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

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Date:   Thu, 7 Sep 2017 13:54:45 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Eric Sandeen <sandeen@...hat.com>,
        Lukas Czerner <lczerner@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Christoph Hellwig <hch@....de>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH 0/9] add ext4 per-inode DAX flag

On Wed, Sep 6, 2017 at 10:07 AM, Ross Zwisler
<ross.zwisler@...ux.intel.com> 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.