linux-kernel - Re: [PATCH] fgraph: Allocate ret_stack

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Message-Id: <20241021164336.79a90af16419aa6e3f065bf3@kernel.org>
Date: Mon, 21 Oct 2024 16:43:36 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux trace kernel
 <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, Mathieu
 Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] fgraph: Allocate ret_stack_list with proper size

On Fri, 18 Oct 2024 21:52:12 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> 
> The ret_stack_list is an array of ret_stack shadow stacks for the function
> graph usage. When the first function graph is enabled, all tasks in the
> system get a shadow stack. The ret_stack_list is a 32 element array of
> pointers to these shadow stacks. It allocates the shadow stack in batches
> (32 stacks at a time), assigns them to running tasks, and continues until
> all tasks are covered.
> 
> When the function graph shadow stack changed from an array of
> ftrace_ret_stack structures to an array of longs, the allocation of
> ret_stack_list went from allocating an array of 32 elements to just a
> block defined by SHADOW_STACK_SIZE. Luckily, that's defined as PAGE_SIZE
> and is much more than enough to hold 32 pointers. But it is way overkill
> for the amount needed to allocate.
> 
> Change the allocation of ret_stack_list back to a kcalloc() of
> FTRACE_RETSTACK_ALLOC_SIZE pointers.

Looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thanks,

> 
> Fixes: 42675b723b484 ("function_graph: Convert ret_stack to a series of longs")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/fgraph.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 43f4e3f57438..41e7a15dcb50 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -1162,7 +1162,8 @@ static int start_graph_tracing(void)
>  	unsigned long **ret_stack_list;
>  	int ret;
>  
> -	ret_stack_list = kmalloc(SHADOW_STACK_SIZE, GFP_KERNEL);
> +	ret_stack_list = kcalloc(FTRACE_RETSTACK_ALLOC_SIZE,
> +				 sizeof(*ret_stack_list), GFP_KERNEL);
>  
>  	if (!ret_stack_list)
>  		return -ENOMEM;
> -- 
> 2.45.2
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>