VFS: delay the dentry name generation on sockets and pipes

1) Introduces a new method in 'struct dentry_operations'.  This method
   called d_dname() might be called from d_path() to build a pathname for
   special filesystems.  It is called without locks.

   Future patches (if we succeed in having one common dentry for all
   pipes/sockets) may need to change prototype of this method, but we now
   use : char *d_dname(struct dentry *dentry, char *buffer, int buflen);

2) Adds a dynamic_dname() helper function that eases d_dname() implementations

3) Defines d_dname method for sockets : No more sprintf() at socket
   creation.  This is delayed up to the moment someone does an access to
   /proc/pid/fd/...

4) Defines d_dname method for pipes : No more sprintf() at pipe
   creation.  This is delayed up to the moment someone does an access to
   /proc/pid/fd/...

A benchmark consisting of 1.000.000 calls to pipe()/close()/close() gives a
*nice* speedup on my Pentium(M) 1.6 Ghz :

3.090 s instead of 3.450 s

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Christoph Hellwig <hch@infradead.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 7 deletions

diff --git a/net/socket.c b/net/socket.c
index 759825b7ca2..98a8f67abbf 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -313,8 +313,19 @@ static int sockfs_delete_dentry(struct dentry *dentry)
 	dentry->d_flags |= DCACHE_UNHASHED;
 	return 0;
 }
+
+/*
+ * sockfs_dname() is called from d_path().
+ */
+static char *sockfs_dname(struct dentry *dentry, char *buffer, int buflen)
+{
+	return dynamic_dname(dentry, buffer, buflen, "socket:[%lu]",
+				dentry->d_inode->i_ino);
+}
+
 static struct dentry_operations sockfs_dentry_operations = {
 	.d_delete = sockfs_delete_dentry,
+	.d_dname  = sockfs_dname,
 };
 
 /*
@@ -354,14 +365,9 @@ static int sock_alloc_fd(struct file **filep)
 
 static int sock_attach_fd(struct socket *sock, struct file *file)
 {
-	struct qstr this;
-	char name[32];
-
-	this.len = sprintf(name, "[%lu]", SOCK_INODE(sock)->i_ino);
-	this.name = name;
-	this.hash = 0;
+	struct qstr name = { .name = "" };
 
-	file->f_path.dentry = d_alloc(sock_mnt->mnt_sb->s_root, &this);
+	file->f_path.dentry = d_alloc(sock_mnt->mnt_sb->s_root, &name);
 	if (unlikely(!file->f_path.dentry))
 		return -ENOMEM;