fanotify: limit number of event merge attempts

Event merges are expensive when event queue size is large, so limit the
linear search to 128 merge tests.

In combination with 128 size hash table, there is a potential to merge
with up to 16K events in the hashed queue.

Link: https://lore.kernel.org/r/20210304104826.3993892-6-amir73il@gmail.com
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fanotify/fanotify.c

@@ -148,13 +148,17 @@ static bool fanotify_should_merge(struct fanotify_event *old,
 	return false;
 }
 
+/* Limit event merges to limit CPU overhead per event */
+#define FANOTIFY_MAX_MERGE_EVENTS 128
+
 /* and the list better be locked by something too! */
 static int fanotify_merge(struct fsnotify_group *group,
 			  struct fsnotify_event *event)
 {
 	struct fanotify_event *old, *new = FANOTIFY_E(event);
 	unsigned int bucket = fanotify_event_hash_bucket(group, new);
 	struct hlist_head *hlist = &group->fanotify_data.merge_hash[bucket];
+	int i = 0;
 
 	pr_debug("%s: group=%p event=%p bucket=%u\n", __func__,
 		 group, event, bucket);
@@ -168,6 +172,8 @@ static int fanotify_merge(struct fsnotify_group *group,
 		return 0;
 
 	hlist_for_each_entry(old, hlist, merge_list) {
+		if (++i > FANOTIFY_MAX_MERGE_EVENTS)
+			break;
 		if (fanotify_should_merge(old, new)) {
 			old->mask |= new->mask;
 			return 1;