From 4b936885ab04dc6e0bb0ef35e0e23c1a7364d9e5 Mon Sep 17 00:00:00 2001
From: Nick Piggin <npiggin@kernel.dk>
Date: Fri, 7 Jan 2011 17:50:07 +1100
Subject: 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>

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index aca8806..9d92b33 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 09ec945..9e6e6db 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 01a7865..cfe3a7f2 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 d719e4d..c0a2ca9 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 991e266..0ee74c3 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;
-- 
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