4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Andy Shevchenko <andy.shevchenko@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Mel Gorman <mgorman@xxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- 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;