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

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Message-ID: <aLtMrlSDP7M5GZ27@google.com>
Date: Fri, 5 Sep 2025 20:48:46 +0000
From: Peilin Ye <yepeilin@...gle.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>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Alexei Starovoitov <ast@...nel.org>,
	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 01:16:06PM -0700, 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>

Tested-by: Peilin Ye <yepeilin@...gle.com>

The repro described in [1] no longer triggers locking issues after
applying this patch and making __bpf_async_init() use __GFP_HIGH
instead of GFP_ATOMIC:

--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1275,7 +1275,7 @@ static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u
        }

        /* allocate hrtimer via map_kmalloc to use memcg accounting */
-       cb = bpf_map_kmalloc_node(map, size, GFP_ATOMIC, map->numa_node);
+       cb = bpf_map_kmalloc_node(map, size, __GFP_HIGH, map->numa_node);
        if (!cb) {
                ret = -ENOMEM;
                goto out;

[1] https://lore.kernel.org/bpf/20250905061919.439648-1-yepeilin@google.com/#t

Thanks,
Peilin Ye