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.

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