Re: [PATCH] dont insert pipe dentries into dentry_hashtable.

[Andrew Morton <akpm@osdl.org>]

On Wed, 22 Nov 2006 18:48:41 +0100
Eric Dumazet <dada1@cosmosbay.com> wrote:

> 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)

The DCACHE_UNHASHED games seem hacky.

Would it be cleaner to define a new dentry.d_flags bit which can be used to
indicate that this is a hashing-not-needed dentry, and to handle that over
in dcache.c?