From: Andrey Ryabinin aryabinin@virtuozzo.com
This patch has been added to the 4.1 stable tree. If you have any objections, please let us know.
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[ Upstream commit d59b1087a98e402ed9a7cc577f4da435f9a555f5 ]
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: Sasha Levin alexander.levin@microsoft.com --- mm/page-writeback.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index eb59f7eea508..308757ae508d 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -967,6 +967,7 @@ static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi, unsigned long pos_ratio; unsigned long step; unsigned long x; + unsigned long shift;
/* * The dirty rate will match the writeout rate in long term, except @@ -1094,11 +1095,11 @@ static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi, * 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;