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SubjectRe: [PATCH 2/2] arm64/mm: add speculative page fault
2018-05-02 22:46 GMT+08:00 Punit Agrawal <punit.agrawal@arm.com>:
> Hi Ganesh,
>
> I was looking at evaluating speculative page fault handling on arm64 and
> noticed your patch.
>
> Some comments below -

Thanks for your review.

>
> Ganesh Mahendran <opensource.ganesh@gmail.com> writes:
>
>> This patch enables the speculative page fault on the arm64
>> architecture.
>>
>> I completed spf porting in 4.9. From the test result,
>> we can see app launching time improved by about 10% in average.
>> For the apps which have more than 50 threads, 15% or even more
>> improvement can be got.
>>
>> Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
>> ---
>> This patch is on top of Laurent's v10 spf
>> ---
>> arch/arm64/mm/fault.c | 38 +++++++++++++++++++++++++++++++++++---
>> 1 file changed, 35 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index 4165485..e7992a3 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -322,11 +322,13 @@ static void do_bad_area(unsigned long addr, unsigned int esr, struct pt_regs *re
>>
>> static int __do_page_fault(struct mm_struct *mm, unsigned long addr,
>> unsigned int mm_flags, unsigned long vm_flags,
>> - struct task_struct *tsk)
>> + struct task_struct *tsk, struct vm_area_struct *vma)
>> {
>> - struct vm_area_struct *vma;
>> int fault;
>>
>> + if (!vma || !can_reuse_spf_vma(vma, addr))
>> + vma = find_vma(mm, addr);
>> +
>
> It would be better to move this hunk to do_page_fault().
>
> It'll help localise the fact that handle_speculative_fault() is a
> stateful call which needs a corresponding can_reuse_spf_vma() to
> properly update the vma reference counting.

Yes, your suggestion is better.

>
>
>> vma = find_vma(mm, addr);
>
> Remember to drop this call in the next version. As it stands the call
> the find_vma() needlessly gets duplicated.

Will fix

>
>> fault = VM_FAULT_BADMAP;
>> if (unlikely(!vma))
>> @@ -371,6 +373,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>> int fault, major = 0;
>> unsigned long vm_flags = VM_READ | VM_WRITE;
>> unsigned int mm_flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
>> + struct vm_area_struct *vma;
>>
>> if (notify_page_fault(regs, esr))
>> return 0;
>> @@ -409,6 +412,25 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>>
>> perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
>>
>> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) {
>
> You don't need the IS_ENABLED() check. The alternate implementation of
> handle_speculative_fault() when CONFIG_SPECULATIVE_PAGE_FAULT is not
> enabled takes care of this.

Will fix

>
>> + fault = handle_speculative_fault(mm, addr, mm_flags, &vma);
>> + /*
>> + * Page fault is done if VM_FAULT_RETRY is not returned.
>> + * But if the memory protection keys are active, we don't know
>> + * if the fault is due to key mistmatch or due to a
>> + * classic protection check.
>> + * To differentiate that, we will need the VMA we no
>> + * more have, so let's retry with the mmap_sem held.
>> + */
>
> As there is no support for memory protection keys on arm64 most of this
> comment can be dropped.

will fix

>
>> + if (fault != VM_FAULT_RETRY &&
>> + fault != VM_FAULT_SIGSEGV) {
>
> Not sure if you need the VM_FAULT_SIGSEGV here.
>
>> + perf_sw_event(PERF_COUNT_SW_SPF, 1, regs, addr);
>> + goto done;
>> + }
>> + } else {
>> + vma = NULL;
>> + }
>> +
>
> If vma is initiliased to NULL during declaration, the else part can be
> dropped.

will fix

>
>> /*
>> * As per x86, we may deadlock here. However, since the kernel only
>> * validly references user space from well defined areas of the code,
>> @@ -431,7 +453,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>> #endif
>> }
>>
>> - fault = __do_page_fault(mm, addr, mm_flags, vm_flags, tsk);
>> + fault = __do_page_fault(mm, addr, mm_flags, vm_flags, tsk, vma);
>> major |= fault & VM_FAULT_MAJOR;
>>
>> if (fault & VM_FAULT_RETRY) {
>> @@ -454,11 +476,21 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>> if (mm_flags & FAULT_FLAG_ALLOW_RETRY) {
>> mm_flags &= ~FAULT_FLAG_ALLOW_RETRY;
>> mm_flags |= FAULT_FLAG_TRIED;
>> +
>> + /*
>> + * Do not try to reuse this vma and fetch it
>> + * again since we will release the mmap_sem.
>> + */
>> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
>> + vma = NULL;
>
> Please drop the IS_ENABLED() check.

will fix

>
> Thanks,
> Punit
>
>> +
>> goto retry;
>> }
>> }
>> up_read(&mm->mmap_sem);
>>
>> +done:
>> +
>> /*
>> * Handle the "normal" (no error) case first.
>> */

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