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 18:53:19 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Guenter Roeck <linux@...ck-us.net>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 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 Tue, 11 Jun 2024 15:43:59 -0700
Guenter Roeck <linux@...ck-us.net> wrote:

> 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.

Hmm, I could remove it.

> 
> >   	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 ?

If mapped is non zero, it limits what it can do. If it enters here as zero,
we are really in a unknown state, so yeah, wrapping will just keep it
limited. Which is a good thing.

Do you want me to add a comment there?

-- Steve

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