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>

2 files changed, 43 insertions, 6 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 268da2e2bc09..2135ab8bb103 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1853,6 +1853,16 @@ char * d_path(struct dentry *dentry, struct vfsmount *vfsmnt,
         struct vfsmount *rootmnt;
         struct dentry *root;
 
+        /*
+         * We have various synthetic filesystems that never get mounted.  On
+         * these filesystems dentries are never used for lookup purposes, and
+         * thus don't need to be hashed.  They also don't need a name until a
+         * user wants to identify the object in /proc/pid/fd/.  The little hack
+         * below allows us to generate a name for these objects on demand:
+         */
+        if (dentry->d_op && dentry->d_op->d_dname)
+                return dentry->d_op->d_dname(dentry, buf, buflen);
+
         read_lock(&current->fs->lock);
         rootmnt = mntget(current->fs->rootmnt);
         root = dget(current->fs->root);
@@ -1866,6 +1876,27 @@ char * d_path(struct dentry *dentry, struct vfsmount *vfsmnt,
 }
 
 /*
+ * Helper function for dentry_operations.d_dname() members
+ */
+char *dynamic_dname(struct dentry *dentry, char *buffer, int buflen,
+                        const char *fmt, ...)
+{
+        va_list args;
+        char temp[64];
+        int sz;
+
+        va_start(args, fmt);
+        sz = vsnprintf(temp, sizeof(temp), fmt, args) + 1;
+        va_end(args);
+
+        if (sz > sizeof(temp) || sz > buflen)
+                return ERR_PTR(-ENAMETOOLONG);
+
+        buffer += buflen - sz;
+        return memcpy(buffer, temp, sz);
+}
+
+/*
  * NOTE! The user-level library version returns a
  * character pointer. The kernel system call just
  * returns the length of the buffer filled (which
diff --git a/fs/pipe.c b/fs/pipe.c
index ebafde7d6aba..3a89592bdf57 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -841,8 +841,18 @@ static int pipefs_delete_dentry(struct dentry *dentry)
         return 0;
 }
 
+/*
+ * pipefs_dname() is called from d_path().
+ */
+static char *pipefs_dname(struct dentry *dentry, char *buffer, int buflen)
+{
+        return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
+                                dentry->d_inode->i_ino);
+}
+
 static struct dentry_operations pipefs_dentry_operations = {
         .d_delete       = pipefs_delete_dentry,
+        .d_dname        = pipefs_dname,
 };
 
 static struct inode * get_pipe_inode(void)
@@ -888,8 +898,7 @@ struct file *create_write_pipe(void)
         struct inode *inode;
         struct file *f;
         struct dentry *dentry;
-        char name[32];
-        struct qstr this;
+        struct qstr name = { .name = "" };
 
         f = get_empty_filp();
         if (!f)
@@ -899,11 +908,8 @@ struct file *create_write_pipe(void)
         if (!inode)
                 goto err_file;
 
-        this.len = sprintf(name, "[%lu]", inode->i_ino);
-        this.name = name;
-        this.hash = 0;
         err = -ENOMEM;
-        dentry = d_alloc(pipe_mnt->mnt_sb->s_root, &this);
+        dentry = d_alloc(pipe_mnt->mnt_sb->s_root, &name);
         if (!dentry)
                 goto err_inode;