linux-kernel - Re: [PATCH v4 01/13] ring-buffer: Allow mapped field to be set without mapping

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Date: Tue, 11 Jun 2024 15:43:59 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Vincent Donnefort <vdonnefort@...gle.com>,
 Joel Fernandes <joel@...lfernandes.org>,
 Daniel Bristot de Oliveira <bristot@...hat.com>,
 Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 suleiman@...gle.com, Thomas Gleixner <tglx@...utronix.de>,
 Vineeth Pillai <vineeth@...byteword.org>,
 Youssef Esmat <youssefesmat@...gle.com>,
 Beau Belgrave <beaub@...ux.microsoft.com>, Alexander Graf <graf@...zon.com>,
 Baoquan He <bhe@...hat.com>, Borislav Petkov <bp@...en8.de>,
 "Paul E. McKenney" <paulmck@...nel.org>, David Howells
 <dhowells@...hat.com>, Mike Rapoport <rppt@...nel.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Tony Luck <tony.luck@...el.com>,
 Ross Zwisler <zwisler@...gle.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v4 01/13] ring-buffer: Allow mapped field to be set
 without mapping

On 6/11/24 12:28, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> In preparation for having the ring buffer mapped to a dedicated location,
> which will have the same restrictions as user space memory mapped buffers,
> allow it to use the "mapped" field of the ring_buffer_per_cpu structure
> without having the user space meta page mapping.
> 
> When this starts using the mapped field, it will need to handle adding a
> user space mapping (and removing it) from a ring buffer that is using a
> dedicated memory range.
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>   kernel/trace/ring_buffer.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 28853966aa9a..78beaccf9c8c 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -5224,6 +5224,9 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
>   {
>   	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
>   
> +	if (!meta)
> +		return;
> +
>   	meta->reader.read = cpu_buffer->reader_page->read;
>   	meta->reader.id = cpu_buffer->reader_page->id;
>   	meta->reader.lost_events = cpu_buffer->lost_events;
> @@ -6167,7 +6170,7 @@ rb_get_mapped_buffer(struct trace_buffer *buffer, int cpu)
>   
>   	mutex_lock(&cpu_buffer->mapping_lock);
>   
> -	if (!cpu_buffer->mapped) {
> +	if (!cpu_buffer->mapped || !cpu_buffer->meta_page) {
>   		mutex_unlock(&cpu_buffer->mapping_lock);
>   		return ERR_PTR(-ENODEV);
>   	}
> @@ -6359,12 +6362,13 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
>   	 */
>   	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>   	rb_setup_ids_meta_page(cpu_buffer, subbuf_ids);
> +

Picky again. Is that a leftover from something ? I don't see an immediate reason
for the added newline.

>   	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>   
>   	err = __rb_map_vma(cpu_buffer, vma);
>   	if (!err) {
>   		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> -		cpu_buffer->mapped = 1;
> +		cpu_buffer->mapped++;
>   		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>   	} else {
>   		kfree(cpu_buffer->subbuf_ids);
> @@ -6403,7 +6407,8 @@ int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
>   	mutex_lock(&buffer->mutex);
>   	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>   
> -	cpu_buffer->mapped = 0;
> +	WARN_ON_ONCE(!cpu_buffer->mapped);
> +	cpu_buffer->mapped--;

This will wrap to UINT_MAX if it was 0. Is that intentional ?

>   
>   	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>