[PATCH v2 3/5] mm,memory_hotplug: Introduce Vmemmap page helpers

[Oscar Salvador]

Introduce a set of functions for Vmemmap pages.
Set of functions:

- {Set,Clear,Check} Vmemmap flag
- Given a vmemmap page, get its vmemmap-head
- Get #nr of vmemmap pages taking into account the current position
  of the page

These functions will be used for the code handling Vmemmap pages.

Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
---
 include/linux/page-flags.h | 34 ++++++++++++++++++++++++++++++++++
 mm/util.c                  |  2 ++
 2 files changed, 36 insertions(+)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index b848517da64c..a8b9b57162b3 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -466,6 +466,40 @@ static __always_inline int __PageMovable(struct page *page)
 				PAGE_MAPPING_MOVABLE;
 }
 
+#define VMEMMAP_PAGE		(~PAGE_MAPPING_FLAGS)
+static __always_inline int PageVmemmap(struct page *page)
+{
+	return PageReserved(page) && (unsigned long)page->mapping == VMEMMAP_PAGE;
+}
+
+static __always_inline int __PageVmemmap(struct page *page)
+{
+	return (unsigned long)page->mapping == VMEMMAP_PAGE;
+}
+
+static __always_inline void __ClearPageVmemmap(struct page *page)
+{
+	__ClearPageReserved(page);
+	page->mapping = NULL;
+}
+
+static __always_inline void __SetPageVmemmap(struct page *page)
+{
+	__SetPageReserved(page);
+	page->mapping = (void *)VMEMMAP_PAGE;
+}
+
+static __always_inline struct page *vmemmap_get_head(struct page *page)
+{
+	return (struct page *)page->freelist;
+}
+
+static __always_inline unsigned long get_nr_vmemmap_pages(struct page *page)
+{
+	struct page *head = vmemmap_get_head(page);
+	return head->private - (page - head);
+}
+
 #ifdef CONFIG_KSM
 /*
  * A KSM page is one of those write-protected "shared pages" or "merged pages"
diff --git a/mm/util.c b/mm/util.c
index 021648a8a3a3..5e20563cdef6 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -607,6 +607,8 @@ struct address_space *page_mapping(struct page *page)
 	mapping = page->mapping;
 	if ((unsigned long)mapping & PAGE_MAPPING_ANON)
 		return NULL;
+	if ((unsigned long)mapping == VMEMMAP_PAGE)
+		return NULL;
 
 	return (void *)((unsigned long)mapping & ~PAGE_MAPPING_FLAGS);
 }
-- 
2.12.3