linux-kernel - Re: [PATCH 04/41] VFS: Remove unnecessary micro-optimization in cached

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Date:	Sun, 29 Nov 2009 21:07:39 -0500
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Jan Blunck <jblunck@...e.de>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Andy Whitcroft <apw@...onical.com>,
	Scott James Remnant <scott@...onical.com>,
	Sandu Popa Marius <sandupopamarius@...il.com>,
	Jan Rekorajski <baggins@...h.mimuw.edu.pl>,
	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	Arnd Bergmann <arnd@...db.de>,
	Vladimir Dronnikov <dronnikov@...il.com>,
	Felix Fietkau <nbd@...nwrt.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/41] VFS: Remove unnecessary micro-optimization in cached_lookup() 

In message <1256152779-10054-5-git-send-email-vaurora@...hat.com>, Valerie Aurora writes:
> From: Jan Blunck <jblunck@...e.de>
> 
> d_lookup() takes rename_lock which is a seq_lock.  This is so cheap
> it's not worth calling lockless __d_lookup() first from
> cache_lookup().  Rename cached_lookup() to cache_lookup() while we're
> there.

Val, this is another patch unrelated to union mounts, an
optimization/simplification of the VFS code.  I think you need to try and
push such VFS patches upstream more quickly, so as to reduce the set of UM
patches you have to maintain.

> Signed-off-by: Jan Blunck <jblunck@...e.de>
> Signed-off-by: Valerie Aurora <vaurora@...hat.com>
> ---
>  fs/namei.c |   13 ++++---------
>  1 files changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index e334f25..9c9ecfa 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -404,15 +404,10 @@ do_revalidate(struct dentry *dentry, struct nameidata *nd)
>   * Internal lookup() using the new generic dcache.
>   * SMP-safe
>   */
> -static struct dentry * cached_lookup(struct dentry * parent, struct qstr * name, struct nameidata *nd)
> +static struct dentry *cache_lookup(struct dentry *parent, struct qstr *name,
> +				   struct nameidata *nd)
>  {
> -	struct dentry * dentry = __d_lookup(parent, name);
> -
> -	/* lockess __d_lookup may fail due to concurrent d_move() 
> -	 * in some unrelated directory, so try with d_lookup
> -	 */
> -	if (!dentry)
> -		dentry = d_lookup(parent, name);
> +	struct dentry *dentry = d_lookup(parent, name);
>  
>  	if (dentry && dentry->d_op && dentry->d_op->d_revalidate)
>  		dentry = do_revalidate(dentry, nd);
> @@ -1191,7 +1186,7 @@ static struct dentry *__lookup_hash(struct qstr *name,
>  			goto out;
>  	}
>  
> -	dentry = cached_lookup(base, name, nd);
> +	dentry = cache_lookup(base, name, nd);
>  	if (!dentry) {
>  		struct dentry *new;
>  
> -- 
> 1.6.3.3
> 
> --
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Erez.
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