Patch "fanotify: limit number of event merge attempts" has been added to the 5.10-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    fanotify: limit number of event merge attempts

to the 5.10-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     fanotify-limit-number-of-event-merge-attempts.patch
and it can be found in the queue-5.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 76608fa6240eaaefbae1926e528eec908054dccd
Author: Amir Goldstein <amir73il@xxxxxxxxx>
Date:   Thu Mar 4 12:48:26 2021 +0200

    fanotify: limit number of event merge attempts
    
    [ Upstream commit b8cd0ee8cda68a888a317991c1e918a8cba1a568 ]
    
    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@xxxxxxxxx
    Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
    Signed-off-by: Jan Kara <jack@xxxxxxx>
    Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 50b3abc062156..754e27ead8742 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -148,6 +148,9 @@ 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)
@@ -155,6 +158,7 @@ static int fanotify_merge(struct fsnotify_group *group,
 	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;