Re: [RFC PATCH 1/2] mm/damon/reclaim: skip damon_call() if ctx has not started

On Sun, 02 Aug 2026 19:04:30 +0000 [email protected] wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] This problem wasn't introduced by this patch, but the patch fixes an indefinite hang in `mm/damon/reclaim.c` when writing to `commit_inputs` before the DAMON context starts, while failing to apply the identical fix to `mm/damon/lru_sort.c`. This leaves `damon_lru_sort` vulnerable to the exact same bug.
> --
> 
> --- Patch [1]: [RFC PATCH 1/2] mm/damon/reclaim: skip damon_call() if ctx has not started ---
> commit 86e90c966d8636445486cfde6487a797b433cb75
> Author: SJ Park <[email protected]>
> 
> mm/damon/reclaim: skip damon_call() if ctx has not started
> 
> This commit addresses an indefinite hang in DAMON reclaim when writing to
> commit_inputs before the context has fully started. It introduces a tracking
> flag to determine whether the DAMON context has successfully started in order
> to safely skip damon_call().
> 
> Cc: <[email protected]> # 7.2.x
> 
> > diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
> > index 09e941d75f67d..45d5557cc575a 100644
> > --- a/mm/damon/reclaim.c
> > +++ b/mm/damon/reclaim.c
> [ ... ]
> > @@ -291,11 +293,8 @@ static int damon_reclaim_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_reclaim_damon_has_started)
> >  		return -EINVAL;
> >  
> >  	err = damon_call(ctx, &control);
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but does the same vulnerability
> exist in mm/damon/lru_sort.c?
> 
> It looks like damon_lru_sort_commit_inputs_store() uses the exact same
> logic that was fixed here in reclaim.c:
> 
> mm/damon/lru_sort.c:damon_lru_sort_commit_inputs_store() {
>     ...
>     /*
>      * Skip damon_call() if ctx is not initialized to avoid
>      * NULL pointer dereference.
>      */
>     if (!ctx)
>         return -EINVAL;
> 
>     err = damon_call(ctx, &control);
>     ...
> }
> 
> If a user writes to commit_inputs for damon_lru_sort before the thread has
> started, could damon_call() blindly invoke wait_for_completion() and hang
> indefinitely without a running thread to signal it?
> 
> Should a similar fix be applied to mm/damon/lru_sort.c to prevent a task
> from hanging in an uninterruptible sleep state?

Yes, and the next patch of this series is fixing it.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1


Thanks,
SJ