linux-kernel - Re: [RFC Aufs2 #5 28/29] export lookup functions

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Date:	Sat, 11 Apr 2009 01:47:15 +0900
From:	hooanon05@...oo.co.jp
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-kernel@...r.kernel.org, greg@...ah.com,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC Aufs2 #5 28/29] export lookup functions 


Christoph Hellwig:
> Strong NACK to all these exports, no one has any buesiness looking at
> these.

Will you tell me why?
If you have ever discussed exporting lookup_hash(), then please tell me
its URL or something.

lookup_hash is necessary for aufs since,

(from the mail "Subject: [RFC Aufs2 #5 01/29] aufs documents")
+Lookup in a Branch
+----------------------------------------------------------------------
+Since aufs has a character of sub-VFS (see Introduction), it operates
+lookup for branches as VFS does. It may be a heavy work. Generally
+speaking struct nameidata is a bigger structure and includes many
+information. But almost all lookup operation in aufs is the simplest
+case, ie. lookup only an entry directly connected to its parent. Digging
+down the directory hierarchy is unnecessary.
+
+VFS has a function lookup_one_len() for that use, but it is not usable
+for a branch filesystem which requires struct nameidata. So aufs
+implements a simple lookup wrapper function. When a branch filesystem
+allows NULL as nameidata, it calls lookup_one_len(). Otherwise it builds
+a simplest nameidata and calls lookup_hash().
+Here aufs applies "a principle in NFSD", ie. if the filesystem supports
+NFS-export, then it has to support NULL as a nameidata parameter for
+->create(), ->lookup() and ->d_revalidate(). So the lookup wrapper in
+aufs tests if ->s_export_op in the branch is NULL or not.


J. R. Okajima
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