linux-kernel - Re: [PATCH v2] d_ino considered harmful

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Message-ID: <20100617185828.GC14389@shell>
Date:	Thu, 17 Jun 2010 14:58:29 -0400
From:	Valerie Aurora <vaurora@...hat.com>
To:	"J. R. Okajima" <hooanon05@...oo.co.jp>
Cc:	David Dillow <dillowda@...l.gov>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Jan Blunck <jblunck@...e.de>,
	Jamie Lokier <jamie@...reable.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Arnd Bergmann <arnd@...db.de>,
	Andreas Dilger <adilger@....com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] d_ino considered harmful

On Fri, Jun 18, 2010 at 03:04:08AM +0900, J. R. Okajima wrote:
> 
> David Dillow:
> > For example, our main Lustre scratch space has over 285 million files in
> > it, and using find -inum takes over 72 hours to walk the tree using
> 	:::
> > Using ne2scan -- which uses libext2fs and combines the inode scan and
> > the name lookup -- takes over 48 hours to generate a list of candidate
> > files for the purge example. With an optimized inode scan and the custom
> 	:::
> 
> While I've never heard of ne2scan, I am interested in this simplified
> problem such as "find the pathname(s) from an inum in a huge fs."
> Is ne2scan essentially equivalent to "debugfs ncheck inum"?
> 
> About Valeris's patch, as long as "ls -i" is useful/helpful,
> > +	/* Use of d_ino without st_dev is always buggy. */
> is not true.

What I'm hearing again and again is that d_ino is useful to improve
performance.  As Andreas put it to me, if d_ino is the same, the
referenced file may or may not be the same, but if it's different, the
files are definitely different.  Only in well-controlled environments
known not to have submounts or bind mounts do people trust d_ino to be
from the same file system as the other entries in a directory.

I only submitted this patch half-seriously - mainly I wanted to find
out how people are using d_ino, and therefore what I need to do for
fallthru directory entries in union mounts.

In order to get the correct inode number for a directory entry
referring to a lower layer file or directory, we have to do a
->lookup() from the fs-specific readdir code (or else require that
fallthrus store an arbitrarily sized integer - which seriously
restricts the implementation).  Now, doing a ->lookup() to get d_ino
makes no sense if we are using d_ino as a way to avoid the cost
stat(), which is mainly the ->lookup().  And you definitely can't use
d_ino by itself in a union mount.

I'm inclined to save the trouble and just return 1 in d_ino for
fallthru directory entries, especially now that I've tested it
system-wide and had no obvious problems.

-VAL
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