[PATCH] dont insert pipe dentries into dentry_hashtable.

[Eric Dumazet <dada1@cosmosbay.com>]

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We currently insert pipe dentries into the global dentry hashtable.
This is *suboptimal* because there is currently no way these entries can be 
used for a lookup(). (/proc/xxx/fd/xxx uses a different mechanism). Inserting 
them in dentry hashtable slow dcache lookups.


To let __dpath() still work correctly (ie not adding a " (deleted)") after 
dentry name, we do : 

- Right after d_alloc(), pretend they are hashed by clearing the 
DCACHE_UNHASHED bit. 

- Call d_instantiate() instead of d_add() : dentry is not inserted in hash 
table.

__dpath() & friends work as intended during dentry lifetime.

- At dismantle time, once dput() must clear the dentry, setting again 
DCACHE_UNHASHED bit inside the custom d_delete() function provided by pipe 
code, so that dput() can just kill_it.

This patch, combined with the next one (avoid RCU for never hashed dentries) 
reduced time of { pipe(p); close(p[0]); close(p[1]);} on my UP machine 
(1.6GHz Pentium-M) from 3.23 us to 2.86 us 
(But this patch does not depend on other patches, only bench results)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

[-- Attachment #2: pipe_nohash_dentry.patch --]
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--- linux-2.6.19-rc6/fs/pipe.c	2006-11-22 17:33:52.000000000 +0100
+++ linux-2.6.19-rc6-ed/fs/pipe.c	2006-11-22 17:53:02.000000000 +0100
@@ -830,7 +830,14 @@ void free_pipe_info(struct inode *inode)
 static struct vfsmount *pipe_mnt __read_mostly;
 static int pipefs_delete_dentry(struct dentry *dentry)
 {
-	return 1;
+	/*
+	 * At creation time, we pretended this dentry was hashed
+	 * (by clearing DCACHE_UNHASHED bit in d_flags)
+	 * At delete time, we restore the truth : not hashed.
+	 * (so that dput() can proceed correctly)
+	 */
+	dentry->d_flags |= DCACHE_UNHASHED;
+	return 0;
 }
 
 static struct dentry_operations pipefs_dentry_operations = {
@@ -891,17 +898,22 @@ struct file *create_write_pipe(void)
 	if (!inode)
 		goto err_file;
 
-	sprintf(name, "[%lu]", inode->i_ino);
+	this.len = sprintf(name, "[%lu]", inode->i_ino);
 	this.name = name;
-	this.len = strlen(name);
-	this.hash = inode->i_ino; /* will go */
+	this.hash = 0;
 	err = -ENOMEM;
 	dentry = d_alloc(pipe_mnt->mnt_sb->s_root, &this);
 	if (!dentry)
 		goto err_inode;
 
 	dentry->d_op = &pipefs_dentry_operations;
-	d_add(dentry, inode);
+	/*
+	 * We dont want to publish this dentry into global dentry hash table. 
+	 * We pretend dentry is already hashed, by unsetting DCACHE_UNHASHED
+	 * This permits a working /proc/$pid/fd/XXX on pipes
+	 */
+	dentry->d_flags &= ~DCACHE_UNHASHED;
+	d_instantiate(dentry, inode);
 	f->f_vfsmnt = mntget(pipe_mnt);
 	f->f_dentry = dentry;
 	f->f_mapping = inode->i_mapping;