linux-kernel - Re: [PATCH] Added CONFIG_VFAT_FS

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Date:	Wed, 1 Jul 2009 21:11:55 +1000
From:	tridge@...ba.org
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Pavel Machek <pavel@....cz>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	john.lanza@...ux.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	Steve French <sfrench@...ibm.com>,
	Mingming Cao <cmm@...ibm.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] Added CONFIG_VFAT_FS_DUALNAMES option

Hi Alan,

 > > What happens on collision? With 60000 entries in directory, there will
 > > be 50% chance of collision. BAD.
 > 
 > Far more surely - its a birthday paradox.

If you want to do it accurately, the maximum number of long filenames
in a VFAT directory is actually 32767. (it isn't 65536, as each long
filename consumes at least two 8.3 entries, plus you lose the . and
.. entries).

With the patch I've posted there are 30 bits of randomness in each
entry. You could do an accurate binomial expansion to get the exact
probability, but a very good approximation using exponentiation comes
out as a 39.3% chance of a single duplicate appearing in a directory
that is fully populated.

As I mentioned to Pavel, this isn't the whole story though. To cause
the bluescreen the duplicate entries need to be accessed by WindowsXP
in quick succession in a particular pattern. This lowers the
probability a lot. Exactly how much is hard to estimate, but
experiments I've done with deliberately higher probabilities (ie. less
bits of randomness) show that the probability of the bluescreen is
_very_ low.

 > Agreed 100%. I'm also not sure it should be called "vfat" when operating
 > in this mode as it's not vfat any more - it needs a new name.

If the code differed significantly between the two implementations I'd
probably agree, but as the two are extremely close I think maintaining
a separate filesystem isn't worth it.

Cheers, Tridge
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