Patch "writeback: use |1 instead of +1 to protect against div by zero" has been added to the 3.14-stable tree

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

writeback: use |1 instead of +1 to protect against div by zero

to the 3.14-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:
writeback-use-1-instead-of-1-to-protect-against-div-by-zero.patch
and it can be found in the queue-3.14 subdirectory.

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

From: Tejun Heo <[email protected]>
Date: Tue, 21 Apr 2015 16:49:13 -0400
Subject: writeback: use |1 instead of +1 to protect against div by zero

From: Tejun Heo <[email protected]>

commit 464d1387acb94dc43ba772b35242345e3d2ead1b upstream.

mm/page-writeback.c has several places where 1 is added to the divisor to prevent division by zero exceptions; however, if the original divisor is equivalent to -1, adding 1 leads to division by zero.

There are three places where +1 is used for this purpose - one in pos_ratio_polynom() and two in bdi_position_ratio(). The second one in bdi_position_ratio() actually triggered div-by-zero oops on a machine running a 3.10 kernel. The divisor is

x_intercept - bdi_setpoint + 1 == span + 1

span is confirmed to be (u32)-1. It isn't clear how it ended up that but it could be from write bandwidth calculation underflow fixed by c72efb658f7c ("writeback: fix possible underflow in write bandwidth calculation").

At any rate, +1 isn't a proper protection against div-by-zero. This patch converts all +1 protections to |1. Note that bdi_update_dirty_ratelimit() was already using |1 before this patch.

---
 mm/page-writeback.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

@@ -601,7 +601,7 @@ static long long pos_ratio_polynom(unsig
 	long x;
 
 	x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
-		    limit - setpoint + 1);
+		      (limit - setpoint) | 1);
 	pos_ratio = x;
 	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
 	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;

@@ -828,7 +828,7 @@ static unsigned long bdi_position_ratio(
 	 * scale global setpoint to bdi's:
 	 *	bdi_setpoint = setpoint * bdi_thresh / thresh
 	 */
-	x = div_u64((u64)bdi_thresh << 16, thresh + 1);
+	x = div_u64((u64)bdi_thresh << 16, thresh | 1);
 	bdi_setpoint = setpoint * (u64)x >> 16;
 	/*
 	 * Use span=(8*write_bw) in single bdi case as indicated by

@@ -843,7 +843,7 @@ static unsigned long bdi_position_ratio(
 
 	if (bdi_dirty < x_intercept - span / 4) {
 		pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
-				    x_intercept - bdi_setpoint + 1);
+				      (x_intercept - bdi_setpoint) | 1);
 	} else
 		pos_ratio /= 4;
 

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

queue-3.14/writeback-use-1-instead-of-1-to-protect-against-div-by-zero.patch