PM / Hibernate: Fix free_unnecessary_pages()

From: Rafael J. Wysocki <[email protected]>

There is a bug in free_unnecessary_pages() that causes it to attempt to free too many pages in some cases, which triggers the BUG_ON() in memory_bm_clear_bit() for copy_bm. Namely, if count_data_pages() is initially greater than alloc_normal, we get to_free_normal equal to 0 and "save" greater from 0. In that case, if the sum of "save" and count_highmem_pages() is greater than alloc_highmem, we subtract a positive number from to_free_normal. Hence, since to_free_normal was 0 before the subtraction and is an unsigned int, the result is converted to a huge positive number that is used as the number of pages to free.

Fix this bug by checking if to_free_normal is actually greater than or equal to the number we're going to subtract from it.

---
 kernel/power/snapshot.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Index: linux-2.6/kernel/power/snapshot.c
===================================================================
@@ -1211,7 +1211,11 @@ static void free_unnecessary_pages(void)
 		to_free_highmem = alloc_highmem - save;
 	} else {
 		to_free_highmem = 0;
-		to_free_normal -= save - alloc_highmem;
+		save -= alloc_highmem;
+		if (to_free_normal > save)
+			to_free_normal -= save;
+		else
+			to_free_normal = 0;
 	}
 
 	memory_bm_position_reset(&copy_bm);