Re: [PATCH 2/7] arm64: mm: accelerate pagefault when VM_FAULT_BADACCESS

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/4/3 13:30, Suren Baghdasaryan wrote:
> On Tue, Apr 2, 2024 at 10:19 PM Suren Baghdasaryan <surenb@google.com> wrote:
>>
>> On Tue, Apr 2, 2024 at 12:53 AM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>>>
>>> The vm_flags of vma already checked under per-VMA lock, if it is a
>>> bad access, directly set fault to VM_FAULT_BADACCESS and handle error,
>>> no need to lock_mm_and_find_vma() and check vm_flags again, the latency
>>> time reduce 34% in lmbench 'lat_sig -P 1 prot lat_sig'.
>>
>> The change makes sense to me. Per-VMA lock is enough to keep
>> vma->vm_flags stable, so no need to retry with mmap_lock.
>>
>>>
>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>
>> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
>>
>>> ---
>>>   arch/arm64/mm/fault.c | 4 +++-
>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>>> index 9bb9f395351a..405f9aa831bd 100644
>>> --- a/arch/arm64/mm/fault.c
>>> +++ b/arch/arm64/mm/fault.c
>>> @@ -572,7 +572,9 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
>>>
>>>          if (!(vma->vm_flags & vm_flags)) {
>>>                  vma_end_read(vma);
>>> -               goto lock_mmap;
>>> +               fault = VM_FAULT_BADACCESS;
>>> +               count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
>>
>> nit: VMA_LOCK_SUCCESS accounting here seems correct to me but
>> unrelated to the main change. Either splitting into a separate patch
>> or mentioning this additional fixup in the changelog would be helpful.
> 
> The above nit applies to all the patches after this one, so I won't
> comment on each one separately. If you decide to split or adjust the
> changelog please do that for each patch.

I will update the change log for each patch, thank for your review and 
suggestion.

> 
>>
>>> +               goto done;
>>>          }
>>>          fault = handle_mm_fault(vma, addr, mm_flags | FAULT_FLAG_VMA_LOCK, regs);
>>>          if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
>>> --
>>> 2.27.0
>>>