Re: [PATCH] blk-throttle: Calculate allowed value only when the throttle is enabled

[Yu Kuai]

Hi,

在 2023/10/07 23:16, Oleg Nesterov 写道:
> On 10/07, Yu Kuai wrote:
> > > > > probably need to remove the mul_u64_u64_div_u64 and check for
> > > > > overflow/potential overflow ourselves?
> > >
> > > probably yes...
> >
> > How about this?
> >
> > diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> > index 1101fb6f6cc8..5482c316a103 100644
> > --- a/block/blk-throttle.c
> > +++ b/block/blk-throttle.c
> > @@ -723,6 +723,10 @@ static unsigned int calculate_io_allowed(u32
> > iops_limit,
> >
> >   static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long
> > jiffy_elapsed)
> >   {
> > +       if (jiffy_elapsed > HZ &&
> > +           bps_limit > mul_u64_u64_div_u64(U64_MAX, (u64)HZ,
> > (u64)jiffy_elapsed);
> > +               return U64_MAX;
> > +
>
> I can't suggest anything better...
>
> but I do not know if it is possible that HZ > jiffy_elapsed. If yes, then
> mul_u64_u64_div_u64() above is not safe too.

Well, 'jiffy_elapsed > HZ' is judged before mul_u64_u64_div_u64().

Overflow can only happen if the above 2 conditions passed, and U64_MAX
is returned.

Thanks,
Kuai

> Oleg.
>
> .