linux-kernel - Re: ext4 change in v3.3-rc2 broke user space

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Message-ID: <20130311213520.GE15478@thunk.org>
Date:	Mon, 11 Mar 2013 17:35:20 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Bryan Whitehead <driver@...ahappy.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: ext4 change in v3.3-rc2 broke user space

On Mon, Mar 11, 2013 at 02:03:51PM -0700, Bryan Whitehead wrote:
> 
> The change is about a year old now but it is finally making its way
> into distribution binaries. As a user it sucks because "ext4" the
> "stable" fs basically breaks. Fortunately for myself I've used xfs as
> suggested by GlusterFS (older versions stated ext4/ext3 was fine).
> However, there are many installs (particularly older installs) that
> used ext4 and are finding they must now run outdated old kernels until
> they can completely reinstall their gluster cluster with non-ext4
> underlying filesystems.

GlusterFS is doing something fundamentally buggy; it is assuming that
high bits of the value returned "telldir" are zero.  This is
inherently non-portable.  This change was used to avoid hash collision
causing problems for applications using telldir/seekdir() on very
large file systems.

So the question was whether it was important to fix a bug for well
behaved, non-buggy applications, or to not fix this bug to accomodate
a buggy application (namely, Gluster).

There were some questions going back and forth about whether Gluster
could work around this non-potable, buggy use of telldir()/seekdir(),
and that's why nothing had been done for a while.

What we'll probably do to accomodate Gluster is to create a new
ext4-specifical ioctl(), which allows the process to get the old
telldir cookie format (i.e., EXT4_IOC_IM_A_BUGGY_APPLICATION :-).

						- Ted
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