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

[Alexandre Ghiti <alex@ghiti.fr>]

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@google.com>
>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@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@rivosinc.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Thanks,

Alex



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