[E][SRU][PATCH 1/1] fanotify: merge duplicate events on parent and child

[Po-Hsu Lin]

From: Amir Goldstein <amir73il at gmail.com>

With inotify, when a watch is set on a directory and on its child, an
event on the child is reported twice, once with wd of the parent watch
and once with wd of the child watch without the filename.

With fanotify, when a watch is set on a directory and on its child, an
event on the child is reported twice, but it has the exact same
information - either an open file descriptor of the child or an encoded
fid of the child.

The reason that the two identical events are not merged is because the
object id used for merging events in the queue is the child inode in one
event and parent inode in the other.

For events with path or dentry data, use the victim inode instead of the
watched inode as the object id for event merging, so that the event
reported on parent will be merged with the event reported on the child.

Link: https://lore.kernel.org/r/20200319151022.31456-9-amir73il@gmail.com
Signed-off-by: Amir Goldstein <amir73il at gmail.com>
Signed-off-by: Jan Kara <jack at suse.cz>
(backported from commit f367a62a7cad2447d835a9f14fc63997a9137246)
[PHLin: backported with the same logic]
Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
---
 fs/notify/fanotify/fanotify.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 5778d13..f1ca308 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -314,7 +314,12 @@ struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
 	if (!event)
 		goto out;
 init: __maybe_unused
-	fsnotify_init_event(&event->fse, inode);
+	/*
+	 * Use the victim inode instead of the watching inode as the id for
+	 * event queue, so event reported on parent is merged with event
+	 * reported on child when both directory and child watches exist.
+	 */
+	fsnotify_init_event(&event->fse, (unsigned long)id);
 	event->mask = mask;
 	if (FAN_GROUP_FLAG(group, FAN_REPORT_TID))
 		event->pid = get_pid(task_pid(current));
-- 
2.7.4