mm/balloon_compaction: redesign ballooned pages management

Sasha Levin reported KASAN splash inside isolate_migratepages_range().
Problem is in the function __is_movable_balloon_page() which tests
AS_BALLOON_MAP in page->mapping->flags.  This function has no protection
against anonymous pages.  As result it tried to check address space flags
inside struct anon_vma.

Further investigation shows more problems in current implementation:

* Special branch in __unmap_and_move() never works:
  balloon_page_movable() checks page flags and page_count.  In
  __unmap_and_move() page is locked, reference counter is elevated, thus
  balloon_page_movable() always fails.  As a result execution goes to the
  normal migration path.  virtballoon_migratepage() returns
  MIGRATEPAGE_BALLOON_SUCCESS instead of MIGRATEPAGE_SUCCESS,
  move_to_new_page() thinks this is an error code and assigns
  newpage->mapping to NULL.  Newly migrated page lose connectivity with
  balloon an all ability for further migration.

* lru_lock erroneously required in isolate_migratepages_range() for
  isolation ballooned page.  This function releases lru_lock periodically,
  this makes migration mostly impossible for some pages.

* balloon_page_dequeue have a tight race with balloon_page_isolate:
  balloon_page_isolate could be executed in parallel with dequeue between
  picking page from list and locking page_lock.  Race is rare because they
  use trylock_page() for locking.

This patch fixes all of them.

Instead of fake mapping with special flag this patch uses special state of
page->_mapcount: PAGE_BALLOON_MAPCOUNT_VALUE = -256.  Buddy allocator uses
PAGE_BUDDY_MAPCOUNT_VALUE = -128 for similar purpose.  Storing mark
directly in struct page makes everything safer and easier.

PagePrivate is used to mark pages present in page list (i.e.  not
isolated, like PageLRU for normal pages).  It replaces special rules for
reference counter and makes balloon migration similar to migration of
normal pages.  This flag is protected by page_lock together with link to
the balloon device.

Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@samsung.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Link: http://lkml.kernel.org/p/53E6CEAA.9020105@oracle.com
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: <stable@vger.kernel.org>	[3.8+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 14 deletions

diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index 6e45a50..52abeeb 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -93,17 +93,12 @@ struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info)
 		 * to be released by the balloon driver.
 		 */
 		if (trylock_page(page)) {
+			if (!PagePrivate(page)) {
+				/* raced with isolation */
+				unlock_page(page);
+				continue;
+			}
 			spin_lock_irqsave(&b_dev_info->pages_lock, flags);
-			/*
-			 * Raise the page refcount here to prevent any wrong
-			 * attempt to isolate this page, in case of coliding
-			 * with balloon_page_isolate() just after we release
-			 * the page lock.
-			 *
-			 * balloon_page_free() will take care of dropping
-			 * this extra refcount later.
-			 */
-			get_page(page);
 			balloon_page_delete(page);
 			spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
 			unlock_page(page);
@@ -187,7 +182,9 @@ static inline void __isolate_balloon_page(struct page *page)
 {
 	struct balloon_dev_info *b_dev_info = page->mapping->private_data;
 	unsigned long flags;
+
 	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	ClearPagePrivate(page);
 	list_del(&page->lru);
 	b_dev_info->isolated_pages++;
 	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
@@ -197,7 +194,9 @@ static inline void __putback_balloon_page(struct page *page)
 {
 	struct balloon_dev_info *b_dev_info = page->mapping->private_data;
 	unsigned long flags;
+
 	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	SetPagePrivate(page);
 	list_add(&page->lru, &b_dev_info->pages);
 	b_dev_info->isolated_pages--;
 	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
@@ -235,12 +234,11 @@ bool balloon_page_isolate(struct page *page)
 		 */
 		if (likely(trylock_page(page))) {
 			/*
-			 * A ballooned page, by default, has just one refcount.
+			 * A ballooned page, by default, has PagePrivate set.
 			 * Prevent concurrent compaction threads from isolating
-			 * an already isolated balloon page by refcount check.
+			 * an already isolated balloon page by clearing it.
 			 */
-			if (__is_movable_balloon_page(page) &&
-			    page_count(page) == 2) {
+			if (balloon_page_movable(page)) {
 				__isolate_balloon_page(page);
 				unlock_page(page);
 				return true;