[for-linus][PATCH 2/3] tracing: Limit trace_seq size to just 8K and not depend on architecture PAGE_SIZE

[Steven Rostedt <rostedt@goodmis.org>]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

The trace_seq buffer is used to print out entire events. It's typically
set to PAGE_SIZE * 2 as there's some events that can be quite large.

As a side effect, writes to trace_marker is limited by both the size of the
trace_seq buffer as well as the ring buffer's sub-buffer size (which is a
power of PAGE_SIZE). By limiting the trace_seq size, it also limits the
size of the largest string written to trace_marker.

trace_seq does not need to be dependent on PAGE_SIZE like the ring buffer
sub-buffers need to be. Hard code it to 8K which is PAGE_SIZE * 2 on most
architectures. This will also limit the size of trace_marker on those
architectures with greater than 4K PAGE_SIZE.

Link: https://lore.kernel.org/all/20240302111244.3a1674be@gandalf.local.home/
Link: https://lore.kernel.org/linux-trace-kernel/20240304191342.56fb1087@gandalf.local.home

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sachin Sant <sachinp@linux.ibm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 include/linux/trace_seq.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/trace_seq.h b/include/linux/trace_seq.h
index 9ec229dfddaa..1ef95c0287f0 100644
--- a/include/linux/trace_seq.h
+++ b/include/linux/trace_seq.h
@@ -9,9 +9,15 @@
 /*
  * Trace sequences are used to allow a function to call several other functions
  * to create a string of data to use.
+ *
+ * Have the trace seq to be 8K which is typically PAGE_SIZE * 2 on
+ * most architectures. The TRACE_SEQ_BUFFER_SIZE (which is
+ * TRACE_SEQ_SIZE minus the other fields of trace_seq), is the
+ * max size the output of a trace event may be.
  */
 
-#define TRACE_SEQ_BUFFER_SIZE	(PAGE_SIZE * 2 - \
+#define TRACE_SEQ_SIZE		8192
+#define TRACE_SEQ_BUFFER_SIZE	(TRACE_SEQ_SIZE - \
 	(sizeof(struct seq_buf) + sizeof(size_t) + sizeof(int)))
 
 struct trace_seq {
-- 
2.43.0