Re: [RFC PATCH -next 16/16] mm/damon/core: handle quota->esz overflow issues

[SeongJae Park <sj@kernel.org>]

On Wed, 13 Aug 2025 13:07:06 +0800 Quanmin Yan <yanquanmin1@huawei.com> wrote:

> In the original quota enforcement implementation, the traffic
> calculation multiplied A by 1000000 due to time unit conversion,
> making it highly prone to overflow on 32-bit systems:
> 
> damos_set_effective_quota
>   if (quota->total_charged_ns)
>     throughput = quota->total_charged_sz * 1000000 /
> 		quota->total_charged_ns;
> 
> Requiring total_charged_sz to be less than 4GB/1000000 is unreasonable.
> Additionally, when overflow occurs and causes quota->esz to become
> extremely small, the subsequent damos_apply_scheme logic permanently
> sets sz to 0, while quota stop updating, ultimately leading to complete
> functional failure:
> 
> damos_apply_scheme
>   if (quota->esz && quota->charged_sz + sz > quota->esz)
>     sz = ALIGN_DOWN(quota->esz - quota->charged_sz, DAMON_MIN_REGION);
> 
> Total charged stats use the unsigned long long data type to reduce
> overflow risk, with data reset capability after overflow occurs.

Thank you for finding this issue!  I don't want to change the data type if
possible, though.  Could replacing the easily-overflowing throughput
calculation with mult_frac() fix the issue?


Thanks,
SJ

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