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

[Gutierrez Asier <gutierrez.asier@huawei-partners.com>]

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.
> 
>>
>> Maybe this suggestion should go in a different patch.
> 
> I agree.
> 
> Thanks,
> SJ
> 
> [...]

-- 
Asier Gutierrez
Huawei