Re: [RFC PATCH] mm/damon/core: trace esz at first setup

[SeongJae Park <sj@kernel.org>]

On Wed, 20 May 2026 01:18:37 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] The monotonic accumulators `total_charged_ns` and `total_charged_sz` will rapidly overflow on 32-bit architectures, breaking the throughput estimation logic.

Not introduced by this patch, so no blocker.

> --
> 
> commit 4cb83c3ed801305fa15e94548668201c275a1037
> Author: SeongJae Park <sj@kernel.org>
> 
> mm/damon/core: trace esz at first setup
> 
> This commit fixes an issue where the initial effective size quota is not
> traced, which leaves tracepoint investigations incomplete when using the
> temporal tuner. It updates the quota adjustment logic to unconditionally
> trace the initial quota value.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 4e223857a0f99..0db6530825d1d 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -2883,6 +2883,8 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
> >  	if (!quota->total_charged_sz && !quota->charged_from) {
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but will total_charged_ns and
> total_charged_sz overflow on 32-bit architectures?
> 
> Since unsigned long is 32 bits on those systems, total_charged_ns will
> overflow after roughly 4.29 seconds of total active execution time.
> 
> Additionally, the throughput calculation in damos_set_effective_quota():
> 
> damos_set_effective_quota() {
>     ...
>     if (quota->total_charged_ns)
>         throughput = mult_frac(quota->total_charged_sz,
>                 1000000, quota->total_charged_ns);
>     ...
> }
> 
> uses mult_frac() which evaluates (x % d) * n internally using the type of x.
> Since the remainder could be up to total_charged_ns - 1, multiplying it by
> 1,000,000 will quickly overflow the 32-bit boundary. This produces incorrect
> throughput estimates and impacts the effective size quota calculations on
> 32-bit systems.

On 32-bit environment, addr_unit can be used to workaround this issue.  On
systems that runs DAMON after a reasonable amount of tests, the issue should
not happen.  I'm thinking about a better solution, though.

Anyway, not a blocker of this patch.


Thanks,
SJ

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