[PATCH] don't insert pipe dentries into dentry_hashtable.

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 slows 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 (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>
Acked-by: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 17 insertions, 5 deletions

diff --git a/fs/pipe.c b/fs/pipe.c
index b1626f269a34..ae36b89b1a37 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -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;