Re: [PATCH] mm/damon/stat: avoid 32-bit overflow in idle percentiles

[Aveline Noir <jm5905938@gmail.com>]

Hi SJ,

On Thu, Oct 15, 2026 at 05:21:29PM -0700, SJ Park wrote:
> This patch is sent as a followup of your previous bug report, right?
> Unless it is very urgent, please make sure the previous discussion is
> completed before making a followup action like sending this patch. As a
> rule of thumb for DAMON, I'd suggest waiting for ~10 hours after the last
> comment on the thread.

The patch was already prepared and additional tests were underway when
your reply to the bug report arrived, so I went ahead and sent it.
I'll leave more time for discussion before sending follow-up patches.

> What is 38?

38 individual VM runs, forming 19 before/after pairs. The 20 same-image
runs formed 10 control pairs.

> Do you have a plan to run more tests to be able to tell that?

I've run longer VM tests, but even same-kernel runs vary substantially,
so the overall patch effect remains unclear. In a fixed-input 32-bit
userspace test with 10 regions over 256 MiB, the median increase was
227 ns/call (+42%). The old code updated 15 entries, versus 101 with the
fix. I haven't separated the extra updates from arithmetic cost.
Maybe we could record region counts and sizes alongside VM timings to
check whether both versions are doing comparable work.

> Any reason to use div64_ul() instead of div_u64()?

total_sz is unsigned long. div_u64() would truncate its divisor on 64-bit
systems if it exceeds U32_MAX. div64_ul() preserves that width and maps
to div_u64() on 32-bit.

Thanks,
Aveline Noir