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@xxxxxxxxxx> 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
