linux-kernel - [PATCH v2 3/6] kernfs: improve kernfs path resolution

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Date:   Wed, 17 Jun 2020 15:37:58 +0800
From:   Ian Kent <raven@...maw.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Tejun Heo <tj@...nel.org>, Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...IV.linux.org.uk>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH v2 3/6] kernfs: improve kernfs path resolution

Now that an rwsem is used by kernfs, take advantage of it to reduce
lookup overhead.

If there are many lookups (possibly many negative ones) there can
be a lot of overhead during path walks.

To reduce lookup overhead avoid allocating a new dentry where possible.

To do this stay in rcu-walk mode where possible and use the dentry cache
handling of negative hashed dentries to avoid allocating (and freeing
shortly after) new dentries on every negative lookup.

Signed-off-by: Ian Kent <raven@...maw.net>
---
 fs/kernfs/dir.c |   87 ++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 72 insertions(+), 15 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 9b315f3b20ee..f4943329e578 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1046,15 +1046,75 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
 {
 	struct kernfs_node *kn;
 
-	if (flags & LOOKUP_RCU)
+	if (flags & LOOKUP_RCU) {
+		kn = kernfs_dentry_node(dentry);
+		if (!kn) {
+			/* Negative hashed dentry, tell the VFS to switch to
+			 * ref-walk mode and call us again so that node
+			 * existence can be checked.
+			 */
+			if (!d_unhashed(dentry))
+				return -ECHILD;
+
+			/* Negative unhashed dentry, this shouldn't happen
+			 * because this case occurs in rcu-walk mode after
+			 * dentry allocation which is followed by a call
+			 * to ->loopup(). But if it does happen the dentry
+			 * is surely invalid.
+			 */
+			return 0;
+		}
+
+		/* Since the dentry is positive (we got the kernfs node) a
+		 * kernfs node reference was held at the time. Now if the
+		 * dentry reference count is still greater than 0 it's still
+		 * positive so take a reference to the node to perform an
+		 * active check.
+		 */
+		if (d_count(dentry) <= 0 || !atomic_inc_not_zero(&kn->count))
+			return -ECHILD;
+
+		/* The kernfs node reference count was greater than 0, if
+		 * it's active continue in rcu-walk mode.
+		 */
+		if (kernfs_active_read(kn)) {
+			kernfs_put(kn);
+			return 1;
+		}
+
+		/* Otherwise, just tell the VFS to switch to ref-walk mode
+		 * and call us again so the kernfs node can be validated.
+		 */
+		kernfs_put(kn);
 		return -ECHILD;
+	}
 
-	/* Always perform fresh lookup for negatives */
-	if (d_really_is_negative(dentry))
-		goto out_bad_unlocked;
+	down_read(&kernfs_rwsem);
 
 	kn = kernfs_dentry_node(dentry);
-	down_read(&kernfs_rwsem);
+	if (!kn) {
+		struct kernfs_node *parent;
+
+		/* If the kernfs node can be found this is a stale negative
+		 * hashed dentry so it must be discarded and the lookup redone.
+		 */
+		parent = kernfs_dentry_node(dentry->d_parent);
+		if (parent) {
+			const void *ns = NULL;
+
+			if (kernfs_ns_enabled(parent))
+				ns = kernfs_info(dentry->d_parent->d_sb)->ns;
+			kn = kernfs_find_ns(parent, dentry->d_name.name, ns);
+			if (kn)
+				goto out_bad;
+		}
+
+		/* The kernfs node doesn't exist, leave the dentry negative
+		 * and return success.
+		 */
+		goto out;
+	}
+
 
 	/* The kernfs node has been deactivated */
 	if (!kernfs_active_read(kn))
@@ -1072,12 +1132,11 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
 	if (kn->parent && kernfs_ns_enabled(kn->parent) &&
 	    kernfs_info(dentry->d_sb)->ns != kn->ns)
 		goto out_bad;
-
+out:
 	up_read(&kernfs_rwsem);
 	return 1;
 out_bad:
 	up_read(&kernfs_rwsem);
-out_bad_unlocked:
 	return 0;
 }
 
@@ -1092,7 +1151,7 @@ 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;
 
 	down_read(&kernfs_rwsem);
@@ -1102,11 +1161,9 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
 
 	kn = kernfs_find_ns(parent, dentry->d_name.name, ns);
 
-	/* no such entry */
-	if (!kn || !kernfs_active(kn)) {
-		ret = NULL;
-		goto out_unlock;
-	}
+	/* no such entry, retain as negative hashed dentry */
+	if (!kn || !kernfs_active(kn))
+		goto out_negative;
 
 	/* attach dentry and inode */
 	inode = kernfs_get_inode(dir->i_sb, kn);
@@ -1114,10 +1171,10 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
 		ret = ERR_PTR(-ENOMEM);
 		goto out_unlock;
 	}
-
+out_negative:
 	/* instantiate and hash dentry */
 	ret = d_splice_alias(inode, dentry);
- out_unlock:
+out_unlock:
 	up_read(&kernfs_rwsem);
 	return ret;
 }