[PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini <aquini@redhat.com>]

This patch fixes the following crash by fixing and enhancing the way 
page->flags are tested to identify a ballooned page.

---8<---
BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
--->8---

The NULL pointer deref was taking place because balloon_page_movable()
page->flags tests were incomplete and we ended up 
inadvertently poking at private pages.

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 include/linux/balloon_compaction.h | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 68893bc..634a19b 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -107,6 +107,23 @@ static inline void balloon_mapping_free(struct address_space *balloon_mapping)
 }
 
 /*
+ * __balloon_page_flags - helper to perform balloon @page ->flags tests.
+ *
+ * As balloon pages are got from Buddy, and we do not play with page->flags
+ * at driver level (exception made when we get the page lock for compaction),
+ * therefore we can safely identify a ballooned page by checking if the
+ * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
+ * This approach also helps on skipping ballooned pages that are locked for
+ * compaction or release, thus mitigating their racy check at
+ * balloon_page_movable()
+ */
+#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
+static inline bool __balloon_page_flags(struct page *page)
+{
+	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
+}
+
+/*
  * __is_movable_balloon_page - helper to perform @page mapping->flags tests
  */
 static inline bool __is_movable_balloon_page(struct page *page)
@@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
 	 * Before dereferencing and testing mapping->flags, lets make sure
 	 * this is not a page that uses ->mapping in a different way
 	 */
-	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
-	    !page_mapped(page))
+	if (__balloon_page_flags(page) && !page_mapped(page) &&
+	    page_count(page) == 1)
 		return __is_movable_balloon_page(page);
 
 	return false;
-- 
1.7.11.7