Re: [PATCH 2/5] ext4: disable merging of uninitialized extents

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Mon, Feb 25, 2013 at 08:07:40PM +0400, Dmitry Monakhov wrote:
> Derived from Jan's patch:http://permalink.gmane.org/gmane.comp.file-systems.ext4/36470
> 
> Merging of uninitialized extents creates all sorts of interesting race
> possibilities when writeback / DIO races with fallocate. Thus
> ext4_convert_unwritten_extents_endio() has to deal with a case where
> extent to be converted needs to be split out first. That isn't nice
> for two reasons:
> 
> 1) It may need allocation of extent tree block so ENOSPC is possible.
> 2) It complicates end_io handling code
> 
> So we disable merging of uninitialized extents which allows us to simplify
> the code. Extents will get merged after they are converted to initialized
> ones.
> 
> Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>

After applied this patch, xfstests #275 will print a warning message.

kernel:EXT4-fs (sda2): failed to convert unwritten extents to written
extents -- potential data loss!  (inode 13, offset 1537212416, size 524288,
error -28)

But IMHO we don't need to worry about it because it is hard to be
trigger.  I hit it because I run xfstests #275 several times.  So just a
note here.

Regards,
                                                - Zheng