* Re: [PATCH] mm/damon/core: support multiple damon_call_control requests
[not found] <20251202021407.11818-1-lienze@kylinos.cn>
@ 2025-12-02 5:29 ` SeongJae Park
2025-12-02 7:55 ` Enze Li
0 siblings, 1 reply; 2+ messages in thread
From: SeongJae Park @ 2025-12-02 5:29 UTC (permalink / raw)
To: Enze Li; +Cc: SeongJae Park, akpm, damon, linux-mm, enze.li, stable
Hello Enze,
First of all, thank you for sharing this patch!
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.
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.
>
> 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.
> To demonstrate
> the effect of this change, I made minor modifications to
> samples/damon/prcl.c by adding a new request alongside the original
> damon_call_control request and performed comparative tests.
>
> Before applying the patch, I observed,
>
> [ 381.661821] damon_sample_prcl: start
> [ 381.668199] damon_sample_prcl: repeat_call_v2
> [ 381.668208] damon_sample_prcl: repeat_call
> [ 381.668211] damon_sample_prcl: wss: 0
> [ 381.675194] damon_sample_prcl: repeat_call
> [ 381.675202] damon_sample_prcl: wss: 0
>
> after applying the patch, I saw,
>
> [ 61.750723] damon_sample_prcl: start
> [ 61.757104] damon_sample_prcl: repeat_call_v2
> [ 61.757106] damon_sample_prcl: repeat_call
> [ 61.757107] damon_sample_prcl: wss: 0
> [ 61.763067] damon_sample_prcl: repeat_call_v2
> [ 61.763069] damon_sample_prcl: repeat_call
> [ 61.763070] damon_sample_prcl: wss: 0
>
> Signed-off-by: Enze Li <lienze@kylinos.cn>
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?
Again, thank you for letting us find this bug.
Thanks,
SJ
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