Patch "kernfs: don't depend on d_find_any_alias() when generating notifications" has been added to the 4.4-stable tree

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

kernfs: don't depend on d_find_any_alias() when generating notifications

to the 4.4-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:
kernfs-don-t-depend-on-d_find_any_alias-when-generating-notifications.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tejun Heo <[email protected]>
Date: Fri, 17 Jun 2016 17:51:17 -0400
Subject: kernfs: don't depend on d_find_any_alias() when generating notifications

From: Tejun Heo <[email protected]>

commit df6a58c5c5aa8ecb1e088ecead3fa33ae70181f1 upstream.

kernfs_notify_workfn() sends out file modified events for the scheduled kernfs_nodes. Because the modifications aren't from userland, it doesn't have the matching file struct at hand and can't use fsnotify_modify(). Instead, it looked up the inode and then used d_find_any_alias() to find the dentry and used fsnotify_parent() and fsnotify() directly to generate notifications.

The assumption was that the relevant dentries would have been pinned if there are listeners, which isn't true as inotify doesn't pin dentries at all and watching the parent doesn't pin the child dentries even for dnotify. This led to, for example, inotify watchers not getting notifications if the system is under memory pressure and the matching dentries got reclaimed. It can also be triggered through /proc/sys/vm/drop_caches or a remount attempt which involves shrinking dcache.

fsnotify_parent() only uses the dentry to access the parent inode, which kernfs can do easily. Update kernfs_notify_workfn() so that it uses fsnotify() directly for both the parent and target inodes without going through d_find_any_alias(). While at it, supply the target file name to fsnotify() from kernfs_node->name.

---
 fs/kernfs/file.c |   28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

@@ -833,21 +833,35 @@ repeat:
 	mutex_lock(&kernfs_mutex);
 
 	list_for_each_entry(info, &kernfs_root(kn)->supers, node) {
+		struct kernfs_node *parent;
 		struct inode *inode;
-		struct dentry *dentry;
 
+		/*
+		 * We want fsnotify_modify() on @kn but as the
+		 * modifications aren't originating from userland don't
+		 * have the matching @file available.  Look up the inodes
+		 * and generate the events manually.
+		 */
 		inode = ilookup(info->sb, kn->ino);
 		if (!inode)
 			continue;
 
-		dentry = d_find_any_alias(inode);
-		if (dentry) {
-			fsnotify_parent(NULL, dentry, FS_MODIFY);
-			fsnotify(inode, FS_MODIFY, inode, FSNOTIFY_EVENT_INODE,
-				 NULL, 0);
-			dput(dentry);
+		parent = kernfs_get_parent(kn);
+		if (parent) {
+			struct inode *p_inode;
+
+			p_inode = ilookup(info->sb, parent->ino);
+			if (p_inode) {
+				fsnotify(p_inode, FS_MODIFY | FS_EVENT_ON_CHILD,
+					 inode, FSNOTIFY_EVENT_INODE, kn->name, 0);
+				iput(p_inode);
+			}
+
+			kernfs_put(parent);
 		}
 
+		fsnotify(inode, FS_MODIFY, inode, FSNOTIFY_EVENT_INODE,
+			 kn->name, 0);
 		iput(inode);
 	}
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/kernfs-don-t-depend-on-d_find_any_alias-when-generating-notifications.patch queue-4.4/cpuset-make-sure-new-tasks-conform-to-the-current-config-of-the-cpuset.patch