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

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Message-ID: <20200414201808.GI6742@magnolia>
Date:   Tue, 14 Apr 2020 13:18:08 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Ira Weiny <ira.weiny@...el.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 01:00:15PM -0700, Ira Weiny wrote:
> 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>

Why stop cutting there? :)

 1. There exists an in-kernel file access mode flag S_DAX that
    corresponds to the statx flag STATX_ATTR_DIRECT_LOAD_STORE.  See the
    manpage for statx(2) for details about this access mode.

--D

> Ira
>