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

[Yu Kuai]

Hi,

在 2023/10/17 4:06, Khazhy Kumykov 写道:
> On Sun, Oct 15, 2023 at 6:47 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > 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).
> I believe 62 is correct, although admittedly it's less "intuitive"
> than the check in mul_u64_u64_div_u64()....
>
> The result overflows if log2(A * B / C) >= 64, so we want to ensure that:
> log2(A) + log2(B) - log2(C) < 64
>
> Given that:
> ilog2(A) <= log2(A) < ilog2(A) + 1  // truncation defn
> It follows that:
> -log2(A) <= -ilog2(A)  // Inverse rule
> log2(A) - 1 < ilog2(A)
>
> Starting from:
> ilog2(A) + ilog2(B) - ilog2(C) <= X
>
> We can show:
> (log2(A) - 1) + (log2(B) - 1) + (-log2(C)) < ilog2(A) + ilog2(B) +
> (-ilog2(C)) // strict inequality here since the substitutions for A
> and B terms are strictly less
> (log2(A) - 1) + (log2(B) - 1) + (-log2(C)) < X
> log2(A) + log2(B) - log2(C) < X + 2
>
> So for X = 62, log2(A) + log2(B) - log2(C) < 64 must be true, and we
> must be safe from overflow.
>
> So... by converse, if ilog2(A) + ilog2(B) - ilog2(C) > 62, we cannot
> guarantee that the result will not overflow - thus we bail out.
>
> // end math

Thanks for the explanation, I understand that, so the problem is that
if the above condition(>62) match, the result may not overflow, but
U64_MAX is returned here, this is not correct but I guess this doesn't
matter in real word, it's impossible that user will issue more than
1<<63 bytes IO in an extended slice.

I'm good with this fix with some comments.

Thanks,
Kuai

> It /is/ less exact than your proposal (sufficient, but not necessary),
> but saves an extra 128bit mul.
>
> I mostly just want us to pick /something/, since 6.6-rc and the LTSs
> with the patch in question are busted currently. :)
>
>
>
> > Thanks,
> > Kuai
> >
> > > +               return U64_MAX;
> > >           return mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed, (u64)HZ);
> > >    }
> > >
> > > .
>
> .