linux-kernel - Re: [RFC, PATCH] dont insert sockets/pipes dentries into dentry

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Message-id: <4548A730.8060507@cosmosbay.com>
Date:	Wed, 01 Nov 2006 14:54:56 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>, Al Viro <viro@....linux.org.uk>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC, PATCH] dont insert sockets/pipes dentries into
 dentry_hashtable.

David Miller a écrit :
> 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.
> 
> 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.
> 

Thank you David and Al for the feedback.

Here is a new version of the patch with your suggestions included.

If necessary, I could split this patch in 3 elementary patches. I chose to
sent it as one patch for ease of discussion.

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

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)

Machines with a lot of sockets/pipes might suffer from longer chains in dentry
hashtable.

Since dentries an unhashed dentry means __dpath() adds a " (deleted)", the 
trick for socket/pipe dentries is to :

- Right after d_alloc(), pretend they are hashed by clearing the 
DCACHE_UNHASHED bit. __dpath() & friends work as intended.

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

- Once dput() must clear the dentry, setting again DCACHE_UNHASHED bit inside 
the custom d_delete() function provided by socket/pipe code.

[patch 1/3] Small optimization to bypass RCU freeing in d_free() for dentries 
that were never hashed (like sockets and pipes). Such dentries dont have to 
wait a RCU grace period.

[patch 2/3] Change socket code to use d_instantiate() instead of d_add()

[patch 3/3] Change pipe code to use d_instantiate() instead of d_add()

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

View attachment "donthash.patch" of type "text/plain" (3862 bytes)