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Date:   Wed, 20 Feb 2019 12:45:31 -0500
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        stable <stable@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 0/7] libnvdimm/pfn: Fix section-alignment padding

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

> On Tue, Feb 12, 2019 at 1:37 PM Dan Williams <dan.j.williams@...el.com> wrote:
>>
>> Lately Linux has encountered platforms that collide Persistent Memory
>> regions between each other, specifically cases where ->start_pad needed
>> to be non-zero. This lead to commit ae86cbfef381 "libnvdimm, pfn: Pad
>> pfn namespaces relative to other regions". That commit allowed
>> namespaces to be mapped with devm_memremap_pages(). However dax
>> operations on those configurations currently fail if attempted within the
>> ->start_pad range because pmem_device->data_offset was still relative to
>> raw resource base not relative to the section aligned resource range
>> mapped by devm_memremap_pages().
>>
>> Luckily __bdev_dax_supported() caught these failures and simply disabled
>> dax. However, to fix this situation a non-backwards compatible change
>> needs to be made to the interpretation of the nd_pfn info-block.
>> ->start_pad needs to be accounted in ->map.map_offset (formerly
>> ->data_offset), and ->map.map_base (formerly ->phys_addr) needs to be
>> adjusted to the section aligned resource base used to establish
>> ->map.map formerly (formerly ->virt_addr).
>>
>> See patch 7 "libnvdimm/pfn: Fix 'start_pad' implementation" for more
>> details, and the ndctl patch series "Improve support + testing for
>> labels + info-blocks" for the corresponding regression test.
>
> Hello valued reviewers, can I plead for a sanity check of at least
> "libnvdimm/pfn: Introduce super-block minimum version requirements"
> and "libnvdimm/pfn: Fix 'start_pad' implementation"? In particular
> Jeff / Johannes this has end user / distro impact in that users may
> lose access to namespaces that are upgraded to v1.3 info-blocks and
> then boot an old kernel. I did not see a way around that sharp edge.

Yes, I'll take a look.

Cheers,
Jeff

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