linux-kernel - Re: [PATCH 1/7] xfs: always use DAX if mount option is used

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Date:   Tue, 26 Sep 2017 23:40:01 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Christoph Hellwig <hch@....de>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH 1/7] xfs: always use DAX if mount option is used

On Tue, Sep 26, 2017 at 11:30:57AM -0600, Ross Zwisler wrote:
> I agree that Christoph's idea about having the system intelligently adjust to
> use DAX based on performance information it gathers about the underlying
> persistent memory (probably via the HMAT on x86_64 systems) is interesting,
> but I think we're still a ways away from that.

So what are the missing blockers for a getting started?

> FWIW, as my patches suggest and Jan observed I think that we should allow
> users to turn on DAX by treating the inode flag and the mount flag as an 'or'
> operation.  i.e. you get DAX if either the mount option is specified or if the
> inode flag is set, and you can continue to manipulate the per-inode flag as
> you want regardless of the mount option.  I think this provides maximum
> flexibility of the mechanism to select DAX without enforcing policy.

IFF we stick to the dax flag that's the only workable way.  The only
major issue I still see with that is that this allows unprivilegued
users to enable DAX on a any file they own / have write access to.
So there isn't really any way to effectively disable the DAX path
by the sysadmin.

> Does it make sense at this point to just start a "dax" man page that can
> contain info about the mount options, inode flags, kernel config options, how
> to get PMDs, etc?  Or does this documentation need to be sprinkled around more
> in existing man pages?

A dax manpage would be good.