mm: add rss counters consistency check

Warn about non-zero rss counters at final mmdrop.

This check will prevent reoccurences of bugs such as that fixed in "mm:
fix rss count leakage during migration".

I didn't hide this check under CONFIG_VM_DEBUG because it rather small and
rss counters cover whole page-table management, so this is a good
invariant.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -511,6 +511,23 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
 	return NULL;
 }
 
+static void check_mm(struct mm_struct *mm)
+{
+	int i;
+
+	for (i = 0; i < NR_MM_COUNTERS; i++) {
+		long x = atomic_long_read(&mm->rss_stat.count[i]);
+
+		if (unlikely(x))
+			printk(KERN_ALERT "BUG: Bad rss-counter state "
+					  "mm:%p idx:%d val:%ld\n", mm, i, x);
+	}
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	VM_BUG_ON(mm->pmd_huge_pte);
+#endif
+}
+
 /*
  * Allocate and initialize an mm_struct.
  */
@@ -538,9 +555,7 @@ void __mmdrop(struct mm_struct *mm)
 	mm_free_pgd(mm);
 	destroy_context(mm);
 	mmu_notifier_mm_destroy(mm);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	VM_BUG_ON(mm->pmd_huge_pte);
-#endif
+	check_mm(mm);
 	free_mm(mm);
 }
 EXPORT_SYMBOL_GPL(__mmdrop);