Re: [PATCH 2/3] powerpc/mm: handle VM_FAULT_RETRY earlier

Laurent Dufour
2017-03-21T13:41:07Z

On 21/03/2017 10:12, Aneesh Kumar K.V wrote:
> Laurent Dufour <ldufour@linux.vnet.ibm.com> writes:
>
>> In do_page_fault() if handle_mm_fault() returns VM_FAULT_RETRY, retry
>> the page fault handling before anything else.
>>
>> This would simplify the handling of the mmap_sem lock in this part of
>> the code.
>>
>> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
>> ---
>> arch/powerpc/mm/fault.c | 67 ++++++++++++++++++++++++++++---------------------
>> 1 file changed, 38 insertions(+), 29 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index ee09604bbe12..2a6bc7e6e69a 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -434,6 +434,26 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>> * the fault.
>> */
>> fault = handle_mm_fault(vma, address, flags);
>> +
>> + /*
>> + * Handle the retry right now, the mmap_sem has been released in that
>> + * case.
>> + */
>> + if (unlikely(fault & VM_FAULT_RETRY)) {
>> + /* We retry only once */
>> + if (flags & FAULT_FLAG_ALLOW_RETRY) {
>> + /*
>> + * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
>> + * of starvation.
>> + */
>> + flags &= ~FAULT_FLAG_ALLOW_RETRY;
>> + flags |= FAULT_FLAG_TRIED;
>> + if (!fatal_signal_pending(current))
>> + goto retry;
>> + }
>> + /* We will enter mm_fault_error() below */
>> + }
>> +
>> if (unlikely(fault & (VM_FAULT_RETRY|VM_FAULT_ERROR))) {
>> if (fault & VM_FAULT_SIGSEGV)
>> goto bad_area;
>> @@ -445,38 +465,27 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>> }
>
> We could make it further simpler, by handling the FAULT_RETRY without
> FLAG_ALLOW_RETRY set earlier. But i guess that can be done later ?

Thanks for the review,

I agree that double checking against VM_FAULT_RETRY is confusing here.

But handling all the retry path in the first if() statement means that
we'll have to handle part of the mm_fault_error() code and segv here...
Unless we can't identify what is really relevant in that retry path.

It would take time to review all that tricky part, but I agree it should
be simplified later.

>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>
>
>>
>> /*
>> - * Major/minor page fault accounting is only done on the
>> - * initial attempt. If we go through a retry, it is extremely
>> - * likely that the page will be found in page cache at that point.
>> + * Major/minor page fault accounting.
>> */
>> - if (flags & FAULT_FLAG_ALLOW_RETRY) {
>> - if (fault & VM_FAULT_MAJOR) {
>> - current->maj_flt++;
>> - perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
>> - regs, address);
>> + if (fault & VM_FAULT_MAJOR) {
>> + current->maj_flt++;
>> + perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
>> + regs, address);
>> #ifdef CONFIG_PPC_SMLPAR
>> - if (firmware_has_feature(FW_FEATURE_CMO)) {
>> - u32 page_ins;
>> -
>> - preempt_disable();
>> - page_ins = be32_to_cpu(get_lppaca()->page_ins);
>> - page_ins += 1 << PAGE_FACTOR;
>> - get_lppaca()->page_ins = cpu_to_be32(page_ins);
>> - preempt_enable();
>> - }
>> -#endif /* CONFIG_PPC_SMLPAR */
>> - } else {
>> - current->min_flt++;
>> - perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
>> - regs, address);
>> - }
>> - if (fault & VM_FAULT_RETRY) {
>> - /* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
>> - * of starvation. */
>> - flags &= ~FAULT_FLAG_ALLOW_RETRY;
>> - flags |= FAULT_FLAG_TRIED;
>> - goto retry;
>> + if (firmware_has_feature(FW_FEATURE_CMO)) {
>> + u32 page_ins;
>> +
>> + preempt_disable();
>> + page_ins = be32_to_cpu(get_lppaca()->page_ins);
>> + page_ins += 1 << PAGE_FACTOR;
>> + get_lppaca()->page_ins = cpu_to_be32(page_ins);
>> + preempt_enable();
>> }
>> +#endif /* CONFIG_PPC_SMLPAR */
>> + } else {
>> + current->min_flt++;
>> + perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
>> + regs, address);
>> }
>>
>> up_read(&mm->mmap_sem);
>> --
>> 2.7.4