Re: [Phishing Risk] [External] Re: [PATCH] blk-iocost: fix very large vtime when iocg activate

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2022/5/17 02:46, Tejun Heo wrote:
> On Mon, May 16, 2022 at 04:40:45PM +0800, Chengming Zhou wrote:
>> When the first iocg activate after blk_iocost_init(), now->vnow
>> maybe smaller than ioc->margins.target, cause very large vtarget
>> since it's u64.
>>
>> 	vtarget = now->vnow - ioc->margins.target;
>> 	atomic64_add(vtarget - vtime, &iocg->vtime);
>>
>> Then the iocg's vtime will be very large too, larger than now->vnow.
> 
> It's a wrapping counter. Please take a look at how time_before64() and
> friends work.

Hi Tejun, below is from the trace of test on original code:

iocost_iocg_activate: [vda:/user.slice] now=38343468:2171657838 vrate=137438 \
period=0->0 vtime=18446744007162209454 weight=6553600/6553600 hweight=65536/65536

The vtime value is very large, much larger than vnow. Maybe the commit message
is a little misleading?

And I take a look at how time_before64() work:

#define time_after64(a,b)	\
	(typecheck(__u64, a) &&	\
	 typecheck(__u64, b) && \
	 ((__s64)((b) - (a)) < 0))
#define time_before64(a,b)	time_after64(b,a)

I still don't get why my changes are wrong. :-)

> 
> Nacked-by: Tejun Heo <tj@kernel.org>
> 
> Again, please spend more effort understanding the code before sending these
> subtle patches.

Ok, will do. This problem is found from the trace of test, then verified fixed
using the trace of the same test with this patch.

Thanks.

> 
> Thanks.
>