Re: [RFC PATCH 2/2] mm/damon/lru_sort: skip damon_call() if ctx has not started

[SJ Park <sj@kernel.org>]

On Sun, 02 Aug 2026 19:08:37 +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] A TOCTOU race between `damon_start()` and `damon_call()` can cause `damon_call()` to loop indefinitely.
> --
> 
> --- Patch [2]: [RFC PATCH 2/2] mm/damon/lru_sort: skip damon_call() if ctx has not started ---
> commit 9a410362ed5a4418640b6bf73c4ee49300c9dc16
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/lru_sort: skip damon_call() if ctx has not started
> 
> This commit prevents damon_call() from hanging when the DAMON context is
> initialized but not yet started. It introduces a boolean flag to track whether
> the context has ever successfully started, and skips the call if it has not.
> 
> > diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
> > index 7e077084cb03a..7df45f9a0b3ae 100644
> > --- a/mm/damon/lru_sort.c
> > +++ b/mm/damon/lru_sort.c
> [ ... ]
> > @@ -364,11 +366,8 @@ static int damon_lru_sort_commit_inputs_store(const char *val,
> >  	if (!commit_inputs_request)
> >  		return 0;
> >  
> > -	/*
> > -	 * Skip damon_call() if ctx is not initialized to avoid
> > -	 * NULL pointer dereference.
> > -	 */
> > -	if (!ctx)
> > +	/* Skip damon_call() if ctx has not successfully started. */
> > +	if (!damon_lru_sort_damon_has_started)
> >  		return -EINVAL;
> >  
> >  	err = damon_call(ctx, &control);
> 
> [Severity: High]
> This is a pre-existing issue, but could there be a TOCTOU race here between
> damon_start() and damon_call() that causes damon_call() to loop indefinitely?
> 
> If a user starts DAMON (setting damon_lru_sort_damon_has_started = true),
> stops it (leaving call_controls_obsolete = true), and then concurrently
> starts DAMON again while calling damon_call() via commit_inputs, can
> damon_call() get stuck?

There is no such concurrent calls.

> 
> Looking at mm/damon/core.c, __damon_start() sets ctx->kdamond before the new
> thread clears ctx->call_controls_obsolete:
> 
> mm/damon/core.c:__damon_start() {
>     ...
>     ctx->kdamond = kthread_run(kdamond_fn, ctx, "kdamond.%d", nr_running_ctxs);
>     ...
> }
> 
> If damon_call() runs concurrently, it checks call_controls_obsolete without
> holding kdamond_lock, sees the stale true value, and jumps to the canceled
> label:
> 
> mm/damon/core.c:damon_call() {
>     ...
>     if (ctx->call_controls_obsolete) {
>         ...
>         goto canceled;
>     }
>     ...
> canceled:
>     while (damon_is_running(ctx))
>         schedule_timeout_idle(msecs_to_jiffies(100));
>     return -ECANCELED;
> }
> 
> Since damon_is_running() returns true (because ctx->kdamond is set), and the
> new healthy thread will run indefinitely, will this task loop forever in
> TASK_IDLE?

So the issue cannot happen in real.  But good point.  Maybe the API or the
documentation can be safer.  I will add this to my TODO list.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260802185430.9940-1-sj@kernel.org?part=2


Thanks,
SJ