tracing: Fix nr_subbufs initialization in simple_ring_buffer_init_mm()

nr_subbufs in the ring buffer metadata is always initialized to zero
because it is assigned from cpu_buffer->nr_pages before the page
initialization loop has run. While nr_subbufs is not currently read
by the kernel, it should reflect the actual buffer geometry in the
meta page for correctness.

Move the assignment after the page loop so that cpu_buffer->nr_pages
holds the final count.

Link: https://patch.msgid.link/20260512135420.99194-1-devnexen@gmail.com
Fixes: 34e5b958bdad ("tracing: Introduce simple_ring_buffer")
Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/simple_ring_buffer.c b/kernel/trace/simple_ring_buffer.c
index 02af2297ae5a955481665ed11a835c66327ddf5d..f731f14d0ff72204cae8bc7d6a7206e7f328b5a5 100644 (file)
--- a/kernel/trace/simple_ring_buffer.c
+++ b/kernel/trace/simple_ring_buffer.c
@@ -395,7 +395,6 @@ int simple_ring_buffer_init_mm(struct simple_rb_per_cpu *cpu_buffer,
 
        memset(cpu_buffer->meta, 0, sizeof(*cpu_buffer->meta));
        cpu_buffer->meta->meta_page_size = PAGE_SIZE;
-       cpu_buffer->meta->nr_subbufs = cpu_buffer->nr_pages;
 
        /* The reader page is not part of the ring initially */
        page = load_page(desc->page_va[0]);
@@ -437,6 +436,7 @@ int simple_ring_buffer_init_mm(struct simple_rb_per_cpu *cpu_buffer,
                return ret;
        }
 
+       cpu_buffer->meta->nr_subbufs = cpu_buffer->nr_pages;
        /* Close the ring */
        bpage->link.next = &cpu_buffer->tail_page->link;
        cpu_buffer->tail_page->link.prev = &bpage->link;