diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 30351f0..ad2a414 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -265,18 +265,19 @@ static void bdi_writeout_fraction(struct backing_dev_info *bdi,
  * This avoids exceeding the total dirty_limit when the floating averages
  * fluctuate too quickly.
  */
-static void
-clip_bdi_dirty_limit(struct backing_dev_info *bdi, long dirty, long *pbdi_dirty)
+static void clip_bdi_dirty_limit(struct backing_dev_info *bdi,
+		unsigned long dirty, unsigned long *pbdi_dirty)
 {
-	long avail_dirty;
+	unsigned long avail_dirty;
 
-	avail_dirty = dirty -
-		(global_page_state(NR_FILE_DIRTY) +
+	avail_dirty = global_page_state(NR_FILE_DIRTY) +
 		 global_page_state(NR_WRITEBACK) +
 		 global_page_state(NR_UNSTABLE_NFS) +
-		 global_page_state(NR_WRITEBACK_TEMP));
+		 global_page_state(NR_WRITEBACK_TEMP);
 
-	if (avail_dirty < 0)
+	if (avail_dirty < dirty)
+		avail_dirty = dirty - avail_dirty;
+	else
 		avail_dirty = 0;
 
 	avail_dirty += bdi_stat(bdi, BDI_RECLAIMABLE) +
@@ -299,10 +300,10 @@ static inline void task_dirties_fraction(struct task_struct *tsk,
  *
  *   dirty -= (dirty/8) * p_{t}
  */
-static void task_dirty_limit(struct task_struct *tsk, long *pdirty)
+static void task_dirty_limit(struct task_struct *tsk, unsigned long *pdirty)
 {
 	long numerator, denominator;
-	long dirty = *pdirty;
+	unsigned long dirty = *pdirty;
 	u64 inv = dirty >> 3;
 
 	task_dirties_fraction(tsk, &numerator, &denominator);