On 12/18/19 8:04 AM, Kirill A. Shutemov wrote:

On Mon, Dec 16, 2019 at 02:25:16PM -0800, John Hubbard wrote:

quoted

An upcoming patch changes and complicates the refcounting and
especially the "put page" aspects of it. In order to keep
everything clean, refactor the devmap page release routines:

* Rename put_devmap_managed_page() to page_is_devmap_managed(),
   and limit the functionality to "read only": return a bool,
   with no side effects.

* Add a new routine, put_devmap_managed_page(), to handle checking
   what kind of page it is, and what kind of refcount handling it
   requires.

* Rename __put_devmap_managed_page() to free_devmap_managed_page(),
   and limit the functionality to unconditionally freeing a devmap
   page.

What the reason to separate put_devmap_managed_page() from
free_devmap_managed_page() if free_devmap_managed_page() has exacly one
caller? Is it preparation for the next patches?

Yes. A later patch, #23, adds another caller: __unpin_devmap_managed_user_page().

...

quoted

@@ -971,7 +971,14 @@ static inline bool put_devmap_managed_page(struct page *page)
  	return false;
  }
  
+bool put_devmap_managed_page(struct page *page);
+
  #else /* CONFIG_DEV_PAGEMAP_OPS */
+static inline bool page_is_devmap_managed(struct page *page)
+{
+	return false;
+}
+
  static inline bool put_devmap_managed_page(struct page *page)
  {
  	return false;

@@ -1028,8 +1035,10 @@ static inline void put_page(struct page *page)
  	 * need to inform the device driver through callback. See
  	 * include/linux/memremap.h and HMM for details.
  	 */
-	if (put_devmap_managed_page(page))
+	if (page_is_devmap_managed(page)) {
+		put_devmap_managed_page(page);

put_devmap_managed_page() has yet another page_is_devmap_managed() check
inside. It looks strange.

Good point, it's an extra unnecessary check. So to clean it up, I'll note
that the "if" check is required here in put_page(), in order to stay out of
non-inlined function calls in the hot path (put_page()). So I'll do the
following:

* Leave the above code as it is here

* Simplify put_devmap_managed_page(), it was trying to do two separate things,
   and those two things have different requirements. So change it to a void
   function, with a WARN_ON_ONCE to assert that page_is_devmap_managed() is true,

* And change the other caller (release_pages()) to do that check.

...

quoted

@@ -1102,3 +1102,27 @@ void __init swap_setup(void)
  	 * _really_ don't want to cluster much more
  	 */
  }
+
+#ifdef CONFIG_DEV_PAGEMAP_OPS
+bool put_devmap_managed_page(struct page *page)
+{
+	bool is_devmap = page_is_devmap_managed(page);
+
+	if (is_devmap) {

Reversing the condition would save you an indentation level.

Yes. Done.

I'll also git-reply with an updated patch so you can see what it looks like.


thanks,
-- 
John Hubbard
NVIDIA