linux-kernel - Re: Linux 2.6.29

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Date:	Wed, 25 Mar 2009 16:19:50 +0100
From:	Bodo Eggert <7eggert@....de>
To:	Theodore Tso <tytso@....edu>, Theodore Tso <tytso@....edu>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

Theodore Tso <tytso@....edu> wrote:

> OK, so there are a couple of solutions to this problem.  One is to use
> ext4 and delayed allocation.  This solves the problem by simply not
> allocating the blocks in the first place, so we don't have to force
> them out to solve the security problem that data=ordered was trying to
> solve.  Simply mounting an ext3 filesystem using ext4, without making
> any change to the filesystem format, should solve the problem.

[...]

> However, these days, nearly all Linux boxes are single user machines,
> so the security concern is much less of a problem.  So maybe the best
> solution for now is to make data=writeback the default.  This solves
> the problem too.  The only problem with this is that there are a lot
> of sloppy application writers out there, and they've gotten lazy about
> using fsync() where it's necessary;

The problem is not having accidential data loss because the inode /happened/
to be written before the data, but having /guaranteed/ data loss in a
60-seconds-window. This is about as acceptable as having a filesystem
replace _any_ data with "deadbeef" on each crash unless fsync was called.

Besides that: If the problem is due to crappy VM writeout (is it?), reducing
security to DOS level is not the answer. You'd want your fs to be usable on
servers, wouldn't you?

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