Subject: Re: jffs2: -ENOSPC when truncating file?!
From: David Woodhouse <dwmw2@infradead.org>
To: =?ISO-8859-1?Q?J=F6rn?= Engel <joern@logfs.org>
Cc: Pavel Machek <pavel@ucw.cz>, linux-mtd@lists.infradead.org,
       kernel list <linux-kernel@vger.kernel.org>
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	 <1203813368.5771.174.camel@shinybook.infradead.org>
	 <20080224065751.GA31293@lazybastard.org>
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Date: Sun, 24 Feb 2008 18:57:32 +0800
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On Sun, 2008-02-24 at 07:57 +0100, Jörn Engel wrote:
> Could a naïve implementation of this get exploited by doing a large
> number of truncates that just shave single bytes off various files?

Yeah, which is why _my_ naïve implementation would do it for
truncate-to-zero instead of just _any_ truncate (which could even be
truncate-to-larger).

A more complex version might allow _any_ transaction to eat into the
ALLOC_DELETION pool if it is ultimately going to reduce the amount of
space taken on the file system -- even overwriting 'real' data with
zeroes which compress better. That's going to be hard to calculate in
the general case though.

If allowing only truncate-to-zero isn't good enough, perhaps we could
allow truncation to use the ALLOC_DELETION pool when it's going to
obsolete at least one full data node. That's not so hard to check.

-- 
dwmw2

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