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>

1 files changed, 3 insertions, 4 deletions

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