Dan Williams [off-list ref] writes:

On Sat, Nov 16, 2019 at 4:15 AM Aneesh Kumar K.V
[off-list ref] wrote:

quoted

....

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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.

quoted

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.

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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.

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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
/sys/bus/nd/devices/region0>

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.

-aneesh