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

[SJ Park <sj@kernel.org>]

On Thu, 13 Aug 2026 17:27:51 +0300 Gutierrez Asier <gutierrez.asier@huawei-partners.com> wrote:

> 
> 
> On 8/13/2026 4:55 PM, SJ Park wrote:
> > 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?
> Good point, I didn't think about it.
> 
> Given that damon_call is not called often, I believe user experience should be
> more important than efficiency. Anyway, you have the last word.

Thank you for the quick answer, Asier.  I understand you are find with keeping
this as is.  Let's keep this as is for now.


Thanks,
SJ

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