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

[Michal Hocko <mhocko@suse.com>]

On Fri 05-09-25 20:48:46, Peilin Ye wrote:
> 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@linux.dev>
> 
> Tested-by: Peilin Ye <yepeilin@google.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);

Why do you need to consume memory reserves? Shouldn't kmalloc_nolock be
used instead here?

>         if (!cb) {
>                 ret = -ENOMEM;
>                 goto out;
> 
> [1] https://lore.kernel.org/bpf/20250905061919.439648-1-yepeilin@google.com/#t
> 
> Thanks,
> Peilin Ye

-- 
Michal Hocko
SUSE Labs