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Date:   Mon, 16 Mar 2020 10:52:24 +0100
From:   Jan Kara <jack@suse.cz>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Dave Chinner <david@fromorbit.com>,
        "Theodore Y. Ts'o" <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5
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On Wed 11-03-20 10:07:18, Dan Williams wrote:
> On Tue, Mar 10, 2020 at 11:30 PM Christoph Hellwig <hch@lst.de> wrote:
> >
> > On Tue, Mar 10, 2020 at 08:36:14PM -0700, Darrick J. Wong wrote:
> > > 1) Leave the inode flag (FS_XFLAG_DAX) as it is, and export the S_DAX
> > > status via statx.  Document that changes to FS_XFLAG_DAX do not take
> > > effect immediately and that one must check statx to find out the real
> > > mode.  If we choose this, I would also deprecate the dax mount option;
> > > send in my mkfs.xfs patch to make it so that you can set FS_XFLAG_DAX on
> > > all files at mkfs time; and we can finally lay this whole thing to rest.
> > > This is the closest to what we have today.
> > >
> > > 2) Withdraw FS_XFLAG_DAX entirely, and let the kernel choose based on
> > > usage patterns, hardware heuristics, or spiteful arbitrariness.
> >
> > 3) Only allow changing FS_XFLAG_DAX on directories or files that do
> > not have blocks allocated to them yet, and side step all the hard
> > problems.
> 
> This sounds reasonable to me.
> 
> As for deprecating the mount option, I think at a minimum it needs to
> continue be accepted as an option even if it is ignored to not break
> existing setups.

Agreed. But that's how we usually deprecate mount options. Also I'd say
that statx() support for reporting DAX state and some education of
programmers using DAX is required before we deprecate the mount option
since currently applications check 'dax' mount option to determine how much
memory they need to set aside for page cache before they consume everything
else on the machine...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR