Messages in this thread | | | Date | Mon, 7 Dec 2015 09:26:21 -0500 | From | Tejun Heo <> | Subject | Re: undefined shift in wb_update_dirty_ratelimit() |
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Hello, Andrey.
On Mon, Dec 07, 2015 at 05:17:06PM +0300, Andrey Ryabinin wrote: > I've hit undefined shift in wb_update_dirty_ratelimit() which does some > mysterious 'step' calculations: > > /* > * Don't pursue 100% rate matching. It's impossible since the balanced > * 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); > > > dirty_ratelimit = INIT_BW and step = 0 results in this: > > [ 5006.957366] ================================================================================ > [ 5006.957798] UBSAN: Undefined behaviour in ../mm/page-writeback.c:1286:7 > [ 5006.958091] shift exponent 25600 is too large for 64-bit type 'long unsigned int'
We prolly should do sth like
shift = dirty_ratelimit / (2 * step = 1); if (shift < BITS_PER_LONG) { step = (step >> shift) + 7 / 8; } else { step = 0; }
Thanks.
-- tejun
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