linux-ext4 - Re: Question about Experimental of Filesystem DAX.

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Date:   Thu, 31 May 2018 09:29:15 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Yasunori Goto <y-goto@...fujitsu.com>, Jan Kara <jack@...e.cz>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        NVDIMM-ML <linux-nvdimm@...ts.01.org>
Subject: Re: Question about Experimental of Filesystem DAX.

On Thu, May 31, 2018 at 8:07 AM, Ross Zwisler
<ross.zwisler@...ux.intel.com> wrote:
> On Thu, May 31, 2018 at 11:27:33AM +0900, Yasunori Goto wrote:
>> Hello,
>>
>>
>> I would like to know about the Experimental message of Filesystem DAX.
>> --------------------------------------------------------
>> DAX enabled. Warning: EXPERIMENTAL, use at your own risk
>> --------------------------------------------------------
>>
>> AFAIK, the final issue of Filesystem DAX is metadata update problem,
>> and it is(will be?) solved by great effort of MAP_SYNC and
>> "fix dma vs truncate/hole-punch" patch set.
>> So, I suppose that the Experimental message can be removed,
>> but I'm not sure.
>>
>> Is it possible?
>> Otherwise, are there any other issues in Filesystem DAX yet?
>>
>> If this is silly question, sorry for noise....
>>
>> Thanks,
>> ---
>> Yasunori Goto
>
> Adding in the XFS and ext4 developers, as it's really their call when to
> remove this notice.
>
> We've talked about this off and on for a long while, but IMHO we should remove
> the EXPERIMENTAL warning.  The last few things that we had on our TODO list
> before this was removed were:
>
> 1) Get consistent handling of the DAX mount option.  We currently have this,
> as both filesystems will behave the same and fall back and remove the DAX
> mount option if it is unsupported by the block device, etc.
>
> 2) Get consistent handling of the DAX inode option.  We currently have this,
> as all DAX behavior now happens through the mount option.  If/when we
> re-enable the per-inode DAX flag we should do it consistently for all DAX
> enabled filesystems.
>
> 3) Make DAX work with other XFS features like reflink, etc.  This one isn't
> done, but we at least disallow DAX with XFS features like reflink where it
> could be an issue.  Darrick, do you still feel like we need to get these
> working together to remove EXPERIMENTAL, or are you happy enough that we're
> keeping them separated and that we're keeping user data safe?
>
> Jan and the other ext4 guys, do you have any additional things you need done
> before removing the EXPERIMENTAL warning from ext4 + DAX?

The one's on my list are:

1/ Get proper support for recovering userspace consumed poison in DAX
mappings (may not make 4.18)

2/ The DAX-DMA vs Truncate fix (queued for 4.18).