Re: [PATCH 0/8] device-dax: sub-division support

[Jeff Moyer <jmoyer@redhat.com>]

Hi, Dan,

Dan Williams <dan.j.williams@intel.com> writes:

>>From [PATCH 6/8] dax: sub-division support:
>
> Device-DAX is a mechanism to establish mappings of performance / feature
> differentiated memory with strict fault behavior guarantees.  With
> sub-division support a platform owner can provision sub-allocations of a
> dax-region into separate devices. The provisioning mechanism follows the
> same scheme as the libnvdimm sub-system in that a 'seed' device is
> created at initialization time that can be resized from zero to become
> enabled.
>
> Unlike the nvdimm sub-system there is no on media labelling scheme
> associated with this partitioning. Provisioning decisions are ephemeral
> / not automatically restored after reboot. While the initial use case of
> device-dax is persistent memory other uses case may be volatile, so the
> device-dax core is unable to assume the underlying memory is pmem.  The
> task of recalling a partitioning scheme or permissions on the device(s)
> is left to userspace.

Can you explain this reasoning in a bit more detail, please?  If you
have specific use cases in mind, that would be helpful.

> For persistent allocations, naming, and permissions automatically
> recalled by the kernel, use filesystem-DAX. For a userspace helper

I'd agree with that guidance if it wasn't for the fact that device dax
was born out of the need to be able to flush dirty data in a safe manner
from userspace.  At best, we're giving mixed guidance to application
developers.

-Jeff

> library and utility for manipulating device-dax instances see libdaxctl
> and the daxctl utility here: https://github.com/pmem/ndctl
>
> ---
>
> Dan Williams (8):
>       dax: add region-available-size attribute
>       dax: add region 'id', 'size', and 'align' attributes
>       dax: register seed device
>       dax: use multi-order radix for resource lookup
>       dax: refactor locking out of size calculation routines
>       dax: sub-division support
>       dax: add / remove dax devices after provisioning
>       dax: add debug for region available_size
>
>
>  drivers/dax/Kconfig |    1 
>  drivers/dax/dax.c   |  747 ++++++++++++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 698 insertions(+), 50 deletions(-)
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