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

[Vincent Donnefort <vdonnefort@google.com>]

On Fri, Aug 07, 2026 at 04:25:33PM +0100, Vincent Donnefort wrote:
> On Fri, Aug 07, 2026 at 10:45:26AM -0400, Steven Rostedt wrote:
> > On Fri, 7 Aug 2026 10:43:12 +0100
> > Vincent Donnefort <vdonnefort@google.com> 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.  
> > > 
> > > 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?
> > 
> > Resizing a persistent ring buffer to a smaller size may be allowed, but I
> > see no point in increasing the size. Making it smaller should allow us to
> > give back a portion of the persistent ring buffer for general usage.
> > 
> > -- Steve
> 
> While I see the appeal to reclaim that memory, I don't think we have any good
> interface for that. There is no nice way to get the list of pages that have been
> freed and we have no control over what part of the ring-buffer is removed.
> 
> Also, as this memory is from a reserved-range, is there really a way to re-inject
> it into the buddy allocator?

free_reserved_page() would do actually. But then it is definitive. 

Happy to implement something like that. That also means that the instance can be
actually freed?

> 
> Perhaps what would make sense for resizing would be to support a CMA pool as a
> persistent buffer?
> 
> -- 
> Vincent

-- 
Vincent