thp: remove PG_buddy

PG_buddy can be converted to _mapcount == -2.  So the PG_compound_lock can
be added to page->flags without overflowing (because of the sparse section
bits increasing) with CONFIG_X86_PAE=y and CONFIG_X86_PAT=y.  This also
has to move the memory hotplug code from _mapcount to lru.next to avoid
any risk of clashes.  We can't use lru.next for PG_buddy removal, but
memory hotplug can use lru.next even more easily than the mapcount
instead.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 13 insertions, 12 deletions

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index a2832c092509..e92f04749fcb 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -82,9 +82,10 @@ static void release_memory_resource(struct resource *res)
 
 #ifdef CONFIG_MEMORY_HOTPLUG_SPARSE
 #ifndef CONFIG_SPARSEMEM_VMEMMAP
-static void get_page_bootmem(unsigned long info,  struct page *page, int type)
+static void get_page_bootmem(unsigned long info,  struct page *page,
+                             unsigned long type)
 {
-        atomic_set(&page->_mapcount, type);
+        page->lru.next = (struct list_head *) type;
         SetPagePrivate(page);
         set_page_private(page, info);
         atomic_inc(&page->_count);
@@ -94,15 +95,16 @@ static void get_page_bootmem(unsigned long info,  struct page *page, int type)
  * so use __ref to tell modpost not to generate a warning */
 void __ref put_page_bootmem(struct page *page)
 {
-        int type;
+        unsigned long type;
 
-        type = atomic_read(&page->_mapcount);
-        BUG_ON(type >= -1);
+        type = (unsigned long) page->lru.next;
+        BUG_ON(type < MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE ||
+               type > MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE);
 
         if (atomic_dec_return(&page->_count) == 1) {
                 ClearPagePrivate(page);
                 set_page_private(page, 0);
-                reset_page_mapcount(page);
+                INIT_LIST_HEAD(&page->lru);
                 __free_pages_bootmem(page, 0);
         }
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e7664b9f706c..9dfe49bceff4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -449,8 +449,8 @@ __find_combined_index(unsigned long page_idx, unsigned int order)
  * (c) a page and its buddy have the same order &&
  * (d) a page and its buddy are in the same zone.
  *
- * For recording whether a page is in the buddy system, we use PG_buddy.
- * Setting, clearing, and testing PG_buddy is serialized by zone->lock.
+ * For recording whether a page is in the buddy system, we set ->_mapcount -2.
+ * Setting, clearing, and testing _mapcount -2 is serialized by zone->lock.
  *
  * For recording page's order, we use page_private(page).
  */
@@ -483,7 +483,7 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
  * as necessary, plus some accounting needed to play nicely with other
  * parts of the VM system.
  * At each level, we keep a list of pages, which are heads of continuous
- * free pages of length of (1 << order) and marked with PG_buddy. Page's
+ * free pages of length of (1 << order) and marked with _mapcount -2. Page's
  * order is recorded in page_private(page) field.
  * So when we are allocating or freeing one, we can derive the state of the
  * other.  That is, if we allocate a small block, and both were   
@@ -5574,7 +5574,6 @@ static struct trace_print_flags pageflag_names[] = {
         {1UL << PG_swapcache,           "swapcache"     },
         {1UL << PG_mappedtodisk,        "mappedtodisk"  },
         {1UL << PG_reclaim,             "reclaim"       },
-        {1UL << PG_buddy,               "buddy"         },
         {1UL << PG_swapbacked,          "swapbacked"    },
         {1UL << PG_unevictable,         "unevictable"   },
 #ifdef CONFIG_MMU
diff --git a/mm/sparse.c b/mm/sparse.c
index 95ac219af379..93250207c5cf 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -671,10 +671,10 @@ static void __kfree_section_memmap(struct page *memmap, unsigned long nr_pages)
 static void free_map_bootmem(struct page *page, unsigned long nr_pages)
 {
         unsigned long maps_section_nr, removing_section_nr, i;
-        int magic;
+        unsigned long magic;
 
         for (i = 0; i < nr_pages; i++, page++) {
-                magic = atomic_read(&page->_mapcount);
+                magic = (unsigned long) page->lru.next;
 
                 BUG_ON(magic == NODE_INFO);