Re: [RFC PATCH 1/4] mm/damon/core: handle NULL ctx parameter in damon_call()

[SJ Park <sj@kernel.org>]

Hi Asier,


Thank you for your review!

On Thu, 13 Aug 2026 11:33:01 +0300 Gutierrez Asier <gutierrez.asier@huawei-partners.com> wrote:

> Hi SJ,
> 
> On 8/13/2026 6:49 AM, SJ Park wrote:
> > When NULL damon_ctx pointer parameter is passed, damon_call() could do
> > NULL dereference.  The caller is responsible to avoid that.  It is easy
> > to forget, and there are many damon_call() callers.  Meanwhile,
> > damon_call() is never meant to be performance critical.  It uses mutex
> > and completion.  Add the NULL pointer check inside damon_call() so that
> > callers can pass the parameter without NULL checks.
> > 
> > Signed-off-by: SJ Park <sj@kernel.org>
> > ---
> >  mm/damon/core.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 92631a36d7b51..5882f9c94c47f 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -2188,6 +2188,8 @@ int damon_kdamond_pid(struct damon_ctx *ctx)
> >   */
> >  int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
> >  {
> > +	if (!ctx)
> > +		return -EINVAL;
> >  	if (!control->repeat)
> >  		init_completion(&control->completion);
> >  	control->canceled = false;
> 
> I know that this is a minor optimization, but how about moving INIT_LIST_HEAD(&control->list)
> to right before list_add_tail?
> 
> It doesn't make sense to initialize the list before checking call_controls_obsolete.

I agree it might be more efficient.

I think my intention of the code was to to keep the call_controls_lock critical
section minimal with access to things that really protected by the mutex.
Moving the simple initialization should be fine.  As you mentioned, it should
be minor change.  I'm bit concerned if it will confuse people if the list head
is also protected by the mutex, though.  What do you think?

> 
> Maybe this suggestion should go in a different patch.

I agree.


Thanks,
SJ

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