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

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Message-ID: <mai3ndkvqrpkfpblkazbyejvpkizrp7dh22374tpkmepfji32o@3troawzsuvqe>
Date: Fri, 5 Sep 2025 14:31:13 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Tejun Heo <tj@...nel.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 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?
>

Thanks for the review. For now only MAX can happen in non-spinning
context. All others only happen in process context. Maybe with BPF OOM,
OOM might be possible in a different context (is that what you are
thinking?). I think we can add the complexity of preserving the event
when the actual need arise.