Commit 464d1387 authored Apr 21, 2015 by Tejun Heo Committed by Jens Axboe Apr 23, 2015

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



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
c72efb65 ("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@kernel.org>
Cc: stable@vger.kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

parent 2a34c087

mm/page-writeback.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -580,7 +580,7 @@ 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;
		@@ -807,7 +807,7 @@ 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
		@@ -822,7 +822,7 @@ 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;