Re: [PATCH] mm/damon/core: prevent unnecessary overflow in damos_set_effective_quota()

[SeongJae Park]

On Thu, 21 Aug 2025 20:55:55 +0800 Quanmin Yan <yanquanmin1@xxxxxxxxxx> 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 run
> for longtime, throughput calculation can overflow and set esz too
> small. As a result, speed of the scheme get unexpectedly slow.
> 
> Fixes: 1cd243030059 ("mm/damon/schemes: implement time quota")
> Cc: <stable@xxxxxxxxxxxxxxx> # 5.16.x
> Signed-off-by: Quanmin Yan <yanquanmin1@xxxxxxxxxx>
> Reported-by: Andrew Paniakin <apanyaki@xxxxxxxxxx>
> Closes: N/A # privately reported

Reviewed-by: SeongJae Park <sj@xxxxxxxxxx>


Thanks,
SJ

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