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From: Dan Williams <dan.j.williams@intel.com>
Date: Wed, 30 Oct 2019 23:30:19 -0700
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Subject: Re: [RFC PATCH 1/4] libnvdimm/namespace: Make namespace size
 validation arch dependent
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
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On Wed, Oct 30, 2019 at 10:35 PM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
[..]
> > True, for the pfn device and the device-dax mapping size, but I'm
> > suggesting adding another instance of alignment control at the raw
> > namespace level. That would need to be disconnected from the
> > device-dax page mapping granularity.
> >
>
> Can you explain what you mean by raw namespace level ? We don't have
> multiple values against which we need to check the alignment of
> namespace start and namespace size.
>
> If you can outline how and where you would like to enforce that check I
> can start working on it.
>

What I mean is that the process of setting up a pfn namespace goes
something like this in shell script form:

1/ echo $size > /sys/bus/nd/devices/$namespace/size
2/ echo $namespace > /sys/bus/nd/devices/$pfn/namespace
3/ echo $pfn_align > /sys/bus/nd/devices/$pfn/align

What I'm suggesting is add an optional 0th step that does:

echo $raw_align > /sys/bus/nd/devices/$namespace/align

Where the raw align needs to be needs to be max($pfn_align,
arch_mapping_granulariy).

So on powerpc where PAGE_SIZE < arch_mapping_granulariy, the following:

cat /sys/bus/nd/devices/$namespace/supported_aligns

...would show the same output as:

cat /sys/bus/nd/devices/$pfn/align

...but with any alignment choice less than arch_mapping_granulariy removed.



All that said, the x86 vmemmap_populate() falls back to use small
pages in some case to get around this constraint. Can't powerpc do the
same? It would seem to be less work than the above proposal.