linux-kernel - Re: [resend PATCH v2 11/33] dm: add dax_device and dax

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Message-ID: <1501363445.3005.1.camel@wdc.com>
Date:   Sat, 29 Jul 2017 21:24:07 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "dan.j.williams@...el.com" <dan.j.williams@...el.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "hch@....de" <hch@....de>,
        "toshi.kani@....com" <toshi.kani@....com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>
Subject: Re: [resend PATCH v2 11/33] dm: add dax_device and dax_operations
 support

On Sat, 2017-07-29 at 12:57 -0700, Dan Williams wrote:
> On Fri, Jul 28, 2017 at 9:17 AM, Bart Van Assche <Bart.VanAssche@....com> wrote:
> > On Mon, 2017-04-17 at 12:09 -0700, Dan Williams wrote:
> > > diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
> > > index b7767da50c26..1de8372d9459 100644
> > > --- a/drivers/md/Kconfig
> > > +++ b/drivers/md/Kconfig
> > > @@ -200,6 +200,7 @@ config BLK_DEV_DM_BUILTIN
> > >  config BLK_DEV_DM
> > >       tristate "Device mapper support"
> > >       select BLK_DEV_DM_BUILTIN
> > > +     select DAX
> > >       ---help---
> > >         Device-mapper is a low level volume manager.  It works by allowing
> > >         people to specify mappings for ranges of logical sectors.  Various
> > 
> > (replying to an e-mail of three months ago)
> > 
> > Hello Dan,
> > 
> > While building a v4.12 kernel I noticed that enabling device mapper support
> > now unconditionally enables DAX. I think there are plenty of systems that use
> > dm but do not need DAX. Have you considered to rework this such that instead
> > of dm selecting DAX that DAX support is only enabled in dm if CONFIG_DAX is
> > enabled?
> 
> I'd rather flip this around and add a CONFIG_DM_DAX that gates whether
> DM enables / links to the DAX core. I'll take a look at a patch.

Thanks! Please also consider to move all DAX-related dm code into a separate
source file such that the number of #ifdef CONFIG_DM_DAX statements can be
kept to an absolute minimum.

Bart.