Re: [PATCH 1/6] ring-buffer: Prevent resizing of persistent ring buffer

[Vincent Donnefort <vdonnefort@google.com>]

On Fri, Aug 07, 2026 at 10:43:12AM +0100, Vincent Donnefort wrote:
> On Fri, Aug 07, 2026 at 09:14:26AM +0100, Vincent Donnefort wrote:
> > On Fri, Aug 07, 2026 at 11:18:08AM +0900, Masami Hiramatsu wrote:
> > > On Thu,  6 Aug 2026 22:13:01 +0100
> > > Vincent Donnefort <vdonnefort@google.com> wrote:
> > > 
> > > > Dynamically resizing a persistent ring buffer is not possible. Disable
> > > > the feature.
> > > 
> > > Is it true? Of course there is meaningless to resize the persistent
> > > ring buffer (because it makes the buffer none-persistent), we are currently
> > > allows user to resize it (like for resizing unused persistent ring buffer)
> > 
> > __rb_allocate_pages() in ring_buffer_resize() would call for a persistent buffer
> > rb_range_buffer(), which IIUC, is just reusing the same ring buffer pages as the
> > one already in the persistent buffer.

With more logs, during a resize form 8 to 16K: 

cpu_buffer->new_pages contains:

[  632.919017] ring_buffer_resize: CPU 0 NEW page 00000000768ccc81
[  632.919828] ring_buffer_resize: CPU 0 NEW page 00000000ee48729c

while cpu_buffer->pages contains:

[  632.921511] ring_buffer_resize: CPU 0 OLD page 000000002c4c6f92
[  632.921917] ring_buffer_resize: CPU 0 OLD page 00000000dadcb510
[  632.922295] ring_buffer_resize: CPU 0 OLD page 00000000768ccc81
[  632.923226] ring_buffer_resize: CPU 0 OLD reader page 000000003b4dd5ba

So two buffer_page point to 00000000768ccc81.

-- 
Vincent

> 
> I have just tried and if reducing the size works, increasing fails in both
> rb_set_head_page() and rb_insert_pages() with a warning, which I believe is
> expected.
> 
> We could improve that, but it feels like it is a lot of work for a meaningless
> feature which we should just disable?
> 
> -- 
> Vincent
> 
> > 
> > So, reducing the size would surely work, however increasing it, would most
> > likely mean having buffer_page pointing to the same buffer_data_page?
> > 
> > Also, the comment
> > 
> >   /* 
> >    * Range mapped buffers have the same restrictions as memory
> >    * mapped ones do.
> >    */
> > 
> > Made me think that resize_disabled was omitted.
> > 
> > > 
> > > Thank you,
> > > 
> > > > 
> > > > Fixes: be68d63a139b ("ring-buffer: Add ring_buffer_alloc_range()")
> > > > Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> > > > 
> > > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > > > index 8e2485bb3aa8..afe75ad2bbf2 100644
> > > > --- a/kernel/trace/ring_buffer.c
> > > > +++ b/kernel/trace/ring_buffer.c
> > > > @@ -2528,6 +2528,8 @@ rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
> > > >  		if (cpu_buffer->ring_meta->head_buffer)
> > > >  			rb_meta_buffer_update(cpu_buffer, bpage);
> > > >  		bpage->range = 1;
> > > > +
> > > > +		atomic_inc(&cpu_buffer->resize_disabled);
> > > >  	} else if (buffer->remote) {
> > > >  		struct ring_buffer_desc *desc = ring_buffer_desc(buffer->remote->desc, cpu);
> > > >  
> > > > -- 
> > > > 2.55.0.654.g21b8a5bc05-goog
> > > > 
> > > 
> > > 
> > > -- 
> > > Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> > -- 
> > Vincent
>