linux-kernel - Re: [PATCH] memcg: damon: get memcg reference before access

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Message-ID: <20251224153731.69411-1-sj@kernel.org>
Date: Wed, 24 Dec 2025 07:37:30 -0800
From: SeongJae Park <sj@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: SeongJae Park <sj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	damon@...ts.linux.dev,
	linux-mm@...ck.org,
	cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] memcg: damon: get memcg reference before access

On Tue, 23 Dec 2025 23:51:55 -0800 Shakeel Butt <shakeel.butt@...ux.dev> wrote:

> On Tue, Dec 23, 2025 at 09:21:47PM -0800, SeongJae Park wrote:
> > On Tue, 23 Dec 2025 19:45:27 -0800 Shakeel Butt <shakeel.butt@...ux.dev> wrote:
[...]
> > > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > > index 4ad5f290d382..89982e0229f0 100644
> > > --- a/mm/damon/core.c
> > > +++ b/mm/damon/core.c
> > > @@ -2051,13 +2051,15 @@ static unsigned long damos_get_node_memcg_used_bp(
> > >  
> > >  	rcu_read_lock();
> > >  	memcg = mem_cgroup_from_id(goal->memcg_id);
> > > -	rcu_read_unlock();
> > > -	if (!memcg) {
> > > +	if (!memcg || !mem_cgroup_tryget(memcg)) {
> > 
> > For this part, I was thinking '!memcg' part seems not technically needed
> > because mem_cgroup_tryget() does the check.  But I think that's just trivial,
> > so this also looks good to me.
> > 
> 
> Hmm !memcg check inside mem_cgroup_tryget() is a weird one. It makes
> mem_cgroup_tryget() to return true for NULL parameter. We can not use
> mem_cgroup_tryget() as is alone here.

Oh, you're right.  I should have read it more carefully.  Thank you for
enlightening me :)


Thanks,
SJ

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