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

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

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.


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