[PATCH v3 1/2] mm/memremap.c: Add pfn_to_devmap_page() to get page in ZONE_DEVICE

[sxwjean]

From: Xiongwei Song <sxwjean@xxxxxxxxx>

when requesting page information by /proc/kpage*, the pages in ZONE_DEVICE
were ignored . We need a function to help on this.

The pfn_to_devmap_page() function like pfn_to_online_page(), but only
concerns the pages in ZONE_DEVICE.

Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Xiongwei Song <sxwjean@xxxxxxxxx>
---
v3: Before returning page pointer, check validity of page by 
    pgmap_pfn_valid().
v2: Simplify pfn_to_devmap_page() as David suggested.
---
 include/linux/memremap.h |  8 ++++++++
 mm/memremap.c            | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index c0e9d35889e8..621723e9c4a5 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -137,6 +137,8 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
 void devm_memunmap_pages(struct device *dev, struct dev_pagemap *pgmap);
 struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
 		struct dev_pagemap *pgmap);
+struct page *pfn_to_devmap_page(unsigned long pfn,
+		struct dev_pagemap **pgmap);
 bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn);
 
 unsigned long vmem_altmap_offset(struct vmem_altmap *altmap);
@@ -166,6 +168,12 @@ static inline struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
 	return NULL;
 }
 
+static inline struct page *pfn_to_devmap_page(unsigned long pfn,
+		struct dev_pagemap **pgmap)
+{
+	return NULL;
+}
+
 static inline bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn)
 {
 	return false;
diff --git a/mm/memremap.c b/mm/memremap.c
index 5a66a71ab591..782309b74d71 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -494,6 +494,25 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
 }
 EXPORT_SYMBOL_GPL(get_dev_pagemap);
 
+/**
+ * pfn_to_devmap_page - get page pointer which belongs to dev_pagemap by @pfn
+ * @pfn: page frame number to lookup page_map
+ * @pgmap: to save pgmap address which is for putting reference
+ *
+ * If @pgmap is non-NULL, then pfn is on ZONE_DEVICE. Meanwhile check if
+ * pfn is valid in @pgmap, if yes return page pointer.
+ */
+struct page *pfn_to_devmap_page(unsigned long pfn, struct dev_pagemap **pgmap)
+{
+	if (pfn_valid(pfn)) {
+		*pgmap = get_dev_pagemap(pfn, NULL);
+		if (*pgmap && pgmap_pfn_valid(*pgmap, pfn))
+			return pfn_to_page(pfn);
+	}
+
+	return NULL;
+}
+
 #ifdef CONFIG_DEV_PAGEMAP_OPS
 void free_devmap_managed_page(struct page *page)
 {
-- 
2.30.2