[PATCH 2/2] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages

[John Hubbard]

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.

This is originally based on a separate patch by Ira Weiny, which
applied to an early version of the put_user_page() experiments.
Since then, JÃrÃme Glisse suggested the refactoring described above.

Cc: Jan Kara <jack@xxxxxxx>
Cc: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Suggested-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
 include/linux/mm.h | 27 ++++++++++++++++++++++++---
 mm/memremap.c      | 16 ++--------------
 2 files changed, 26 insertions(+), 17 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a2adf95b3f9c..96228376139c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -967,9 +967,10 @@ static inline bool is_zone_device_page(const struct page *page)
 #endif
 
 #ifdef CONFIG_DEV_PAGEMAP_OPS
-void __put_devmap_managed_page(struct page *page);
+void free_devmap_managed_page(struct page *page);
 DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
-static inline bool put_devmap_managed_page(struct page *page)
+
+static inline bool page_is_devmap_managed(struct page *page)
 {
 	if (!static_branch_unlikely(&devmap_managed_key))
 		return false;
@@ -978,7 +979,6 @@ static inline bool put_devmap_managed_page(struct page *page)
 	switch (page->pgmap->type) {
 	case MEMORY_DEVICE_PRIVATE:
 	case MEMORY_DEVICE_FS_DAX:
-		__put_devmap_managed_page(page);
 		return true;
 	default:
 		break;
@@ -986,6 +986,27 @@ static inline bool put_devmap_managed_page(struct page *page)
 	return false;
 }
 
+static inline bool put_devmap_managed_page(struct page *page)
+{
+	bool is_devmap = page_is_devmap_managed(page);
+
+	if (is_devmap) {
+		int count = page_ref_dec_return(page);
+
+		/*
+		 * devmap page refcounts are 1-based, rather than 0-based: if
+		 * refcount is 1, then the page is free and the refcount is
+		 * stable because nobody holds a reference on the page.
+		 */
+		if (count == 1)
+			free_devmap_managed_page(page);
+		else if (!count)
+			__put_page(page);
+	}
+
+	return is_devmap;
+}
+
 #else /* CONFIG_DEV_PAGEMAP_OPS */
 static inline bool put_devmap_managed_page(struct page *page)
 {
diff --git a/mm/memremap.c b/mm/memremap.c
index e899fa876a62..2ba773859031 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -411,20 +411,8 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
 EXPORT_SYMBOL_GPL(get_dev_pagemap);
 
 #ifdef CONFIG_DEV_PAGEMAP_OPS
-void __put_devmap_managed_page(struct page *page)
+void free_devmap_managed_page(struct page *page)
 {
-	int count = page_ref_dec_return(page);
-
-	/* still busy */
-	if (count > 1)
-		return;
-
-	/* only triggered by the dev_pagemap shutdown path */
-	if (count == 0) {
-		__put_page(page);
-		return;
-	}
-
 	/* notify page idle for dax */
 	if (!is_device_private_page(page)) {
 		wake_up_var(&page->_refcount);
@@ -461,5 +449,5 @@ void __put_devmap_managed_page(struct page *page)
 	page->mapping = NULL;
 	page->pgmap->ops->page_free(page);
 }
-EXPORT_SYMBOL(__put_devmap_managed_page);
+EXPORT_SYMBOL(free_devmap_managed_page);
 #endif /* CONFIG_DEV_PAGEMAP_OPS */
-- 
2.24.0