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 16:53:43 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Steven Rostedt <rostedt@...dmis.org>
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 6/11/24 15:53, Steven Rostedt wrote:
> 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?
> 

Maybe. I just wondered if something like
	if (!WARN_ON_ONCE(!cpu_buffer->mapped))
		cpu_buffer->mapped--;

would be better than wrapping because 'mapped' is used as flag elsewhere,
but then I can see that it is also manipulated in __rb_inc_dec_mapped(),
and that it is checked against UINT_MAX there (and not decremented if it is 0).

Maybe explain why sometimes __rb_inc_dec_mapped() is called to increment
or decrement ->mapped, and sometimes it id done directly ? I can see that
the function also acquires the buffer mutex, which isn't needed at the places
where mapped is incremented/decremented directly, but common code would
still be nice, and it is odd to see over/underflows handled sometimes but
not always.

Thanks,
Guenter