diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a0a84ff46ecd..a0d66a056e59 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2734,10 +2734,44 @@ trace_event_buffer_lock_reserve(struct trace_buffer **current_rb,
 	if (!tr->no_filter_buffering_ref &&
 	    (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 */
+		/*
+		 * Filtering is on, so try to use the per cpu buffer first.
+		 * This buffer will simulate a ring_buffer_event,
+		 * where the type_len is zero and the array[0] will
+		 * hold the full length.
+		 * (see include/linux/ring-buffer.h for details on
+		 *  how the ring_buffer_event is structured).
+		 *
+		 * Using a temp buffer during filtering and copying it
+		 * on a matched filter is quicker than writing directly
+		 * into the ring buffer and then discarding it when
+		 * it doesn't match. That is because the discard
+		 * requires several atomic operations to get right.
+		 * Copying on match and doing nothing on a failed match
+		 * is still quicker than no copy on match, but having
+		 * to discard out of the ring buffer on a failed match.
+		 */
 		int max_len = PAGE_SIZE - struct_size(entry, array, 1);
 
 		val = this_cpu_inc_return(trace_buffered_event_cnt);
+
+		/*
+		 * Preemption is disabled, but interrupts and NMIs
+		 * can still come in now. If that happens after
+		 * the above increment, then it will have to go
+		 * back to the old method of allocating the event
+		 * on the ring buffer, and if the filter fails, it
+		 * will have to call ring_buffer_discard_commit()
+		 * to remove it.
+		 *
+		 * Need to also check the unlikely case that the
+		 * length is bigger than the temp buffer size.
+		 * If that happens, then the reserve is pretty much
+		 * guaranteed to fail, as the ring buffer currently
+		 * only allows events less than a page. But that may
+		 * change in the future, so let the ring buffer reserve
+		 * handle the failure in that case.
+		 */
 		if (val == 1 && likely(len <= max_len)) {
 			trace_event_setup(entry, type, trace_ctx);
 			entry->array[0] = len;