Re: [PATCH 4/7] tracing: account ring buffer pages and trace_array metadata to memcg

[Vincent Donnefort]

On Fri, Nov 06, 2026 at 09:18:26PM +0800, Tao Cui wrote:
> From: Tao Cui <cuitao@xxxxxxxxxx>
> 
> Creating an instance immediately allocates two full sets of per-cpu
> ring buffers, array buffer plus snapshot, about 1.375MB per cpu per set
> on the default config, plus the trace_array and its name.  None of it
> is charged: neither trace.c nor ring_buffer.c passes __GFP_ACCOUNT,
> while user_events in the same directory accounts everything it
> allocates.
> 
> Instance creation is currently gated by tracefs file permissions
> alone, so a
> rootful container with the host tracefs bound in can create instances
> until the host runs out of memory.  Charge the ring buffer pages and
> the trace_array metadata to the creating cgroup.
> 
> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
> ---
>  kernel/trace/ring_buffer.c | 8 +++++++-
>  kernel/trace/trace.c       | 6 +++---
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 1fd28a72e6ee..676c059c112c 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -441,7 +441,13 @@ static struct buffer_data_page *alloc_cpu_data(int cpu, int order)
>  	 * gracefully without invoking oom-killer and the system is not
>  	 * destabilized.
>  	 */
> -	mflags = GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_COMP | __GFP_ZERO;
> +	/*
> +	 * The subbuffers are charged to the memcg of whoever caused the
> +	 * allocation: the creator of a trace instance, or root at boot for
> +	 * the global trace.
> +	 */
> +	mflags = GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_COMP | __GFP_ZERO |
> +		  __GFP_ACCOUNT;
>  
>  	page = alloc_pages_node(cpu_to_node(cpu), mflags, order);
>  	if (!page)

Hum, by default tracefs is root only. 

And even if made accessible to other users: 

  1. The life span of a trace instance is not tied to the caller.
  2. The ring buffer is shared by multiple users.

Overall I am not convinced charging the creator makes much sense.

Eventually a few allocations for iterators could be GFP_ACCOUNT.

-- 
Vincent

> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index e8c4efa8107b..04b60b9cc4f5 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -8693,11 +8693,11 @@ trace_array_create_systems(const char *name, const char *systems,
>  	int ret;
>  
>  	ret = -ENOMEM;
> -	tr = kzalloc_obj(*tr);
> +	tr = kzalloc_obj(*tr, GFP_KERNEL_ACCOUNT);
>  	if (!tr)
>  		return ERR_PTR(ret);
>  
> -	tr->name = kstrdup(name, GFP_KERNEL);
> +	tr->name = kstrdup(name, GFP_KERNEL_ACCOUNT);
>  	if (!tr->name)
>  		goto out_free_tr;
>  
> @@ -8708,7 +8708,7 @@ trace_array_create_systems(const char *name, const char *systems,
>  		goto out_free_tr;
>  
>  	if (systems) {
> -		tr->system_names = kstrdup_const(systems, GFP_KERNEL);
> +		tr->system_names = kstrdup_const(systems, GFP_KERNEL_ACCOUNT);
>  		if (!tr->system_names)
>  			goto out_free_tr;
>  	}
> -- 
> 2.43.0
>