Commit d59b1087 authored by Andrey Ryabinin's avatar Andrey Ryabinin Committed by Linus Torvalds

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: default avatarAndrey 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent b72d0ffb
......@@ -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;
......
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