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

[Suren Baghdasaryan <surenb@google.com>]

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.
>
> > +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.
>
> > +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.

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.
>
> > +     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
> >
> >