[PATCH v10 1/4] ring-buffer: Allow splice reads on static buffers

[Vincent Donnefort <vdonnefort@google.com>]

ring_buffer_read_page() rejects splice (full=1) reads on static buffers
(that is user-mapped, persistent or remote) because !read check assumes
unread pages must be swapped. However for those buffers we have no other
choice than memcpy the data.

For the memcpy case, only return an error when the writer is still on
the reader page for the splice interface to wait.

Fixes: 117c39200d9d ("ring-buffer: Introducing ring-buffer mapping functions")
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b0963ac6fd16..84fd4cdd486f 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7193,15 +7193,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 		unsigned int event_size;
 		unsigned int flags = 0;
 
-		/*
-		 * If a full page is expected, this can still be returned
-		 * if there's been a previous partial read and the
-		 * rest of the page can be read and the commit page is off
-		 * the reader page.
-		 */
-		if (full &&
-		    (!read || (len < (size - read)) ||
-		     cpu_buffer->reader_page == cpu_buffer->commit_page))
+		/* If a full page is requested, it cannot be the commit page */
+		if (full && cpu_buffer->reader_page == cpu_buffer->commit_page)
 			return -1;
 
 		if (len > (size - read))
-- 
2.55.0.979.g7e5102b832-goog