[3.13.y-ckt stable] Patch "writeback: use |1 instead of +1 to protect against div by zero" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt22.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From ccdb53aa536cc492722349bae527b6c86c0c8209 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj at kernel.org>
Date: Tue, 21 Apr 2015 16:49:13 -0400
Subject: writeback: use |1 instead of +1 to protect against div by zero

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.

Signed-off-by: Tejun Heo <tj at kernel.org>
Reviewed-by: Jan Kara <jack at suse.cz>
Signed-off-by: Jens Axboe <axboe at fb.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 mm/page-writeback.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 1802b62..25b940d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -601,7 +601,7 @@ static long long pos_ratio_polynom(unsigned long setpoint,
 	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(struct backing_dev_info *bdi,
 	 * 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(struct backing_dev_info *bdi,

 	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;

--
1.9.1