Hi,
在 2023/10/14 5:51, Khazhy Kumykov 写道:
Looking at the generic mul_u64_u64_div_u64 impl, it doesn't handle
overflow of the final result either, as far as I can tell. So while on
x86 we get a DE, on non-x86 we just get the wrong result.
(Aside: after 8d6bbaada2e0 ("blk-throttle: prevent overflow while
calculating wait time"), setting a very-high bps_limit would probably
also cause this crash, no?)
Would it be possible to have a "check_mul_u64_u64_div_u64_overflow()",
where if the result doesn't fit in u64, we indicate (and let the
caller choose what to do? Here we should just return U64_MAX)?
Absent that, maybe we can take inspiration from the generic
mul_u64_u64_div_u64? (Forgive the paste)
static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long jiffy_elapsed)
{
+ /* Final result probably won't fit in u64 */
+ if (ilog2(bps_limit) + ilog2(jiffy_elapsed) - ilog2(HZ) > 62)
I'm not sure, but this condition looks necessary, but doesn't look
sufficient, for example, jiffy_elapsed cound be greater than HZ, while
ilog2(jiffy_elapsed) is equal to ilog2(HZ).
Thanks,
Kuai
+ return U64_MAX;
return mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed, (u64)HZ);
}
.
