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 <[email protected]> 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 <[email protected]>
> > >
> > > 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) <[email protected]>
>
> --
> Vincent
