[HMM 08/18] mm/ZONE_DEVICE: special case put_page() for device private pages v2

["Jérôme Glisse" <jglisse@redhat.com>]

A ZONE_DEVICE page that reach a refcount of 1 is free ie no longer
have any user. For device private pages this is important to catch
and thus we need to special case put_page() for this.

Changed since v1:
  - use static key to disable special code path in put_page() by
    default
  - uninline put_zone_device_private_page()
  - fix build issues with some kernel config related to header
    inter-dependency

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
---
 include/linux/memremap.h | 13 +++++++++++++
 include/linux/mm.h       | 31 ++++++++++++++++++++++---------
 kernel/memremap.c        | 19 ++++++++++++++++++-
 mm/page_alloc.c          |  6 ++++++
 4 files changed, 59 insertions(+), 10 deletions(-)

diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 0fcf840..0e0d2e6 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -125,6 +125,14 @@ struct dev_pagemap {
 void *devm_memremap_pages(struct device *dev, struct resource *res,
 		struct percpu_ref *ref, struct vmem_altmap *altmap);
 struct dev_pagemap *find_dev_pagemap(resource_size_t phys);
+
+static inline bool is_zone_device_page(const struct page *page);
+
+static inline bool is_device_private_page(const struct page *page)
+{
+	return is_zone_device_page(page) &&
+		page->pgmap->type == MEMORY_DEVICE_PRIVATE;
+}
 #else
 static inline void *devm_memremap_pages(struct device *dev,
 		struct resource *res, struct percpu_ref *ref,
@@ -143,6 +151,11 @@ static inline struct dev_pagemap *find_dev_pagemap(resource_size_t phys)
 {
 	return NULL;
 }
+
+static inline bool is_device_private_page(const struct page *page)
+{
+	return false;
+}
 #endif
 
 /**
diff --git a/include/linux/mm.h b/include/linux/mm.h
index a825dab..7f0656f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -23,6 +23,7 @@
 #include <linux/page_ext.h>
 #include <linux/err.h>
 #include <linux/page_ref.h>
+#include <linux/memremap.h>
 
 struct mempolicy;
 struct anon_vma;
@@ -788,25 +789,25 @@ static inline bool is_zone_device_page(const struct page *page)
 {
 	return page_zonenum(page) == ZONE_DEVICE;
 }
-
-static inline bool is_device_private_page(const struct page *page)
-{
-	/* See MEMORY_DEVICE_PRIVATE in include/linux/memory_hotplug.h */
-	return ((page_zonenum(page) == ZONE_DEVICE) &&
-		(page->pgmap->type == MEMORY_DEVICE_PRIVATE));
-}
 #else
 static inline bool is_zone_device_page(const struct page *page)
 {
 	return false;
 }
+#endif
 
-static inline bool is_device_private_page(const struct page *page)
+#ifdef CONFIG_DEVICE_PRIVATE
+void put_zone_device_private_page(struct page *page);
+#else
+static inline void put_zone_device_private_page(struct page *page)
 {
-	return false;
 }
 #endif
 
+static inline bool is_device_private_page(const struct page *page);
+
+DECLARE_STATIC_KEY_FALSE(device_private_key);
+
 static inline void get_page(struct page *page)
 {
 	page = compound_head(page);
@@ -822,6 +823,18 @@ static inline void put_page(struct page *page)
 {
 	page = compound_head(page);
 
+	/*
+	 * For private device pages we need to catch refcount transition from
+	 * 2 to 1, when refcount reach one it means the private device page is
+	 * free and we need to inform the device driver through callback. See
+	 * include/linux/memremap.h and HMM for details.
+	 */
+	if (static_branch_unlikely(&device_private_key) &&
+	    unlikely(is_device_private_page(page))) {
+		put_zone_device_private_page(page);
+		return;
+	}
+
 	if (put_page_testzero(page))
 		__put_page(page);
 }
diff --git a/kernel/memremap.c b/kernel/memremap.c
index cd596d4..b9baa6c 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -11,7 +11,6 @@
  * General Public License for more details.
  */
 #include <linux/radix-tree.h>
-#include <linux/memremap.h>
 #include <linux/device.h>
 #include <linux/types.h>
 #include <linux/pfn_t.h>
@@ -464,3 +463,21 @@ struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
 	return pgmap ? pgmap->altmap : NULL;
 }
 #endif /* CONFIG_ZONE_DEVICE */
+
+
+#ifdef CONFIG_DEVICE_PRIVATE
+void put_zone_device_private_page(struct page *page)
+{
+	int count = page_ref_dec_return(page);
+
+	/*
+	 * If refcount is 1 then page is freed and refcount is stable as nobody
+	 * holds a reference on the page.
+	 */
+	if (count == 1)
+		page->pgmap->page_free(page, page->pgmap->data);
+	else if (!count)
+		__put_page(page);
+}
+EXPORT_SYMBOL(put_zone_device_private_page);
+#endif /* CONFIG_DEVICE_PRIVATE */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e7521ec..2a9dce5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -72,6 +72,12 @@
 #include <asm/div64.h>
 #include "internal.h"
 
+/*
+ * Device private memory see HMM (Documentation/vm/hmm.txt) or hmm.h
+ */
+DEFINE_STATIC_KEY_FALSE(device_private_key);
+EXPORT_SYMBOL(device_private_key);
+
 /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
 static DEFINE_MUTEX(pcp_batch_high_lock);
 #define MIN_PERCPU_PAGELIST_FRACTION	(8)
-- 
2.9.4