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Subject[PATCH 4.4 27/87] mm/page-writeback: fix dirty_ratelimit calculation
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andrey Ryabinin <aryabinin@virtuozzo.com>

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.

Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -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;

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