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SubjectRe: dm: enable opt-out of device-mapper dax support
On Tue, Aug 1, 2017 at 12:45 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> On Tue, Aug 1, 2017 at 12:02 PM, Mike Snitzer <snitzer@redhat.com> wrote:
>> On Tue, Aug 01 2017 at 2:12pm -0400,
>> Dan Williams <dan.j.williams@intel.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@redhat.com>
>>> Cc: Mike Snitzer <snitzer@redhat.com>
>>> Reported-by: Bart Van Assche <Bart.VanAssche@wdc.com>
>>> Signed-off-by: Dan Williams <dan.j.williams@intel.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?

Actually, no, I was thrown off by Bart's suggestion to move code
around. I can handle this all by deleting "select DAX" and adding more
stubbed out dax helpers.

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