[PATCH 2/2] make pipefs do lazy i_ino assignment and hashing

[Jeff Layton <jlayton@redhat.com>]

This patch updates pipefs to do defer assigning an i_ino value to its inodes
until someone actually tries to stat it. This allows us to have unique i_ino
values for the inodes here, without the performance impact for anyone who
doesn't actually care about it.

Since we don't have an i_ino value at pipe creation time, we need something
else to stuff into the dentry name. Here, I'm using the pointer address of
the inode xor'ed with a random value. There are certainly better hashing
schemes, so if someone wants to propose a better way to do this, then I'm
open to looking at it (maybe halfmd4?).

This patch should apply cleanly to the current -mm.

Signed-off-by: Jeff Layton <jlayton@redhat.com>

diff --git a/fs/pipe.c b/fs/pipe.c
index 9b3cb34..76dc0e1 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -16,6 +16,7 @@
 #include <linux/uio.h>
 #include <linux/highmem.h>
 #include <linux/pagemap.h>
+#include <linux/random.h>
 
 #include <asm/uaccess.h>
 #include <asm/ioctls.h>
@@ -35,6 +36,8 @@
  * -- Manfred Spraul <manfred@colorfullife.com> 2002-05-09
  */
 
+static unsigned long pipefs_dname_obfuscator;
+
 /* Drop the inode semaphore and wait for a pipe event, atomically */
 void pipe_wait(struct pipe_inode_info *pipe)
 {
@@ -844,6 +847,10 @@ static struct dentry_operations pipefs_dentry_operations = {
 	.d_delete	= pipefs_delete_dentry,
 };
 
+static struct inode_operations pipefs_inode_operations = {
+	.getattr	= lazy_getattr,
+};
+
 static struct inode * get_pipe_inode(void)
 {
 	struct inode *inode = new_inode(pipe_mnt->mnt_sb);
@@ -859,6 +866,7 @@ static struct inode * get_pipe_inode(void)
 
 	pipe->readers = pipe->writers = 1;
 	inode->i_fop = &rdwr_pipe_fops;
+	inode->i_op = &pipefs_inode_operations;
 
 	/*
 	 * Mark the inode dirty from the very beginning,
@@ -871,8 +879,7 @@ static struct inode * get_pipe_inode(void)
 	inode->i_uid = current->fsuid;
 	inode->i_gid = current->fsgid;
 	inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
-	inode->i_ino = iunique(pipe_mnt->mnt_sb, 1);
-	insert_inode_hash(inode);
+	inode->i_ino = 0;
 
 	return inode;
 
@@ -900,7 +907,8 @@ struct file *create_write_pipe(void)
 	if (!inode)
 		goto err_file;
 
-	this.len = sprintf(name, "[%lu]", inode->i_ino);
+	this.len = sprintf(name, "[%lu]",
+		   (unsigned long) inode ^ pipefs_dname_obfuscator);
 	this.name = name;
 	this.hash = 0;
 	err = -ENOMEM;
@@ -1033,6 +1041,8 @@ static int __init init_pipe_fs(void)
 {
 	int err = register_filesystem(&pipe_fs_type);
 
+	get_random_bytes(&pipefs_dname_obfuscator, sizeof(unsigned long));
+
 	if (!err) {
 		pipe_mnt = kern_mount(&pipe_fs_type);
 		if (IS_ERR(pipe_mnt)) {
-
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