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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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