[RFC PATCH 1/4] libnvdimm/namespace: Make namespace size validation arch dependent

[Dan Williams]

On Mon, Nov 18, 2019 at 1:52 AM Aneesh Kumar K.V
<aneesh.kumar at linux.ibm.com> wrote:
>
> Dan Williams <dan.j.williams at intel.com> writes:
>
> > On Sat, Nov 16, 2019 at 4:15 AM Aneesh Kumar K.V
> > <aneesh.kumar at linux.ibm.com> wrote:
> >>
>
> ....
>
>
> >>
> >> Considering the direct-map map size is not going to be user selectable,
> >> do you agree that we can skip the above step 0 configuration you
> >> suggested.
> >>
> >> The changes proposed in the patch series essentially does the rest.
> >>
> >> 1) It validate the size against the arch specific limit during
> >> namespace creation. (part of step 1)
> >
> > This validation is a surprise failure to ndctl.
> >
> >> 2) It also disable initializing a region if it find the size not
> >> correctly aligned as per the platform requirement.
> >
> > There needs to be a way for the user to discover the correct alignment
> > that the kernel will accept.
> >
> >> 3) Direct map  mapping size is different from supported_alignment for a
> >> namespace. The supported alignment controls what possible PAGE SIZE user want the
> >> namespace to be mapped to user space.
> >
> > No, the namespace alignment is different than the page mapping size.
> > The alignment is only interpreted as a mapping size at the device-dax
> > level, otherwise at the raw namespace level it's just an arbitrary
> > alignment.
> >
> >> With the above do you think the current patch series is good?
> >
> > I don't think we've quite converged on a solution.
>
> How about we make it a property of seed device. ie,
> we add `supported_size_align` RO attribute to the seed device. ndctl can
> use this to validate the size value. So this now becomes step0
>
> sys/bus/nd/devices/region0> cat btt0.0/supported_size_align
> 16777216
> /sys/bus/nd/devices/region0> cat pfn0.0/supported_size_align
> 16777216
> /sys/bus/nd/devices/region0> cat dax0.0/supported_size_align
> 16777216

Why on those devices and not namespace0.0?

> We follow that up with validating the size value written to size
> attribute(step 1).
>
> While initializing the namespaces already present in a region we again
> validate the size and if not properly aligned we mark the region
> disabled.

The region might have a mix of namespaces, some aligned and some not,
only the misaligned namespaces should fail to enable. The region
should otherwise enable successfully.