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

From: Jia He <[email-masked]>
Date: Sat, 21 Sep 2019 21:19:34 +0800

[On behalf of [email-masked]]

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)

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