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

[Kefeng Wang]

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." ?
>
> >   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.

> > > +     if (!(vmlf->fault_flags & FAULT_FLAG_USER))
> > > +             return -EINVAL;
> > > +
> > > +     vma = lock_vma_under_rcu(vmlf->mm, vmlf->address);
> > > +     if (!vma)
> > > +             return -EINVAL;
> > > +
> > > +     if (arch_vma_check_access(vma, vmlf)) {
> > > +             vma_end_read(vma);
> > > +             return -EINVAL;
> > > +     }
> > > +
> > > +     fault = handle_mm_fault(vma, vmlf->address,
> > > +                             vmlf->fault_flags | FAULT_FLAG_VMA_LOCK,
> > > +                             vmlf->regs);
> > > +     *ret = fault;
> > > +
> > > +     if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> > > +             vma_end_read(vma);
> > > +
> > > +     if ((fault & VM_FAULT_RETRY))
> > > +             count_vm_vma_lock_event(VMA_LOCK_RETRY);
> > > +     else
> > > +             count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> > > +
> > > +     return 0;
> > > +}
> > > +
> > >   #endif /* CONFIG_PER_VMA_LOCK */
> > >
> > >   #ifndef __PAGETABLE_P4D_FOLDED
> > > --
> > > 2.27.0