[patch] epoll use a single inode ...

[Davide Libenzi]

Epoll does not keep any private data attached to its inode, so there'd be 
no need to allocate one inode per fd. For epoll, the inode is just a 
placeholder for the file operations and could be shared by all instances.
I'd like to use the same optimization even for the upcoming file-based 
objects, so if you see problems let me know.
One that Al was pointing out was that an fstat(2) over an epoll fd would 
show the same st_ino. IMO that should be fine since an fstat(2) over an 
epoll fd is not something you want to do in any case and expecting 
meaningfull results.



Signed-off-by: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>


- Davide



eventpoll.c |   36 ++++++++++++++++++++++++++++++++----
1 file changed, 32 insertions(+), 4 deletions(-)



Index: linux-2.6.20.ep2/fs/eventpoll.c
===================================================================
--- linux-2.6.20.ep2.orig/fs/eventpoll.c	2007-03-04 14:40:01.000000000 -0800
+++ linux-2.6.20.ep2/fs/eventpoll.c	2007-03-05 13:03:52.000000000 -0800
@@ -258,6 +258,7 @@
 		   int maxevents, long timeout);
 static int eventpollfs_delete_dentry(struct dentry *dentry);
 static struct inode *ep_eventpoll_inode(void);
+static struct inode *ep_create_inode(void);
 static int eventpollfs_get_sb(struct file_system_type *fs_type,
 			      int flags, const char *dev_name,
 			      void *data, struct vfsmount *mnt);
@@ -279,6 +280,9 @@
 /* Virtual fs used to allocate inodes for eventpoll files */
 static struct vfsmount *eventpoll_mnt __read_mostly;
 
+/* Placeholder inode for eventpoll fds */
+static struct inode *eventpoll_inode;
+
 /* File callbacks that implement the eventpoll file behaviour */
 static const struct file_operations eventpoll_fops = {
 	.release	= ep_eventpoll_close,
@@ -763,15 +767,18 @@
 	 * using the inode number.
 	 */
 	error = -ENOMEM;
-	sprintf(name, "[%lu]", inode->i_ino);
+	sprintf(name, "[%p]", ep);
 	this.name = name;
 	this.len = strlen(name);
-	this.hash = inode->i_ino;
+	this.hash = 0;
 	dentry = d_alloc(eventpoll_mnt->mnt_sb->s_root, &this);
 	if (!dentry)
 		goto eexit_4;
 	dentry->d_op = &eventpollfs_dentry_operations;
-	d_add(dentry, inode);
+	/* Do not publish this dentry inside the global dentry hash table */
+	dentry->d_flags &= ~DCACHE_UNHASHED;
+	d_instantiate(dentry, inode);
+
 	file->f_path.mnt = mntget(eventpoll_mnt);
 	file->f_path.dentry = dentry;
 	file->f_mapping = inode->i_mapping;
@@ -1555,6 +1562,11 @@
 
 static int eventpollfs_delete_dentry(struct dentry *dentry)
 {
+	/*
+	 * We faked vfs to believe the dentry was hashed when we created it.
+	 * Now we restore the flag so that dput() will work correctly.
+	 */
+	dentry->d_flags |= DCACHE_UNHASHED;
 
 	return 1;
 }
@@ -1562,6 +1574,17 @@
 
 static struct inode *ep_eventpoll_inode(void)
 {
+
+	return igrab(eventpoll_inode);
+}
+
+/*
+ * A single inode exist for all eventpoll files. On the contrary of pipes,
+ * eventpoll inodes has no per-instance data associated, so we can avoid
+ * the allocation of multiple of them.
+ */
+static struct inode *ep_create_inode(void)
+{
 	int error = -ENOMEM;
 	struct inode *inode = new_inode(eventpoll_mnt->mnt_sb);
 
@@ -1626,10 +1649,14 @@
 
 	/* Mount the above commented virtual file system */
 	eventpoll_mnt = kern_mount(&eventpoll_fs_type);
-	error = PTR_ERR(eventpoll_mnt);
 	if (IS_ERR(eventpoll_mnt))
 		goto epanic;
 
+	/* Create the single instance of inode for all eventpoll fds */
+	eventpoll_inode = ep_create_inode();
+	if (IS_ERR(eventpoll_inode))
+		goto epanic;
+
 	DNPRINTK(3, (KERN_INFO "[%p] eventpoll: successfully initialized.\n",
 			current));
 	return 0;
@@ -1642,6 +1669,7 @@
 static void __exit eventpoll_exit(void)
 {
 	/* Undo all operations done inside eventpoll_init() */
+	iput(eventpoll_inode);
 	unregister_filesystem(&eventpoll_fs_type);
 	mntput(eventpoll_mnt);
 	kmem_cache_destroy(pwq_cache);
-
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