Re: [RFC PATCH mm-next v2 12/12] mm/damon/core: prevent unnecessary overflow in damos_set_effective_quota()

[SeongJae Park <sj@kernel.org>]

+ Andrew

On Wed, 20 Aug 2025 16:06:22 +0800 Quanmin Yan <yanquanmin1@huawei.com> wrote:

> On 32-bit systems, the throughput calculation in function
> damos_set_effective_quota() is prone to unnecessary
> multiplication overflow. Using mult_frac() to fix it.

Andrew Paniakin also recently found and privately reported this issue, on 64
bit systems.  This can also happen on 64-bit systems, once the charged size
exceeds ~17 TiB.  On systems running for long time in production, this issue
can actually happen.

More specifically, when a DAMOS scheme having the time quota runs for long
time, throughput calculation can overflow and set esz too small.  As a result,
speed of the scheme get unexpectedly slow.

Quanmin, could you please add the above paragraph on the commit message?  Also,
I think this fix deserves Cc: stable@ and has no reason to be a part of this
patch series.  Could you please add appripriate tags like below and post again
separately?

    Fixes: 1cd243030059 ("mm/damon/schemes: implement time quota")
    Reported-by: Andrew Paniakin <apanyaki@amazon.com>
    Closes: N/A # privately reported
    Cc: <stable@vger.kernel.org> # 5.16.x

> 
> Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com>
> ---
>  mm/damon/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 980e271e42e9..38b5f842ef30 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2102,8 +2102,8 @@ static void damos_set_effective_quota(struct damos_quota *quota)
>  
>  	if (quota->ms) {
>  		if (quota->total_charged_ns)
> -			throughput = quota->total_charged_sz * 1000000 /
> -				quota->total_charged_ns;
> +			throughput = mult_frac(quota->total_charged_sz, 1000000,
> +							quota->total_charged_ns);
>  		else
>  			throughput = PAGE_SIZE * 1024;
>  		esz = min(throughput * quota->ms, esz);
> -- 
> 2.43.0


Thanks,
SJ