Re: [PATCH rfc -next 01/10] mm: add a generic VMA lock-based page fault handler

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

On 2023/7/14 9:52, Kefeng Wang wrote:
> 
> 
> On 2023/7/14 4:12, Suren Baghdasaryan wrote:
>> On Thu, Jul 13, 2023 at 9:15 AM Matthew Wilcox <willy@infradead.org> 
>> wrote:
>>>
>>>> +int try_vma_locked_page_fault(struct vm_locked_fault *vmlf, 
>>>> vm_fault_t *ret)
>>>> +{
>>>> +     struct vm_area_struct *vma;
>>>> +     vm_fault_t fault;
>>>
>>>
>>> On Thu, Jul 13, 2023 at 05:53:29PM +0800, Kefeng Wang wrote:
>>>> +#define VM_LOCKED_FAULT_INIT(_name, _mm, _address, _fault_flags, 
>>>> _vm_flags, _regs, _fault_code) \
>>>> +     _name.mm                = _mm;                  \
>>>> +     _name.address           = _address;             \
>>>> +     _name.fault_flags       = _fault_flags;         \
>>>> +     _name.vm_flags          = _vm_flags;            \
>>>> +     _name.regs              = _regs;                \
>>>> +     _name.fault_code        = _fault_code
>>>
>>> More consolidated code is a good idea; no question.  But I don't think
>>> this is the right way to do it.
> 
> I agree it is not good enough, but the arch's vma check acess has
> different implementation, some use vm flags, some need fault code and
> regs, and some use both :(
> 
>>>
>>>> +int __weak arch_vma_check_access(struct vm_area_struct *vma,
>>>> +                              struct vm_locked_fault *vmlf);
>>>
>>> This should be:
>>>
>>> #ifndef vma_check_access
>>> bool vma_check_access(struct vm_area_struct *vma, )
>>> {
>>>          return (vma->vm_flags & vm_flags) == 0;
>>> }
>>> #endif
>>>
>>> and then arches which want to do something different can just define
>>> vma_check_access.
> 
> Ok, I could convert to use this way.
> 
>>>
>>>> +int try_vma_locked_page_fault(struct vm_locked_fault *vmlf, 
>>>> vm_fault_t *ret)
>>>> +{
>>>> +     struct vm_area_struct *vma;
>>>> +     vm_fault_t fault;
>>>
>>> Declaring the vmf in this function and then copying it back is just 
>>> wrong.
>>> We need to declare vm_fault_t earlier (in the arch fault handler) and
>>> pass it in.
> 
> Actually I passed the vm_fault_t *ret(in the arch fault handler), we
> could directly use *ret instead of a new local variable, and no copy.
>>
>> Did you mean to say "we need to declare vmf (struct vm_fault) earlier
>> (in the arch fault handler) and pass it in." ?

After recheck the code, I think Matthew' idea is 'declare vmf (struct 
vm_fault) earlier' like Suren said, not vm_fault_t, right? will try 
this, thanks.

>>
>>>   I don't think that creating struct vm_locked_fault is the
>>> right idea either.
> 
> As mentioned above for vma check access, we need many arguments for a 
> function, a new struct looks possible better, is there better solution
> or any suggestion?
> 
> Thanks.
>