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

Jia He
2019-09-21T13:19:57Z

[On behalf of justin.he@arm.com]

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)