mm/page-writeback: fix dirty_ratelimit calculation (d59b1087) · Commits · 戴 / test

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>

mm/page-writeback.c

+6 −5

Original line number	Diff line number	Diff line
		@@ -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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