Patch "mm/page-writeback: fix dirty_ratelimit calculation" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

mm/page-writeback: fix dirty_ratelimit calculation

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-page-writeback-fix-dirty_ratelimit-calculation.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andrey Ryabinin <[email protected]>
Date: Tue, 15 Mar 2016 14:55:27 -0700
Subject: mm/page-writeback: fix dirty_ratelimit calculation

From: Andrey Ryabinin <[email protected]>

commit d59b1087a98e402ed9a7cc577f4da435f9a555f5 upstream.

Calculation of dirty_ratelimit sometimes is not correct. E.g. initial values of dirty_ratelimit == INIT_BW and step == 0, lead to the following result:

   UBSAN: Undefined behaviour in ../mm/page-writeback.c:1286:7
   shift exponent 25600 is too large for 64-bit type 'long unsigned int'

The fix is straightforward - make step 0 if the shift exponent is too big.

---
 mm/page-writeback.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

@@ -1162,6 +1162,7 @@ static void wb_update_dirty_ratelimit(st
 	unsigned long balanced_dirty_ratelimit;
 	unsigned long step;
 	unsigned long x;
+	unsigned long shift;
 
 	/*
 	 * The dirty rate will match the writeout rate in long term, except

@@ -1286,11 +1287,11 @@ static void wb_update_dirty_ratelimit(st
 	 * rate itself is constantly fluctuating. So decrease the track speed
 	 * when it gets close to the target. Helps eliminate pointless tremors.
 	 */
-	step >>= dirty_ratelimit / (2 * step + 1);
-	/*
-	 * Limit the tracking speed to avoid overshooting.
-	 */
-	step = (step + 7) / 8;
+	shift = dirty_ratelimit / (2 * step + 1);
+	if (shift < BITS_PER_LONG)
+		step = DIV_ROUND_UP(step >> shift, 8);
+	else
+		step = 0;
 
 	if (dirty_ratelimit < balanced_dirty_ratelimit)
 		dirty_ratelimit += step;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/net-mac80211-debugfs.c-prevent-build-failure-with-config_ubsan-y.patch queue-4.4/mm-page-writeback-fix-dirty_ratelimit-calculation.patch