linux-ext4 - Re: [PATCH V7 9/9] Documentation/dax: Update Usage section

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Date:   Tue, 14 Apr 2020 13:00:15 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Jeff Moyer <jmoyer@...hat.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V7 9/9] Documentation/dax: Update Usage section

On Tue, Apr 14, 2020 at 12:57:54PM -0700, Darrick J. Wong wrote:
> On Tue, Apr 14, 2020 at 12:04:57PM -0700, Dan Williams wrote:
> > On Tue, Apr 14, 2020 at 9:15 AM Darrick J. Wong <darrick.wong@...cle.com> wrote:

[snip]

> > > > > +
> > > > > +Enabling DAX on xfs
> > > > > +-------------------
> > > > > +
> > > > > +Summary
> > > > > +-------
> > > > > +
> > > > > + 1. There exists an in-kernel access mode flag S_DAX that is set when
> > > > > +    file accesses go directly to persistent memory, bypassing the page
> > > > > +    cache.
> > > >
> > > > I had reserved some quibbling with this wording, but now that this is
> > > > being proposed as documentation I'll let my quibbling fly. "dax" may
> > > > imply, but does not require persistent memory nor does it necessarily
> > > > "bypass page cache". For example on configurations that support dax,
> > > > but turn off MAP_SYNC (like virtio-pmem), a software flush is
> > > > required. Instead, if we're going to define "dax" here I'd prefer it
> > > > be a #include of the man page definition that is careful (IIRC) to
> > > > only talk about semantics and not backend implementation details. In
> > > > other words, dax is to page-cache as direct-io is to page cache,
> > > > effectively not there, but dig a bit deeper and you may find it.
> > >
> > > Uh, which manpage?  Are you talking about the MAP_SYNC documentation?
> > 
> > No, I was referring to the proposed wording for STATX_ATTR_DAX.
> > There's no reason for this description to say anything divergent from
> > that description.
> 
> Ahh, ok.  Something like this, then:
> 
>  1. There exists an in-kernel access mode flag S_DAX.  When set, the
>     file is in the DAX (cpu direct access) state.  DAX state attempts to
>     minimize software cache effects for both I/O and memory mappings of
>     this file.  The S_DAX state is exposed to userspace via the
>     STATX_ATTR_DAX statx flag.
> 
>     See the STATX_ATTR_DAX in the statx(2) manpage for more information.

We crossed in the ether!!!  I propose even less details here...  Leave all the
details to the man page.

<quote>
1. There exists an in-kernel access mode flag S_DAX that is set when file
    accesses is enabled for 'DAX'.  Applications must call statx to discover
    the current S_DAX state (STATX_ATTR_DAX).  See the man page for statx for
    more details.
</quote>

Ira