[PATCH 5/7] fanotify: limit number of event merge attempts

[Amir Goldstein <amir73il@gmail.com>]

Event merges are expensive when event queue size is large.
Limit the linear search to 128 merge tests.
In combination with 128 hash lists, there is a potential to
merge with up to 16K events in the hashed queue.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/notify/fanotify/fanotify.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 12df6957e4d8..6d3807012851 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/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;
-- 
2.25.1