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From: Oliver <oohall@gmail.com>
Date: Wed, 28 Nov 2018 00:27:11 +1100
Message-ID: <CAOSf1CH1yGws3c0aOJZCLvsUxf5mvqtkQpe3CY7KS7UdKjMjuQ@mail.gmail.com>
Subject: Re: [PATCH 2/4] ndctl/namespace: Check for seed namespaces earlier
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To: Dan Williams <dan.j.williams@intel.com>
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On Mon, Nov 26, 2018 at 7:12 AM Dan Williams <dan.j.williams@intel.com> wrote:
>
> On Mon, Nov 19, 2018 at 12:11 AM Oliver O'Halloran <oohall@gmail.com> wrote:
> >
> > When creating an fsdax or devdax namespace we need to verify that the
> > seed namespaces exist. This patch reworks the validation so that it's
> > done earlier to simplify the subsequent patches in the series.
> >
> > No functional changes.
>
> It does appear to have a functional change. do_setup_pfn() supports
> the case of statically allocated namespaces in NDCTL_NS_MODE_MEMORY
> mode.

Hmm, ok. Up until now I had assumed that as far as ndctl was concerned
NS_MODE_MEMORY was synonymous with a pfn namespace.

> This is what one gets by default with "legacy" pmem defined as
> E820-type-12 memory. In that case the kernel assumes that the
> resulting memmap is always small enough to be allocated from DRAM and
> there is no need to use a dynamic  pfn device. So, if I'm not
> mistaken, the deletion of do_setup_pfn() loses that special case
> handling.

>>From what I see, the main difference is that ndctl would fail
validation when fsdax mode is specified and there's no pfn namespace
support in the kernel. I agree that's not great, but I'm not sure what
we should be doing here. The current behaviour will silently ignore -a
if "-m fsdax -M mem" is specified in the reconfigure case, which
doesn't seem great either.
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