[for-linus][PATCH 06/13] ring-buffer: Fix ring_buffer_read_page() copying only one event per page

[Steven Rostedt]

From: David Carlier <devnexen@xxxxxxxxx>

Commit 8928e4a3be34 ("ring-buffer: Show persistent buffer dropped events
in trace_pipe file") split the "commit" variable in
ring_buffer_read_page() into "commit" (raw) and "size" (masked page
size), but the inner copy loop's terminator was changed to compare rpos
against "event_size" instead of "size".

rpos is the cumulative read offset within the page; event_size is the
length of the single event just copied. The loop thus breaks after the
first event, so only one event is copied per call. This regresses the
per-event memcpy path (partial reads, the active commit page, and
mapped/remote buffers) used by splice/trace_pipe_raw and mmap consumers
into a one-event-at-a-time read.

Compare rpos against the page size as the original code did.

Link: https://patch.msgid.link/20260616175538.111628-1-devnexen@xxxxxxxxx
Fixes: 8928e4a3be34 ("ring-buffer: Show persistent buffer dropped events in trace_pipe file")
Signed-off-by: David Carlier <devnexen@xxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/trace/ring_buffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 3ec173e22eeb..4b994aea9e61 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7183,7 +7183,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 			rpos = reader->read;
 			pos += event_size;
 
-			if (rpos >= event_size)
+			if (rpos >= size)
 				break;
 
 			event = rb_reader_event(cpu_buffer);
-- 
2.53.0