From: Matthew Wilcox <willy@infradead.org>
To: Jingbo Xu <jefflexu@linux.alibaba.com>
Cc: shr@devkernel.io, akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, joseph.qi@linux.alibaba.com,
linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org
Subject: Re: [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting max_ratio
Date: Tue, 19 Dec 2023 13:01:46 +0000 [thread overview]
Message-ID: <ZYGUOslxxwe1sNzR@casper.infradead.org> (raw)
In-Reply-To: <5460aaf1-44f6-475f-b980-cb9058cc1df4@linux.alibaba.com>
On Tue, Dec 19, 2023 at 01:58:21PM +0800, Jingbo Xu wrote:
> On 12/19/23 12:06 PM, Matthew Wilcox wrote:
> > On Tue, Dec 19, 2023 at 10:42:46AM +0800, Jingbo Xu wrote:
> >> } else {
> >> bdi->max_ratio = max_ratio;
> >> - bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
> >> + bdi->max_prop_frac = div64_u64(FPROP_FRAC_BASE * max_ratio,
> >> + 100 * BDI_RATIO_SCALE);
> >> }
> >
> > Why use div64_u64 here?
> >
> > FPROP_FRAC_BASE is an unsigned long. max_ratio is an unsigned int, so
> > the numerator is an unsigned long. BDI_RATIO_SCALE is 10,000, so the
> > numerator is an unsigned int. There's no 64-bit arithmetic needed here.
>
> Yes, div64_u64() is actually not needed here. So it seems
>
> bdi->max_prop_frac = FPROP_FRAC_BASE * max_ratio / 100 / BDI_RATIO_SCALE;
>
> is adequate?
I'd rather spell that as:
bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) /
(100 * BDI_RATIO_SCALE);
It's closer to how you'd write it out mathematically and so it reads
more easily. At least for me.
next prev parent reply other threads:[~2023-12-19 13:02 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-19 2:42 [PATCH v2 0/2] mm: fix arithmetic for bdi min_ratio and Jingbo Xu
2023-12-19 2:42 ` [PATCH v2 1/2] mm: fix arithmetic for bdi min_ratio Jingbo Xu
2023-12-19 2:42 ` [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting max_ratio Jingbo Xu
2023-12-19 4:06 ` Matthew Wilcox
2023-12-19 5:58 ` Jingbo Xu
2023-12-19 13:01 ` Matthew Wilcox [this message]
2023-12-19 14:07 ` Jingbo Xu
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