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Subject[PATCH] [NET] dont insert socket dentries into dentry_hashtable.
Date
We currently insert socket 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 socket
code, so that dput() can just kill_it.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
--- linux-2.6.19-rc6/net/socket.c 2006-11-22 17:33:41.000000000 +0100
+++ linux-2.6.19-rc6-ed/net/socket.c 2006-11-22 18:28:12.000000000 +0100
@@ -306,7 +306,14 @@ static struct file_system_type sock_fs_t

static int sockfs_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 sockfs_dentry_operations = {
.d_delete = sockfs_delete_dentry,
@@ -354,14 +361,20 @@ static int sock_attach_fd(struct socket

this.len = sprintf(name, "[%lu]", SOCK_INODE(sock)->i_ino);
this.name = name;
- this.hash = SOCK_INODE(sock)->i_ino;
+ this.hash = 0;

file->f_dentry = d_alloc(sock_mnt->mnt_sb->s_root, &this);
if (unlikely(!file->f_dentry))
return -ENOMEM;

file->f_dentry->d_op = &sockfs_dentry_operations;
- d_add(file->f_dentry, SOCK_INODE(sock));
+ /*
+ * We dont want to push 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 sockets
+ */
+ file->f_dentry->d_flags &= ~DCACHE_UNHASHED;
+ d_instantiate(file->f_dentry, SOCK_INODE(sock));
file->f_vfsmnt = mntget(sock_mnt);
file->f_mapping = file->f_dentry->d_inode->i_mapping;
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