Re: [RFC, PATCH] dont insert sockets/pipes dentries into dentry_hashtable.

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <dada1@cosmosbay.com>
Date: Tue, 31 Oct 2006 19:48:48 +0100

> We currently insert sockets/pipes dentries into the global dentry
> hashtable.  This is *useless* because there is currently no way
> these entries can be used for a lookup(). (/proc/xxx/fd/xxx uses a
> different mechanism)

It turns out that while procfs uses a different "mechanism", those
procfs symlinks do point to the real socket dentry, so when you
readlink() on it you do d_path() on the real socket dentry.

If you unhash these things, I'm pretty sure you'll see an ugly
"(deleted)" at the end of the symlink string for /proc/$pid/fd/$X
files that are sockets or something like that.

Al Viro just suggested a way around this to me:

1) Just mark the dentry HASHED by hand in the dentry flags, but don't
   actually hash it.

2) Create a special dentry->d_deleted method for sockets that returns
   0 and clears by hand the HASHED flag bit in the dentry (see what
   dput() does when this happens).

It's an abuse but it will work.