[PATCH v2 5/7] riscv: mm: accelerate pagefault when badaccess

[Kefeng Wang]

On 2024/4/11 1:28, Alexandre Ghiti wrote:
> On 10/04/2024 10:07, Kefeng Wang wrote:
>>
>>
>> On 2024/4/10 15:32, Alexandre Ghiti wrote:
>>> Hi Kefeng,
>>>
>>> On 03/04/2024 10:38, Kefeng Wang wrote:
>>>> The access_error() of vma already checked under per-VMA lock, if it
>>>> is a bad access, directly handle error, no need to retry with mmap_lock
>>>> again. Since the page faut is handled under per-VMA lock, count it as
>>>> a vma lock event with VMA_LOCK_SUCCESS.
>>>>
>>>> Reviewed-by: Suren Baghdasaryan <surenb at google.com>
>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang at huawei.com>
>>>> ---
>>>>   arch/riscv/mm/fault.c | 5 ++++-
>>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
>>>> index 3ba1d4dde5dd..b3fcf7d67efb 100644
>>>> --- a/arch/riscv/mm/fault.c
>>>> +++ b/arch/riscv/mm/fault.c
>>>> @@ -292,7 +292,10 @@ void handle_page_fault(struct pt_regs *regs)
>>>>       if (unlikely(access_error(cause, vma))) {
>>>>           vma_end_read(vma);
>>>> -        goto lock_mmap;
>>>> +        count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
>>>> +        tsk->thread.bad_cause = SEGV_ACCERR;
>>>
>>>
>>> I think we should use the cause variable here instead of SEGV_ACCERR, 
>>> as bad_cause is a riscv internal status which describes the real 
>>> fault that happened.
>>
>> Oh, I see, it is exception causes on riscv, so it should be
>>
>> diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
>> index b3fcf7d67efb..5224f3733802 100644
>> --- a/arch/riscv/mm/fault.c
>> +++ b/arch/riscv/mm/fault.c
>> @@ -293,8 +293,8 @@ void handle_page_fault(struct pt_regs *regs)
>>         if (unlikely(access_error(cause, vma))) {
>>                 vma_end_read(vma);
>>                 count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
>> -               tsk->thread.bad_cause = SEGV_ACCERR;
>> -               bad_area_nosemaphore(regs, code, addr);
>> +               tsk->thread.bad_cause = cause;
>> +               bad_area_nosemaphore(regs, SEGV_ACCERR, addr);
>>                 return;
>>         }
>>
>> Hi Alex, could you help to check it?
>>
>> Hi Andrew, please help to squash it after Alex ack it.
>>
>> Thanks both.
> 
> 
> So I have just tested Kefeng's fixup on my usual CI and with a simple 
> program that triggers such bad access, everything went fine so with the 
> fixup applied:
> 
> Reviewed-by: Alexandre Ghiti <alexghiti at rivosinc.com>
> Tested-by: Alexandre Ghiti <alexghiti at rivosinc.com>

Great, thanks.

> 
> Thanks,
> 
> Alex
> 
> 
> 
>>
>>
>>>
>>> Thanks,
>>>
>>> Alex
>>>
>>>
>>>> +        bad_area_nosemaphore(regs, code, addr);
>>>> +        return;
>>>>       }
>>>>       fault = handle_mm_fault(vma, addr, flags | 
>>>> FAULT_FLAG_VMA_LOCK, regs);