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

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:

quoted

On Fri, Aug 07, 2026 at 11:18:08AM +0900, Masami Hiramatsu wrote:

quoted

On Thu,  6 Aug 2026 22:13:01 +0100
Vincent Donnefort [off-list ref] wrote:

quoted

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

quoted

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.

quoted

Thank you,

quoted

Fixes: be68d63a139b ("ring-buffer: Add ring_buffer_alloc_range()")
Signed-off-by: Vincent Donnefort <redacted>

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) [off-list ref]

-- 
Vincent