Re: [PATCH v7 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

Kirill A. Shutemov
2019-09-20T17:00:35Z

On Fri, Sep 20, 2019 at 08:53:00AM -0700, Matthew Wilcox wrote:
> On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote:
> > -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
> > +static inline int cow_user_page(struct page *dst, struct page *src,
> > + struct vm_fault *vmf)
> > {
>
> Can we talk about the return type here?
>
> > + } else {
> > + /* Other thread has already handled the fault
> > + * and we don't need to do anything. If it's
> > + * not the case, the fault will be triggered
> > + * again on the same address.
> > + */
> > + pte_unmap_unlock(vmf->pte, vmf->ptl);
> > + return -1;
> ...
> > + return 0;
> > }
>
> So -1 for "try again" and 0 for "succeeded".
>
> > + if (cow_user_page(new_page, old_page, vmf)) {
>
> Then we use it like a bool. But it's kind of backwards from a bool because
> false is success.
>
> > + /* COW failed, if the fault was solved by other,
> > + * it's fine. If not, userspace would re-fault on
> > + * the same address and we will handle the fault
> > + * from the second attempt.
> > + */
> > + put_page(new_page);
> > + if (old_page)
> > + put_page(old_page);
> > + return 0;
>
> And we don't use the return value; in fact we invert it.
>
> Would this make more sense:
>
> static inline bool cow_user_page(struct page *dst, struct page *src,
> struct vm_fault *vmf)
> ...
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> return false;
> ...
> return true;
> ...
> if (!cow_user_page(new_page, old_page, vmf)) {
>
> That reads more sensibly for me.

I like this idea too.

--
Kirill A. Shutemov