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>

3 files changed, 14 insertions, 2 deletions

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..cfe3a7f 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);