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

[Jia He]

[On behalf of justin.he@xxxxxxx]

Hi Matthew

On 2019/9/20 23:53, 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.  We could also go with returning a
> vm_fault_t, but that would be more complex than needed today, I think.

Ok, will change the return type to bool as you suggested.
Thanks

---
Cheers,
Justin (Jia He)