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

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Date:   Sat, 29 Jul 2017 12:57:19 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "toshi.kani@....com" <toshi.kani@....com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "hch@....de" <hch@....de>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [resend PATCH v2 11/33] dm: add dax_device and dax_operations support

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.