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

Mike

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