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SubjectRe: dm: enable opt-out of device-mapper dax support
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?

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