mm/page-writeback: fix dirty_ratelimit calculation

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>

1 files changed, 6 insertions, 5 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 6fe7d15bd1f7..d782cbab735a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1169,6 +1169,7 @@ static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
         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
@@ -1293,11 +1294,11 @@ static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
          * 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;