linux-kernel - Re: [PATCH 3/3] [fs/sysv] V7: Add support for non-PDP11 v7 filesystems

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Date:	Tue, 20 Jul 2010 12:41:25 +0200
From:	Lubomir Rintel <lkundrak@...sk>
To:	Christoph Hellwig <hch@....de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] [fs/sysv] V7: Add support for non-PDP11 v7
 filesystems

On Mon, 2010-07-19 at 16:58 +0200, Christoph Hellwig wrote:
> On Mon, Jul 19, 2010 at 07:16:42PM +0200, Lubomir Rintel wrote:
> > A mount-time option was added that makes it possible to override the
> > endianness and an attempt is made to autodetect it (which seems easy,
> > given the disk addresses are 3-byte.
> > 
> > No attempt is made to detect big-endian filesystems -- were there any?
> > Tested with PDP-11 v7 filesystems and PC-IX maintenance floppy.
> 
> Do you actually need the mount option?  We get away just fine with
> it for sysv filesystems.  And if not I'd be consistent and accept the
> options for both sysv and v7 filesystems.

Well, there's no reliable way to detect endiannes of a v7 filesystem
unless we do a deep check as fsck would do (there are cases where we can
be sure that a filesystem is not of a certain bytesex, which is what the
current sanity check does and what's abused for the lousy
autodetection).

In super.c it looks like xenix and sysv use a magic number which the
byte order can be determined from, thus the option would be useless
there.

Coherent seems to always use PDP-11 bytesex. I can not check at the
time, but I'm almost sure it never run on such machines (was PC and 68k
only?), so I suspect the coherent kernel might have always done the
translation to native byte order. I think I have some coherent (for PC)
floppies at home, so I can check tomorrow.

> > +	/* plausibility check on root inode: it is a directory,
> > +	   with a nonzero size that is a multiple of 16 */
> > +	if ((bh2 = sb_bread(sb, 2)) == NULL) {
> > +		return 0;
> > +	}
> 
> A little style nitpick, this should be:
> 
> 	bh2 = sb_bread(sb, 2);
> 	if (!bh)
> 		return 0;
> 

I actually did not write this myself, but merely moved from
v7_fill_super(). It would probably a good idea to keep it as it is (not
to obfuscate the changes) and fix it up in a separate commit if is
worth.

Take care,
Lubo

-- 
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                                     -- Stephen Smoogen

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