fs: improve scalability of pseudo filesystems

Regardless of how much we possibly try to scale dcache, there is likely
always going to be some fundamental contention when adding or removing children
under the same parent. Pseudo filesystems do not seem need to have connected
dentries because by definition they are disconnected.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

5 files changed, 16 insertions, 3 deletions

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index aca8806fa206..9d92b33da8a0 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -102,7 +102,7 @@ struct file *anon_inode_getfile(const char *name,
 	this.name = name;
 	this.len = strlen(name);
 	this.hash = 0;
-	path.dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
+	path.dentry = d_alloc_pseudo(anon_inode_mnt->mnt_sb, &this);
 	if (!path.dentry)
 		goto err_module;
 
diff --git a/fs/dcache.c b/fs/dcache.c
index 09ec945f3c98..9e6e6db76869 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1330,6 +1330,18 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
 }
 EXPORT_SYMBOL(d_alloc);
 
+struct dentry *d_alloc_pseudo(struct super_block *sb, const struct qstr *name)
+{
+	struct dentry *dentry = d_alloc(NULL, name);
+	if (dentry) {
+		dentry->d_sb = sb;
+		dentry->d_parent = dentry;
+		dentry->d_flags |= DCACHE_DISCONNECTED;
+	}
+	return dentry;
+}
+EXPORT_SYMBOL(d_alloc_pseudo);
+
 struct dentry *d_alloc_name(struct dentry *parent, const char *name)
 {
 	struct qstr q;
diff --git a/fs/pipe.c b/fs/pipe.c
index 01a786567810..cfe3a7f2ee21 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -999,7 +999,7 @@ struct file *create_write_pipe(int flags)
 		goto err;
 
 	err = -ENOMEM;
-	path.dentry = d_alloc(pipe_mnt->mnt_sb->s_root, &name);
+	path.dentry = d_alloc_pseudo(pipe_mnt->mnt_sb, &name);
 	if (!path.dentry)
 		goto err_inode;
 	path.mnt = mntget(pipe_mnt);
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index d719e4de8046..c0a2ca97c72f 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -211,6 +211,7 @@ extern void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op
 
 /* allocate/de-allocate */
 extern struct dentry * d_alloc(struct dentry *, const struct qstr *);
+extern struct dentry * d_alloc_pseudo(struct super_block *, const struct qstr *);
 extern struct dentry * d_splice_alias(struct inode *, struct dentry *);
 extern struct dentry * d_add_ci(struct dentry *, struct inode *, struct qstr *);
 extern struct dentry * d_obtain_alias(struct inode *);
diff --git a/net/socket.c b/net/socket.c
index 991e266bc7ae..0ee74c325320 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -361,7 +361,7 @@ static int sock_alloc_file(struct socket *sock, struct file **f, int flags)
 	if (unlikely(fd < 0))
 		return fd;
 
-	path.dentry = d_alloc(sock_mnt->mnt_sb->s_root, &name);
+	path.dentry = d_alloc_pseudo(sock_mnt->mnt_sb, &name);
 	if (unlikely(!path.dentry)) {
 		put_unused_fd(fd);
 		return -ENOMEM;