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, 31 insertions, 11 deletions

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 31c237a00c48..24376fe7ee68 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -13,12 +13,16 @@ struct mem_section;
 #ifdef CONFIG_MEMORY_HOTPLUG
 
 /*
- * Types for free bootmem.
- * The normal smallest mapcount is -1. Here is smaller value than it.
+ * Types for free bootmem stored in page->lru.next. These have to be in
+ * some random range in unsigned long space for debugging purposes.
  */
-#define SECTION_INFO            (-1 - 1)
-#define MIX_SECTION_INFO        (-1 - 2)
-#define NODE_INFO               (-1 - 3)
+enum {
+        MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE = 12,
+        SECTION_INFO = MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE,
+        MIX_SECTION_INFO,
+        NODE_INFO,
+        MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE = NODE_INFO,
+};
 
 /*
  * pgdat resizing functions
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2ec5138badab..7ab7d2b60041 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -397,6 +397,27 @@ static inline void init_page_count(struct page *page)
         atomic_set(&page->_count, 1);
 }
 
+/*
+ * PageBuddy() indicate that the page is free and in the buddy system
+ * (see mm/page_alloc.c).
+ */
+static inline int PageBuddy(struct page *page)
+{
+        return atomic_read(&page->_mapcount) == -2;
+}
+
+static inline void __SetPageBuddy(struct page *page)
+{
+        VM_BUG_ON(atomic_read(&page->_mapcount) != -1);
+        atomic_set(&page->_mapcount, -2);
+}
+
+static inline void __ClearPageBuddy(struct page *page)
+{
+        VM_BUG_ON(!PageBuddy(page));
+        atomic_set(&page->_mapcount, -1);
+}
+
 void put_page(struct page *page);
 void put_pages_list(struct list_head *pages);
 
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 4ca1241ef94e..0db8037e2725 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -48,9 +48,6 @@
  * struct page (these bits with information) are always mapped into kernel
  * address space...
  *
- * PG_buddy is set to indicate that the page is free and in the buddy system
- * (see mm/page_alloc.c).
- *
  * PG_hwpoison indicates that a page got corrupted in hardware and contains
  * data with incorrect ECC bits that triggered a machine check. Accessing is
  * not safe since it may cause another machine check. Don't touch!
@@ -96,7 +93,6 @@ enum pageflags {
         PG_swapcache,           /* Swap page: swp_entry_t in private */
         PG_mappedtodisk,        /* Has blocks allocated on-disk */
         PG_reclaim,             /* To be reclaimed asap */
-        PG_buddy,               /* Page is free, on buddy lists */
         PG_swapbacked,          /* Page is backed by RAM/swap */
         PG_unevictable,         /* Page is "unevictable"  */
 #ifdef CONFIG_MMU
@@ -233,7 +229,6 @@ PAGEFLAG(OwnerPriv1, owner_priv_1) TESTCLEARFLAG(OwnerPriv1, owner_priv_1)
  * risky: they bypass page accounting.
  */
 TESTPAGEFLAG(Writeback, writeback) TESTSCFLAG(Writeback, writeback)
-__PAGEFLAG(Buddy, buddy)
 PAGEFLAG(MappedToDisk, mappedtodisk)
 
 /* PG_readahead is only used for file reads; PG_reclaim is only for writes */
@@ -461,7 +456,7 @@ static inline int PageTransCompound(struct page *page)
 #define PAGE_FLAGS_CHECK_AT_FREE \
         (1 << PG_lru     | 1 << PG_locked    | \
          1 << PG_private | 1 << PG_private_2 | \
-         1 << PG_buddy   | 1 << PG_writeback | 1 << PG_reserved | \
+         1 << PG_writeback | 1 << PG_reserved | \
          1 << PG_slab    | 1 << PG_swapcache | 1 << PG_active | \
          1 << PG_unevictable | __PG_MLOCKED | __PG_HWPOISON | \
          __PG_COMPOUND_LOCK)