tracing: Simplify the max length test when using the filtering temp b…

…uffer

When filtering trace events, a temp buffer is used because the extra copy
from the temp buffer into the ring buffer is still faster than the direct
write into the ring buffer followed by a discard if the filter does not
match.

But the data that can be stored in the temp buffer is a PAGE_SIZE minus the
ring buffer event header. The calculation of that header size is complex,
but using the helper macro "struct_size()" can simplify it.

Link: https://lore.kernel.org/stable/CAHk-=whKbJkuVmzb0hD3N6q7veprUrSpiBHRxVY=AffWZPtxmg@mail.gmail.com/

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -2735,8 +2735,10 @@ trace_event_buffer_lock_reserve(struct trace_buffer **current_rb,
 	    (trace_file->flags & (EVENT_FILE_FL_SOFT_DISABLED | EVENT_FILE_FL_FILTERED)) &&
 	    (entry = this_cpu_read(trace_buffered_event))) {
 		/* Try to use the per cpu buffer first */
+		int max_len = PAGE_SIZE - struct_size(entry, array, 1);
+
 		val = this_cpu_inc_return(trace_buffered_event_cnt);
-		if ((len < (PAGE_SIZE - sizeof(*entry) - sizeof(entry->array[0]))) && val == 1) {
+		if (val == 1 && likely(len <= max_len)) {
 			trace_event_setup(entry, type, trace_ctx);
 			entry->array[0] = len;
 			return entry;