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, Jérôme Glisse suggested the refactoring described above.

Cc: Christoph Hellwig <hch@lst.de>
Suggested-by: Jérôme Glisse <redacted>
Reviewed-by: Dan Williams <redacted>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h | 17 +++++++++++++----
 mm/memremap.c      | 16 ++--------------
 mm/swap.c          | 24 ++++++++++++++++++++++++
 3 files changed, 39 insertions(+), 18 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c97ea3b694e6..77a4df06c8a7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -952,9 +952,10 @@ 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;

@@ -963,7 +964,6 @@ 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;

@@ -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);
                return;
+       }

        if (put_page_testzero(page))
                __put_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);