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

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Message-ID: <aL6htMt-jHAaCGLv@tiehlicka>
Date: Mon, 8 Sep 2025 11:28:20 +0200
From: Michal Hocko <mhocko@...e.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	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 05-09-25 13:16:06, Shakeel Butt 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.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  include/linux/memcontrol.h | 23 ++++++++++++++++-------
>  mm/memcontrol.c            |  7 ++++---
>  2 files changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 9dc5b52672a6..054fa34c936a 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -993,22 +993,25 @@ static inline void count_memcg_event_mm(struct mm_struct *mm,
>  	count_memcg_events_mm(mm, idx, 1);
>  }
>  
> -static inline void memcg_memory_event(struct mem_cgroup *memcg,
> -				      enum memcg_memory_event event)
> +static inline void __memcg_memory_event(struct mem_cgroup *memcg,
> +					enum memcg_memory_event event,
> +					bool allow_spinning)
>  {
>  	bool swap_event = event == MEMCG_SWAP_HIGH || event == MEMCG_SWAP_MAX ||
>  			  event == MEMCG_SWAP_FAIL;
>  
>  	atomic_long_inc(&memcg->memory_events_local[event]);

Doesn't this involve locking on 32b? I guess we do not care all that
much but we might want to bail out early on those arches for
!allow_spinning

> -	if (!swap_event)
> +	if (!swap_event && allow_spinning)
>  		cgroup_file_notify(&memcg->events_local_file);
>  
>  	do {
>  		atomic_long_inc(&memcg->memory_events[event]);
> -		if (swap_event)
> -			cgroup_file_notify(&memcg->swap_events_file);
> -		else
> -			cgroup_file_notify(&memcg->events_file);
> +		if (allow_spinning) {
> +			if (swap_event)
> +				cgroup_file_notify(&memcg->swap_events_file);
> +			else
> +				cgroup_file_notify(&memcg->events_file);
> +		}
>  
>  		if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
>  			break;
> @@ -1018,6 +1021,12 @@ static inline void memcg_memory_event(struct mem_cgroup *memcg,
>  		 !mem_cgroup_is_root(memcg));
>  }
>  
> +static inline void memcg_memory_event(struct mem_cgroup *memcg,
> +				      enum memcg_memory_event event)
> +{
> +	__memcg_memory_event(memcg, event, true);
> +}
> +
>  static inline void memcg_memory_event_mm(struct mm_struct *mm,
>  					 enum memcg_memory_event event)
>  {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 257d2c76b730..dd5cd9d352f3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2306,12 +2306,13 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	bool drained = false;
>  	bool raised_max_event = false;
>  	unsigned long pflags;
> +	bool allow_spinning = gfpflags_allow_spinning(gfp_mask);
>  
>  retry:
>  	if (consume_stock(memcg, nr_pages))
>  		return 0;
>  
> -	if (!gfpflags_allow_spinning(gfp_mask))
> +	if (!allow_spinning)
>  		/* Avoid the refill and flush of the older stock */
>  		batch = nr_pages;
>  
> @@ -2347,7 +2348,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	if (!gfpflags_allow_blocking(gfp_mask))
>  		goto nomem;
>  
> -	memcg_memory_event(mem_over_limit, MEMCG_MAX);
> +	__memcg_memory_event(mem_over_limit, MEMCG_MAX, allow_spinning);
>  	raised_max_event = true;
>  
>  	psi_memstall_enter(&pflags);
> @@ -2414,7 +2415,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	 * a MEMCG_MAX event.
>  	 */
>  	if (!raised_max_event)
> -		memcg_memory_event(mem_over_limit, MEMCG_MAX);
> +		__memcg_memory_event(mem_over_limit, MEMCG_MAX, allow_spinning);
>  
>  	/*
>  	 * The allocation either can't fail or will lead to more memory
> -- 
> 2.47.3
> 

-- 
Michal Hocko
SUSE Labs