[PATCH] tracing: Fix nr_subbufs initialization in simple_ring_buffer_init_mm()

[David Carlier]

In simple_ring_buffer_init_mm(), meta->nr_subbufs is assigned from
cpu_buffer->nr_pages at line 398, but cpu_buffer was just zeroed by
memset() at line 390. The actual page count is only computed later in
the loop (lines 410-429), so nr_subbufs is always set to 0.

This field is part of struct trace_buffer_meta (UAPI), exposed to
consumers who rely on it for buffer geometry calculations (e.g.,
data_len = subbuf_size * nr_subbufs). A value of 0 breaks ring buffer
consumption entirely.

Fix by using desc->nr_page_va directly, which holds the correct total
page count (reader + ring pages) and is available at this point. This
matches the UAPI documentation: "Number of subbufs in the ring-buffer,
including the reader."

Fixes: 34e5b958bdad ("tracing: Introduce simple_ring_buffer")
Signed-off-by: David Carlier <devnexen@xxxxxxxxx>
---
 kernel/trace/simple_ring_buffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/simple_ring_buffer.c b/kernel/trace/simple_ring_buffer.c
index 02af2297ae5a..e991a0d8def2 100644
--- a/kernel/trace/simple_ring_buffer.c
+++ b/kernel/trace/simple_ring_buffer.c
@@ -395,7 +395,7 @@ 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;
+	cpu_buffer->meta->nr_subbufs = desc->nr_page_va;
 
 	/* The reader page is not part of the ring initially */
 	page = load_page(desc->page_va[0]);
-- 
2.53.0