Subject: Re: ext2/3 using ext4 module mkdir IO error on pmem DAX mount

From: Ross Zwisler <[email-masked]>
Date: Wed, 2 Mar 2016 17:28:24 -0700

On Wed, Mar 02, 2016 at 02:43:04PM +0800, Xiong Zhou wrote:
> Hi,
> 
> mkdir failed IO error on pmem DAX ext2/3 fs mount using ext4 module.
> 
> This happends only on -next tree, not on Linus' tree,
> at least from 4.5.0-rc5-next-20160224.

I was able to reproduce this and bisect it to the following commit:

commit 1f2d779fed21 ("ext4: optimize group search for inode allocation")

The fact that DAX was enabled in this failure was a red herring - it fails
without DAX as well.  The key is just that we're mounting an ext2 formatted
filesystem using the ext4 module, I think.

- Ross