[PATCH] mm: make compound page destructor handling explicit

Currently we we use the lru head link of the second page of a compound page
to hold its destructor.  This was ok when it was purely an internal
implmentation detail.  However, hugetlbfs overrides this destructor
violating the layering.  Abstract this out as explicit calls, also
introduce a type for the callback function allowing them to be type
checked.  For each callback we pre-declare the function, causing a type
error on definition rather than on use elsewhere.

[akpm@osdl.org: cleanups]
Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

4 files changed, 22 insertions, 4 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0e266fe1b4c4..a17b147c61e7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -296,6 +296,24 @@ void put_pages_list(struct list_head *pages);
 void split_page(struct page *page, unsigned int order);
 
 /*
+ * Compound pages have a destructor function.  Provide a
+ * prototype for that function and accessor functions.
+ * These are _only_ valid on the head of a PG_compound page.
+ */
+typedef void compound_page_dtor(struct page *);
+
+static inline void set_compound_page_dtor(struct page *page,
+                                                compound_page_dtor *dtor)
+{
+        page[1].lru.next = (void *)dtor;
+}
+
+static inline compound_page_dtor *get_compound_page_dtor(struct page *page)
+{
+        return (compound_page_dtor *)page[1].lru.next;
+}
+
+/*
  * Multiple processes may "see" the same page. E.g. for untouched
  * mappings of /dev/null, all processes see the same page full of
  * zeroes, and text pages of executables and shared libraries have
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2911a364481e..0ccc7f230252 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -109,7 +109,7 @@ static int alloc_fresh_huge_page(void)
         if (nid == MAX_NUMNODES)
                 nid = first_node(node_online_map);
         if (page) {
-                page[1].lru.next = (void *)free_huge_page;      /* dtor */
+                set_compound_page_dtor(page, free_huge_page);
                 spin_lock(&hugetlb_lock);
                 nr_huge_pages++;
                 nr_huge_pages_node[page_to_nid(page)]++;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index dc8753bdd47e..d539f83c62b6 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -230,7 +230,7 @@ static void prep_compound_page(struct page *page, unsigned long order)
         int i;
         int nr_pages = 1 << order;
 
-        page[1].lru.next = (void *)free_compound_page;  /* set dtor */
+        set_compound_page_dtor(page, free_compound_page);
         page[1].lru.prev = (void *)order;
         for (i = 0; i < nr_pages; i++) {
                 struct page *p = page + i;
diff --git a/mm/swap.c b/mm/swap.c
index d9a3770d8f3c..017e72ca9bbb 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -57,9 +57,9 @@ static void put_compound_page(struct page *page)
 {
         page = (struct page *)page_private(page);
         if (put_page_testzero(page)) {
-                void (*dtor)(struct page *page);
+                compound_page_dtor *dtor;
 
-                dtor = (void (*)(struct page *))page[1].lru.next;
+                dtor = get_compound_page_dtor(page);
                 (*dtor)(page);
         }
 }