Re: [PATCH 0/9] add ext4 per-inode DAX flag

From: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
To: Dan Williams <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Cc: Eric Sandeen <sandeen-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Theodore Ts'o <tytso-3s7WtUTddSA@public.gmane.org>,
	"Darrick J. Wong"
	<darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>,
	Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>,
	"linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org"
	<linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org>,
	Dave Chinner <david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org>,
	"linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>,
	linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Andreas Dilger
	<adilger.kernel-m1MBpc4rdrD3fQ9qLvQP4Q@public.gmane.org>,
	Lukas Czerner <lczerner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	linux-ext4 <linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Andrew Morton
	<akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Subject: Re: [PATCH 0/9] add ext4 per-inode DAX flag
Date: Thu, 7 Sep 2017 15:13:03 -0600	[thread overview]
Message-ID: <20170907211303.GA23212@linux.intel.com> (raw)
In-Reply-To: <CAPcyv4hfhDT9NFRXL+MT5epiqWHJ0RLraV4P3CZ4EJM6L-s0Nw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

On Thu, Sep 07, 2017 at 01:54:45PM -0700, Dan Williams wrote:
> On Wed, Sep 6, 2017 at 10:07 AM, Ross Zwisler
> <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org> wrote:
> > On Tue, Sep 05, 2017 at 09:12:35PM -0500, Eric Sandeen wrote:
> >> On 9/5/17 5:35 PM, Ross Zwisler wrote:
> >> > The original intent of this series was to add a per-inode DAX flag to ext4
> >> > so that it would be consistent with XFS.  In my travels I found and fixed
> >> > several related issues in both ext4 and XFS.
> >>
> >> Hi Ross -
> >>
> >> hch had a lot of reasons to nuke the dax flag from orbit, and we just
> >> /disabled/ it in xfs due to its habit of crashing the kernel...
> >
> > Ah, sorry, I wasn't CC'd on those threads and missed them.  For any interested
> > bystanders:
> >
> > https://www.spinics.net/lists/linux-ext4/msg57840.html
> > https://www.spinics.net/lists/linux-xfs/msg09831.html
> > https://www.spinics.net/lists/linux-xfs/msg10124.html
> >
> >> so a couple questions:
> >>
> >> 1) does this series pass hch's "test the per-inode DAX flag" fstest?
> >
> > Nope, it has the exact same problems as the XFS per-inode DAX flag.
> >
> >> 2) do we have an agreement that we need this flag at all, or is this
> >>    just a parity item because xfs has^whad a per-inode flag?
> >
> > It was for parity, and because it allows admins finer grained control over
> > their system.  Basically all things discussed in response to Lukas's original
> > patch in the first link above.
> 
> I think it's more than parity. When pmem is slower than page cache it
> is actively harmful to have DAX enabled globally for a filesystem. So,
> not only should we push for per-inode DAX control, we should also push
> to deprecate the mount option. I agree with Christoph that we should
> try to automatically and transparently enable DAX where it makes
> sense, but we also need a finer-grained mechanism than a mount flag to
> force the behavior one way or the other.

Yep, agreed.  I'll play with how to make this work after I've sorted out all
the data corruptions I've found. :)