Re: [PATCH] mm/damon/core: support multiple damon_call_control requests

[Enze Li <lienze@kylinos.cn>]

Hi SJ,

Thanks for your review and quick reply!

On Mon, Dec 01 2025 at 09:29:55 PM -0800, SeongJae Park wrote:

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> On Tue,  2 Dec 2025 10:14:07 +0800 Enze Li <lienze@kylinos.cn> wrote:
>
>> The current implementation only supports repeated calls to a single
>> damon_call_control request per context.
>
> I understand "repeated calls to a single damon_call_control" means "single
> damon_call_control object having ->repeat set as true".  Let me call it "repeat
> mode damon_call_control object".
>
> This is not an intentionally designed limitation but a bug.  damon_call()
> allows callers adding multiple repeat mode damon_call_control objects per
> context.  Technically, it adds any requested damon_call_control object to the
> per-context linked list, regardless of the number of repeat mode objects on the
> list.  But, the consumer of the damon_call_control objects list,
> kdamond_call(), moves the repeat mode objects from the per-context list to a
> temporal list (repeat_controls), and then move only the first repeat mode entry
> from the temporal list to the per-context list.
>
> If there were multiple repeat mode objects in the per-context list, what
> happens to the remaining repeat mode damon_call_control objects on the temporal
> list?  Nothing.  As a result, the memory for the objects are leaked.
> Definitely this is a bug.

Thank you for the detailed explanation -- it really clarified the design
for me.

>
> Luckily there is no such multiple repeat mode damon_call() requests, so no
> upstream kernel user is exposed to the memory leak bug in real.  But the bug is
> a bug.  We should fix this.
>
>> This limitation introduces
>> inefficiencies for scenarios that require registering multiple deferred
>> operations.
>
> I'm not very convinced with the above reasoning because 1. it is not a matter
> of inefficiency but a clear memory leak bug.  2. there is no damon_call()
> callers that want to have multiple deferred operations with high efficiency, at
> the moment.  In my opinion, the above sentence is better to be just dropped.
>

Agreed.  I will rework the patch description for the next revision.

>> 
>> This patch modifies the implementation of kdamond_call() to support
>> repeated calls to multiple damon_call_control requests.
>
> This change is rquired for fixing the bug, though.
>

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>
> Assuming we agree on the fact this is a fix of the bug, I think we should add
> below tags.
>
> Fixes: 43df7676e550 ("mm/damon/core: introduce repeat mode damon_call()")
> Cc: <stable@vger.kernel.org> # 6.17.x
>
>> ---
>>  mm/damon/core.c | 20 +++++++++++++-------
>>  1 file changed, 13 insertions(+), 7 deletions(-)
>> 
>> diff --git a/mm/damon/core.c b/mm/damon/core.c
>> index 109b050c795a..66b5bae44f22 100644
>> --- a/mm/damon/core.c
>> +++ b/mm/damon/core.c
>> @@ -2526,13 +2526,19 @@ static void kdamond_call(struct damon_ctx *ctx, bool cancel)
>>  			list_add(&control->list, &repeat_controls);
>>  		}
>>  	}
>> -	control = list_first_entry_or_null(&repeat_controls,
>> -			struct damon_call_control, list);
>> -	if (!control || cancel)
>> -		return;
>> -	mutex_lock(&ctx->call_controls_lock);
>> -	list_add_tail(&control->list, &ctx->call_controls);
>> -	mutex_unlock(&ctx->call_controls_lock);
>> +	while (true) {
>> +		control = list_first_entry_or_null(&repeat_controls,
>> +				struct damon_call_control, list);
>> +		if (!control)
>> +			break;
>> +		/* Unlink from the repeate_controls list. */
>> +		list_del(&control->list);
>> +		if (cancel)
>> +			continue;
>> +		mutex_lock(&ctx->call_controls_lock);
>> +		list_add(&control->list, &ctx->call_controls);
>> +		mutex_unlock(&ctx->call_controls_lock);
>> +	}
>
> This looks good enough to fix the bug.
>
> Could you please resend this patch after rewording the commit message as
> appripriate for the bug fix, adding points I listed above?

Okay, I'll resend this patch shortly.

Best Regards,
Enze

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