Re: [RFC PATCH 3/4] mm/damon/reclaim: remove unnecessary damon_call() param validation

[SJ Park <sj@kernel.org>]

On Thu, 13 Aug 2026 04:01:40 +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] Concurrent invocation of `damon_call()` during kthread startup leads to an infinite kernel loop.
> --
> 
> --- Patch [3]: [RFC PATCH 3/4] mm/damon/reclaim: remove unnecessary damon_call() param validation ---
> commit 07f6c5438f6069b0b8225e26e3726d6aa4e3da09
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/reclaim: remove unnecessary damon_call() param validation
> 
> DAMON_RECLAIM avoids passing NULL or unstarted damon_ctx to damon_call()
> with its own validation. The validation is no longer needed, because
> the DAMON core layer now handles the corner cases itself. Remove the
> unnecessary check.
> 
> > diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
> > index 45d5557cc575a..42a2c9cb13431 100644
> > --- a/mm/damon/reclaim.c
> > +++ b/mm/damon/reclaim.c
> [ ... ]
> > @@ -293,10 +291,6 @@ static int damon_reclaim_commit_inputs_store(const char *val,
> >  	if (!commit_inputs_request)
> >  		return 0;
> >  
> > -	/* Skip damon_call() if ctx has not successfully started. */
> > -	if (!damon_reclaim_damon_has_started)
> > -		return -EINVAL;
> > -
> 
> [Severity: High]
> This is a pre-existing issue, but does removing this validation expose an
> infinite loop in the DAMON core?
> 
> If userspace writes to this sysfs file concurrently with damon_start(),
> damon_call() in mm/damon/core.c can hit a race condition. The core
> initializes ctx->call_controls_obsolete to true. If damon_call() runs
> before the kthread fully starts, it can read call_controls_obsolete == true
> and jump to the canceled label:
> 
> mm/damon/core.c:damon_call() {
>     ...
>     if (ctx->call_controls_obsolete) {
>         mutex_unlock(&ctx->call_controls_lock);
>         goto canceled;
>     }
>     ...
> canceled:
>     while (damon_is_running(ctx))
>         schedule_timeout_idle(msecs_to_jiffies(100));
>     return -ECANCELED;
> }
> 
> If damon_start() concurrently sets ctx->kdamond to non-NULL,
> damon_is_running(ctx) becomes true. Because the kthread has just started,
> it runs indefinitely, and damon_call() gets trapped in the while loop
> forever.

Because of the param_lock, that race is impossible.

> 
> >  	err = damon_call(ctx, &control);
> >  
> >  	return err ? err : control.return_code;
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260813035001.97364-1-sj@kernel.org?part=3


Thanks,
SJ