[PATCH 8/9] mm: add API for setting a grace period cookie on compound pages

[Michel Lespinasse <walken@google.com>]

This commit adds the page_get_gp_cookie() / page_gp_cookie_elapsed()
API to be used on compound pages. page_get_gp_cookie() sets a cookie
on a page and page_gp_cookie_elapsed() returns true if an rcu grace
period has elapsed since the cookie was set.

page_clear_gp_cookie() is called before freeing compound pages so that
their state is always returned to a given standard (as enforced by
free_pages_check() in mm/page_alloc.c)

Signed-off-by: Michel Lespinasse <walken@google.com>
---
 include/linux/mm.h       |   22 ++++++++++++++++++++++
 include/linux/mm_types.h |    6 +++++-
 mm/page_alloc.c          |    1 +
 3 files changed, 28 insertions(+), 1 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9ff5f2d..2649b59 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -494,6 +494,28 @@ static inline void set_compound_order(struct page *page, unsigned long order)
 	page[1].lru.prev = (void *)order;
 }
 
+static inline void page_get_gp_cookie(struct page *page)
+{
+	VM_BUG_ON(!PageHead(page));
+	rcu_get_gp_cookie(&page[1].thp_create_timestamp);
+}
+
+static inline bool page_gp_cookie_elapsed(struct page *page)
+{
+	VM_BUG_ON(!PageHead(page));
+	return rcu_gp_cookie_elapsed(&page[1].thp_create_timestamp);
+}
+
+static inline void page_clear_gp_cookie(struct page *page)
+{
+	VM_BUG_ON(!PageHead(page));
+	VM_BUG_ON(offsetof(struct page, thp_create_timestamp) !=
+		  offsetof(struct page, mapping));
+	VM_BUG_ON(sizeof(page->thp_create_timestamp) !=
+		  sizeof(page->mapping));
+	page[1].mapping = 0;
+}
+
 #ifdef CONFIG_MMU
 /*
  * Do pte_mkwrite, but only if the vma says VM_WRITE.  We do this when
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 027935c..a6f99aa 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -12,6 +12,7 @@
 #include <linux/completion.h>
 #include <linux/cpumask.h>
 #include <linux/page-debug-flags.h>
+#include <linux/rcupdate.h>
 #include <asm/page.h>
 #include <asm/mmu.h>
 
@@ -66,7 +67,10 @@ struct page {
 	    spinlock_t ptl;
 #endif
 	    struct kmem_cache *slab;	/* SLUB: Pointer to slab */
-	    struct page *first_page;	/* Compound tail pages */
+	    struct {	/* Compound tail pages */
+		struct page *first_page;
+		struct rcu_cookie thp_create_timestamp;
+	    };
 	};
 	union {
 		pgoff_t index;		/* Our offset within mapping. */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4e8985a..dc42355 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -342,6 +342,7 @@ out:
 
 static void free_compound_page(struct page *page)
 {
+	page_clear_gp_cookie(page);
 	__free_pages_ok(page, compound_order(page));
 }
 
-- 
1.7.3.1