>  /*
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index 4c422632047b..7b3cc4c30eab 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -291,9 +291,36 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>>  	if (is_write && is_user)
>>  		store_update_sp = store_updates_sp(regs);
>>  
>> -	if (is_user)
>> +	if (is_user) {
>>  		flags |= FAULT_FLAG_USER;
>>  
>> +#if defined(__HAVE_ARCH_CALL_SPF)
>> +		/* let's try a speculative page fault without grabbing the
>> +		 * mmap_sem.
>> +		 */
>> +
>> +		/*
>> +		 * flags is set later based on the VMA's flags, for the common
>> +		 * speculative service, we need some flags to be set.
>> +		 */
>> +		if (is_write)
>> +			flags |= FAULT_FLAG_WRITE;
>> +
>> +		fault = handle_speculative_fault(mm, address, flags);
>> +		if (!(fault & VM_FAULT_RETRY || fault & VM_FAULT_ERROR)) {
>> +			perf_sw_event(PERF_COUNT_SW_SPF_DONE, 1,
>> +				      regs, address);
>> +			goto done;
> 
> Why we should retry with classical page fault on VM_FAULT_ERROR ?
> We should always return VM_FAULT_RETRY in case there is a clear
> collision some where which requires retry with classical method
> and return VM_FAULT_ERROR in cases where we know that it cannot
> be retried and fail for good. Should not handle_speculative_fault()
> be changed to accommodate this ?

There is no need to change handle_speculative_fault(), it should return
VM_FAULT_RETRY when a retry is required. If VM_FAULT_ERROR is return, we
should be able to jump to the block dealing with VM_FAULT_ERROR and calling
vm_fault_error().


> 
>> +		}
>> +
>> +		/*
>> +		 * Resetting flags since the following code assumes
>> +		 * FAULT_FLAG_WRITE is not set.
>> +		 */
>> +		flags &= ~FAULT_FLAG_WRITE;
>> +#endif /* defined(__HAVE_ARCH_CALL_SPF) */
> 
> Setting and resetting of FAULT_FLAG_WRITE seems confusing. Why you
> say that some flags need to be set for handle_speculative_fault()
> function. Could you elaborate on this ?

FAULT_FLAG_WRITE is required to handle write access. In the case we retry
with the classical path, the flag is reset and will be set later if
!is_exec and is_write.