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

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Date:   Thu, 31 May 2018 10:46:36 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Yasunori Goto <y-goto@...fujitsu.com>, Jan Kara <jack@...e.cz>,
        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 09:29:15AM -0700, Dan Williams wrote:
> 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.

<nod>

As an aside, I wonder if Christoph's musings about "just have the kernel
determine the appropriate dax/non-dax setting from the acpi tables and
skip the inode flag entirely" ever got resolved?

> > 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.

The behavior of the inode flag isn't all that consistent.  ext4 doesn't
support it at all.  On XFS, you can set or clear FS_XFLAG_DAX on a
directory which will propagate the setting to any files created in that
directory.

However, if you set or clear it on a file we update the on-disk inode
but we can't change the in-core state flag (S_DAX) until the next
in-core inode instantiation.  It's weird that users can change the flag
but the intended behavior changes won't happen until some ... time ...
in the future??

> > 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?

Yes, reflink and dax still need to work together.  I've not heard any
good arguments for why page sharing + copy on write are fundamentally
incompatible with the dax model, or why dax users will never, ever
require reflink.

The recent thread between Jan and Dan make me wonder if making mappings
share struct pages is going to be a nightmare to add to the mm code,
though...

Also: ideally XFS would also be able to consume poison event
notifications from the pmem so that it can try to deal with metadata
loss, but that's probably a separate effort.

--D

> > 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).
> --
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