fanotify: limit number of event merge attempts

commit b8cd0ee upstream.

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

[Stable backport notes] The following statement from upstream commit is
irrelevant for backport:
-
-In combination with 128 size hash table, there is a potential to merge
-with up to 16K events in the hashed queue.
-
[Stable backport notes] The problem is as old as fanotify and described
in the linked cover letter "Performance improvement for fanotify merge".
This backported patch fixes the performance issue at the cost of merging
fewer potential events.  Fixing the performance issue is more important
than preserving the "event merge" behavior, which was not predictable in
any way that applications could rely on.

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>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/linux-fsdevel/20210202162010.305971-1-amir73il@gmail.com/
Link: https://lore.kernel.org/linux-fsdevel/20210915163334.GD6166@quack2.suse.cz/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/notify/fanotify/fanotify.c

@@ -129,11 +129,15 @@ static bool fanotify_should_merge(struct fsnotify_event *old_fsn,
 	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 list_head *list, struct fsnotify_event *event)
 {
 	struct fsnotify_event *test_event;
 	struct fanotify_event *new;
+	int i = 0;
 
 	pr_debug("%s: list=%p event=%p\n", __func__, list, event);
 	new = FANOTIFY_E(event);
@@ -147,6 +151,8 @@ static int fanotify_merge(struct list_head *list, struct fsnotify_event *event)
 		return 0;
 
 	list_for_each_entry_reverse(test_event, list, list) {
+		if (++i > FANOTIFY_MAX_MERGE_EVENTS)
+			break;
 		if (fanotify_should_merge(test_event, event)) {
 			FANOTIFY_E(test_event)->mask |= new->mask;
 			return 1;