linux-kernel - Re: dm: enable opt-out of device-mapper dax support

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Date:   Tue, 1 Aug 2017 12:45:05 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     Bart Van Assche <Bart.VanAssche@....com>, dm-devel@...hat.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alasdair Kergon <agk@...hat.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: dm: enable opt-out of device-mapper dax support

On Tue, Aug 1, 2017 at 12:02 PM, Mike Snitzer <snitzer@...hat.com> wrote:
> On Tue, Aug 01 2017 at  2:12pm -0400,
> Dan Williams <dan.j.williams@...el.com> wrote:
>
>> Now that dax is no longer a default property of a block-device, i.e.
>> ->direct_access() is not a block-device operation, we optionally enable
>> device-mapper dax support with a new CONFIG_DM_DAX option.
>>
>> All the dax operations helpers are moved to a new file,
>> drivers/md/dm-dax.c, that is optionally compiled when CONFIG_DM_DAX=y.
>> Otherwise, we stub out all the operations with NULL function pointers
>> and nop wrappers for the core dax routines.
>>
>> Cc: Alasdair Kergon <agk@...hat.com>
>> Cc: Mike Snitzer <snitzer@...hat.com>
>> Reported-by: Bart Van Assche <Bart.VanAssche@....com>
>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>> ---
>>  drivers/md/Kconfig     |   14 +++
>>  drivers/md/Makefile    |    1
>>  drivers/md/dm-dax.c    |  227 ++++++++++++++++++++++++++++++++++++++++++++++++
>>  drivers/md/dm-dax.h    |   73 +++++++++++++++
>>  drivers/md/dm-linear.c |   56 ------------
>>  drivers/md/dm-snap.c   |    9 --
>>  drivers/md/dm-stripe.c |   89 -------------------
>>  drivers/md/dm-target.c |    7 -
>>  drivers/md/dm.c        |  105 ++--------------------
>>  drivers/md/dm.h        |   34 +++++++
>>  10 files changed, 363 insertions(+), 252 deletions(-)
>>  create mode 100644 drivers/md/dm-dax.c
>>  create mode 100644 drivers/md/dm-dax.h
>>
>> diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
>> index 4a249ee86364..bf27b435f7cd 100644
>> --- a/drivers/md/Kconfig
>> +++ b/drivers/md/Kconfig
>> @@ -200,7 +200,6 @@ 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
>> @@ -214,6 +213,19 @@ config BLK_DEV_DM
>>
>>         If unsure, say N.
>>
>> +config DM_DAX
>> +     bool "Direct access (DAX) support"
>> +     depends on BLK_DEV_DM
>> +     default BLK_DEV_PMEM
>> +     select DAX
>> +     ---help---
>> +       Enable DAX support for the device-mapper linear and stripe
>> +       targets for use with DAX capable block devices like /dev/pmemN.
>> +       If you have a DAX capable block device and have enabled
>> +       filesystem DAX support (CONFIG_FS_DAX), then say Y.
>> +
>> +       If unsure, say N.
>> +
>
> I'm questioning the need to have yet another Kbuild CONFIG option.  If
> the user has enabled CONFIG_BLK_DEV_PMEM and CONFIG_FS_DAX (DAX already
> gets selected by CONFIG_FS_DAX) then shouldn't the DM capabilities just
> be enabled?
>
> Guess I'm just skeptical of: why do we want to move to a model where
> users need to opt-in to DM support for DAX?
>
> I also _really_ don't like each target's DAX support being colocated in
> drivers/md/dm-dax.c
>
> This all looks and feels like a serious step backwards.

Ok, you want ifdef'd sections of DAX code in each target and make
DM_DAX a silent option that gets enabled with BLK_DEV_PMEM, anything
else?