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

[Vincent Donnefort <vdonnefort@google.com>]

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.

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