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

[Kefeng Wang]

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@xxxxxxxxxxxxx> 
> > 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.