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Subject: [PATCH v7 3/6] kernfs: use VFS negative dentry caching
From:   Ian Kent <raven@themaw.net>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>
Cc:     Eric Sandeen <sandeen@sandeen.net>, Fox Chen <foxhlchen@gmail.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Al Viro <viro@ZenIV.linux.org.uk>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Tue, 15 Jun 2021 18:26:21 +0800
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In-Reply-To: <162375263398.232295.14755578426619198534.stgit@web.messagingengine.com>
References: <162375263398.232295.14755578426619198534.stgit@web.messagingengine.com>
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If there are many lookups for non-existent paths these negative lookups
can lead to a lot of overhead during path walks.

The VFS allows dentries to be created as negative and hashed, and caches
them so they can be used to reduce the fairly high overhead alloc/free
cycle that occurs during these lookups.

Use the kernfs node parent revision to identify if a change has been
made to the containing directory so that the negative dentry can be
discarded and the lookup redone.

Signed-off-by: Ian Kent <raven@themaw.net>
---
 fs/kernfs/dir.c |   52 ++++++++++++++++++++++++++++++++--------------------
 1 file changed, 32 insertions(+), 20 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 48704c5b6a072..f9352c9e90581 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1039,9 +1039,28 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
 	if (flags & LOOKUP_RCU)
 		return -ECHILD;
 
-	/* Always perform fresh lookup for negatives */
-	if (d_really_is_negative(dentry))
-		goto out_bad_unlocked;
+	/* Negative hashed dentry? */
+	if (d_really_is_negative(dentry)) {
+		struct kernfs_node *parent;
+
+		/* If the kernfs parent node has changed discard and
+		 * proceed to ->lookup.
+		 */
+		spin_lock(&dentry->d_lock);
+		parent = kernfs_dentry_node(dentry->d_parent);
+		if (parent) {
+			if (kernfs_dir_changed(parent, dentry)) {
+				spin_unlock(&dentry->d_lock);
+				return 0;
+			}
+		}
+		spin_unlock(&dentry->d_lock);
+
+		/* The kernfs parent node hasn't changed, leave the
+		 * dentry negative and return success.
+		 */
+		return 1;
+	}
 
 	kn = kernfs_dentry_node(dentry);
 	mutex_lock(&kernfs_mutex);
@@ -1067,7 +1086,6 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
 	return 1;
 out_bad:
 	mutex_unlock(&kernfs_mutex);
-out_bad_unlocked:
 	return 0;
 }
 
@@ -1082,33 +1100,27 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
 	struct dentry *ret;
 	struct kernfs_node *parent = dir->i_private;
 	struct kernfs_node *kn;
-	struct inode *inode;
+	struct inode *inode = NULL;
 	const void *ns = NULL;
 
 	mutex_lock(&kernfs_mutex);
-
 	if (kernfs_ns_enabled(parent))
 		ns = kernfs_info(dir->i_sb)->ns;
 
 	kn = kernfs_find_ns(parent, dentry->d_name.name, ns);
-
-	/* no such entry */
-	if (!kn || !kernfs_active(kn)) {
-		ret = NULL;
-		goto out_unlock;
-	}
-
 	/* attach dentry and inode */
-	inode = kernfs_get_inode(dir->i_sb, kn);
-	if (!inode) {
-		ret = ERR_PTR(-ENOMEM);
-		goto out_unlock;
+	if (kn && kernfs_active(kn)) {
+		inode = kernfs_get_inode(dir->i_sb, kn);
+		if (!inode)
+			inode = ERR_PTR(-ENOMEM);
 	}
-
-	/* instantiate and hash dentry */
+	/* Needed only for negative dentry validation */
+	if (!inode)
+		kernfs_set_rev(parent, dentry);
+	/* instantiate and hash (possibly negative) dentry */
 	ret = d_splice_alias(inode, dentry);
- out_unlock:
 	mutex_unlock(&kernfs_mutex);
+
 	return ret;
 }