Re: [PATCH v2] memcg: skip cgroup_file_notify if spinning is not allowed

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

On Mon, Sep 22, 2025 at 04:55:09PM -0700, Andrew Morton wrote:
> On Mon, 22 Sep 2025 16:39:53 -0700 Shakeel Butt <shakeel.butt@linux.dev> wrote:
> 
> > On Mon, Sep 22, 2025 at 04:04:43PM -0700, Andrew Morton wrote:
> > > On Mon, 22 Sep 2025 15:02:03 -0700 Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > > 
> > > > Generally memcg charging is allowed from all the contexts including NMI
> > > > where even spinning on spinlock can cause locking issues. However one
> > > > call chain was missed during the addition of memcg charging from any
> > > > context support. That is try_charge_memcg() -> memcg_memory_event() ->
> > > > cgroup_file_notify().
> > > > 
> > > > The possible function call tree under cgroup_file_notify() can acquire
> > > > many different spin locks in spinning mode. Some of them are
> > > > cgroup_file_kn_lock, kernfs_notify_lock, pool_workqeue's lock. So, let's
> > > > just skip cgroup_file_notify() from memcg charging if the context does
> > > > not allow spinning.
> > > > 
> > > > Alternative approach was also explored where instead of skipping
> > > > cgroup_file_notify(), we defer the memcg event processing to irq_work
> > > > [1]. However it adds complexity and it was decided to keep things simple
> > > > until we need more memcg events with !allow_spinning requirement.
> > > > 
> > > > Link: https://lore.kernel.org/all/5qi2llyzf7gklncflo6gxoozljbm4h3tpnuv4u4ej4ztysvi6f@x44v7nz2wdzd/ [1]
> > > > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> > > > Acked-by: Michal Hocko <mhocko@suse.com>
> > > 
> > > Fixes a possible kernel deadlock, yes?
> > > 
> > > Is a cc:stable appropriate and can we identify a Fixes: target?
> > > 
> > > Thanks.
> > > 
> > > (Did it ever generate lockdep warnings?)
> > 
> > The report is here:
> > https://lore.kernel.org/all/20250905061919.439648-1-yepeilin@google.com/
> > 
> > I am not sure about the Fixes tag though or more like which one to put
> > in the Fixes as we recently started supporting memcg charging for NMI
> > context or allowing bpf programs to do memcg charged allocations in
> > recursive context (see the above report for this recursive call chain).
> > There is no single commit which can be blamed here.
> 
> I tend to view the Fixes: as us suggesting which kernel versions should
> be patched.  I'm suspecting that's 6.16+, so using the final relevant
> patch in that release as a Fixes: target would work.
> 

Sounds good. Let use the following.

Fixes: 3ac4638a734a ("memcg: make memcg_rstat_updated nmi safe")