linux-kernel - Re: Fix(es) for ext2 fsync bug

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Date:	Wed, 14 Feb 2007 16:08:49 -0500
From:	sfaibish <sfaibish@....com>
To:	"Valerie Henson" <val_henson@...ux.intel.com>,
	linux-fsdevel@...r.kernel.org
Cc:	"Can Sar" <csar@...nford.edu>, "Junfeng Yang" <junfeng@...il.com>,
	"Dawson Engler" <engler@....stanford.edu>,
	"Theodore Ts'o" <tytso@....edu>,
	"kernel list" <linux-kernel@...r.kernel.org>
Subject: Re: Fix(es) for ext2 fsync bug

Val,

Maybe it is not only our (FS people) problem. We probably need to
bring the kernel people judge as ext2 and ext3 are the base Linux FS.
I add the kernel list for opinion.

/Sorin

On Wed, 14 Feb 2007 14:54:54 -0500, Valerie Henson  
<val_henson@...ux.intel.com> wrote:

> Just some quick notes on possible ways to fix the ext2 fsync bug that
> eXplode found.  Whether or not anyone will bother to implement it is
> another matter.
>
> Background: The eXplode file system checker found a bug in ext2 fsync
> behavior.  Do the following: truncate file A, create file B which
> reallocates one of A's old indirect blocks, fsync file B.  If you then
> crash before file A's metadata is all written out, fsck will complete
> the truncate for file A... thereby deleting file B's data.  So fsync
> file B doesn't guarantee data is on disk after a crash.  Details:
>
> http://www.stanford.edu/~engler/explode-osdi06.pdf
>
> Two possible solutions I can think of:
>
> * Rearrange order of duplicate block checking and fixing file size in
>   fsck.  Not sure how hard this is. (Ted?)
>
> * Keep a set of "still allocated on disk" block bitmaps that gets
>   flushed whenever a sync happens.  Don't allocate these blocks.
>   Journaling file systems already have to do this.
>
> -VAL
> -
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