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, 8 insertions, 6 deletions

diff --git a/fs/proc/page.c b/fs/proc/page.c
index b06c674624e6..6d8e6a9e93ab 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -116,15 +116,17 @@ u64 stable_page_flags(struct page *page)
         if (PageHuge(page))
                 u |= 1 << KPF_HUGE;
 
-        u |= kpf_copy_bit(k, KPF_LOCKED,        PG_locked);
-
         /*
-         * Caveats on high order pages:
-         * PG_buddy will only be set on the head page; SLUB/SLQB do the same
-         * for PG_slab; SLOB won't set PG_slab at all on compound pages.
+         * Caveats on high order pages: page->_count will only be set
+         * -1 on the head page; SLUB/SLQB do the same for PG_slab;
+         * SLOB won't set PG_slab at all on compound pages.
          */
+        if (PageBuddy(page))
+                u |= 1 << KPF_BUDDY;
+
+        u |= kpf_copy_bit(k, KPF_LOCKED,        PG_locked);
+
         u |= kpf_copy_bit(k, KPF_SLAB,          PG_slab);
-        u |= kpf_copy_bit(k, KPF_BUDDY,         PG_buddy);
 
         u |= kpf_copy_bit(k, KPF_ERROR,         PG_error);
         u |= kpf_copy_bit(k, KPF_DIRTY,         PG_dirty);