linux-kernel - [PATCH] kernfs: don't create a negative dentry if inactive node exists

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Date:   Wed, 29 Sep 2021 11:04:34 +0800
From:   Ian Kent <raven@...maw.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>
Cc:     Hou Tao <houtao1@...wei.com>, David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] kernfs: don't create a negative dentry if inactive node
 exists

In kernfs_iop_lookup() a negative dentry is created if there's no kernfs
node associated with the dentry or the node is inactive.

But inactive kernfs nodes are meant to be invisible to the VFS and
creating a negative dentry for these can have unexpected side effects
when the node transitions to an active state.

The point of creating negative dentries is to avoid the expensive
alloc/free cycle that occurs if there are frequent lookups for kernfs
attributes that don't exist. So kernfs nodes that are not yet active
should not result in a negative dentry being created so when they
transition to an active state VFS lookups can create an associated
dentry is a natural way.

Signed-off-by: Ian Kent <raven@...maw.net>
---
 fs/kernfs/dir.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index ba581429bf7b..a957c944cf3a 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1111,7 +1111,14 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,

 	kn = kernfs_find_ns(parent, dentry->d_name.name, ns);
 	/* attach dentry and inode */
-	if (kn && kernfs_active(kn)) {
+	if (kn) {
+		/* Inactive nodes are invisible to the VFS so don't
+		 * create a negative.
+		 */
+		if (!kernfs_active(kn)) {
+			up_read(&kernfs_rwsem);
+			return NULL;
+		}
 		inode = kernfs_get_inode(dir->i_sb, kn);
 		if (!inode)
 			inode = ERR_PTR(-ENOMEM);