Re: [PATCH] mm/damon/stat: avoid overflow in estimated_memory_bandwidth calculation

[SJ Park <sj@kernel.org>]

On Mon, 2 Nov 2026 18:04:23 +0800 Wang Guangke <gkwang@linx-info.com> wrote:

> Hi SJ,
> Thanks for your review.
> 
> On Sun, Nov 01, 2026 at 11:13:43AM -0800, SJ Park wrote:
> > Hello Wang,
> > 
> > On Mon,  2 Nov 2026 02:00:01 +0800 Wang Guangke <gkwang@linx-info.com> wrote:
> > 
> > > The calculation of estimated_memory_bandwidth computes
> > > access_bytes * USEC_PER_MSEC * MSEC_PER_SEC / aggr_interval directly.
> > > On 32-bit systems, the intermediate product
> > > access_bytes * USEC_PER_MSEC * MSEC_PER_SEC may overflow before the
> > > division is performed.
> > 
> > Do you find this issue from real usage?  Or, jsut theoretical?  Could you
> > please clarify it, and user impacts of the issue?
> 
> I found it by code inspection, not from real usage on a 32-bit system.

Thank you for clarifying.

[...]
> > > +	estimated_memory_bandwidth =
> > > +		bytes_per_interval * USEC_PER_MSEC * MSEC_PER_SEC +
> > > +		div64_u64(remainder * USEC_PER_MSEC * MSEC_PER_SEC,
> > > +				c->attrs.aggr_interval);
> > 
> > Let's use USEC_PER_SEC instead of 'USEC_PER_MSEC * MSEC_PER_SEC'.
> 
> Thanks, I will rename the variable to bytes_per_us, and use USEC_PER_SEC
> instead of 'USEC_PER_MSEC * MSEC_PER_SEC'.

Thank you for accepting my humble suggestions.  Looking forward to v2.


Thanks,
SJ

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