linux-kernel - Re: [PATCH] memcg: skip cgroup_file_notify if spinning is not allowed

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Message-ID: <uattgslnbiit2yhz5hzh6g7mc4g4nnbkqospnjuqikzliskyo4@z5i66op4jhdp>
Date: Fri, 5 Sep 2025 14:35:01 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Tejun Heo <tj@...nel.org>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>, 
	Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Muchun Song <muchun.song@...ux.dev>, 
	Alexei Starovoitov <ast@...nel.org>, Peilin Ye <yepeilin@...gle.com>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] memcg: skip cgroup_file_notify if spinning is not allowed

On Fri, Sep 05, 2025 at 11:25:34AM -1000, Tejun Heo wrote:
> On Fri, Sep 05, 2025 at 02:20:46PM -0700, Roman Gushchin wrote:
> > Shakeel Butt <shakeel.butt@...ux.dev> writes:
> > 
> > > 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.
> > 
> > Hmm, what about OOM events? Losing something like MEMCG_LOW doesn't look
> > like a bit deal, but OOM events can be way more important.
> > 
> > Should we instead preserve the event (e.g. as a pending_event_mask) and
> > raise it on the next occasion / from a different context?
> 
> Maybe punt with an irq_work?

Oh good suggestion, I will check irq_work as well. Though I think that
can be a followup work.