linux-kernel - Re: [PATCH] UBIFS: fill f

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Date:	Mon, 01 Sep 2008 12:28:00 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	dedekind@...radead.org
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Adrian Hunter <ext-adrian.hunter@...ia.com>
Subject: Re: [PATCH] UBIFS: fill f_fsid

On Mon, 2008-09-01 at 14:16 +0300, Artem Bityutskiy wrote:
> On Mon, 2008-09-01 at 10:43 +0100, David Woodhouse wrote:
> > > UBIFS stores 16-bit UUID in the superblock, and it is a good
> > > idea to return part of it in 'f_fsid' filed of kstatfs structure.
> > 
> > I think you mean a 16-byte UUID, not 16-bit?
> 
> Oh, right.
> 
> > 
> > > +	memcpy(&buf->f_fsid, c->uuid, sizeof(__kernel_fsid_t));
> > 
> > For btrfs I xor the first 64 bits with the second 64 bits, and put
> > _that_ into f_fsid. You're just putting the first 64 bits in and
> > ignoring the second 64 bits. Neither is really _better_ than the other;
> > you just alter the circumstances in which you get collisions. But I
> > suppose we might as well be consistent about how we do it?
> 
> Well, xor-ing should not make the random UUID more random, but for
> consistency purposes sure, I may use xor as well.
> 
> > Alternatively, there's space in the struct statfs to export a couple
> > more uint32_ts, and thus the _whole_ of the uuid. Perhaps we should do
> > that?
> 
> You mean f_spare? I do not think any application would use it so I doubt
> it is good idea to use it.

Well, it wouldn't be called f_spare any more, and applications would
know that it contains the rest of the UUID if it's non-zero.

I'd fix nfsd to fix it immediately (like as I just fixed nfsd to use the
first 64 bits of f_fsid when it's non-zero).

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation



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