linux-kernel - [PATCH] dont insert pipe dentries into dentry

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Date:	Wed, 22 Nov 2006 18:48:41 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Al Viro <viro@....linux.org.uk>, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>
Subject: [PATCH] dont 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 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@...mosbay.com>

View attachment "pipe_nohash_dentry.patch" of type "text/plain" (1495 bytes)