Re: [PATCH] memcg: damon: get memcg reference before access

[Shakeel Butt <shakeel.butt@linux.dev>]

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@linux.dev> wrote:
> 
> > The commit b74a120bcf507 ("mm/damon/core: implement
> > DAMOS_QUOTA_NODE_MEMCG_USED_BP") added accesses to memcg structure
> > without getting reference to it. This is unsafe. Let's get the reference
> > before accessing the memcg.
> 
> Thank you for catching and fixing this!
> 
> Nit.  On the subject, could we use 'mm/damon/core:' prefix instead of 'memcg:
> damon:' for keeping the file's commit log subjects consistent?

Done.

> 
> > 
> > Fixes: b74a120bcf507 ("mm/damon/core: implement DAMOS_QUOTA_NODE_MEMCG_USED_BP")
> 
> I was firsty thinking we might need to Cc: stable@.  But I realized the broken
> commit has merged into 6.19-rc1.  So Cc: stable@ is not needed.
> 
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> 
> Other than the tirivial subject prefix inconsistency, looks good to me.
> 
> Reviewed-by: SeongJae Park <sj@kernel.org>
>

Thanks.

> > ---
> >  mm/damon/core.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > 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.