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From: Dan Williams <dan.j.williams@intel.com>
Date: Wed, 8 Nov 2017 13:26:32 -0800
Message-ID: <CAPcyv4hR7DQ98ZCqqeyD2ihO0jWpQqPv_+s4v6iVaiNWrv96vw@mail.gmail.com>
Subject: Re: [dm-devel] [PATCH] vmalloc: introduce vmap_pfn for persistent memory
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Sender: owner-linux-mm@kvack.org
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To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>, "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>, Christoph Hellwig <hch@lst.de>, Linux MM <linux-mm@kvack.org>, dm-devel@redhat.com, Ross Zwisler <ross.zwisler@linux.intel.com>, Laura Abbott <labbott@redhat.com>, "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>

On Wed, Nov 8, 2017 at 12:26 PM, Mikulas Patocka <mpatocka@redhat.com> wrote:
> On Wed, 8 Nov 2017, Christoph Hellwig wrote:
>
>> Can you start by explaining what you actually need the vmap for?
>
> It is possible to use lvm on persistent memory. You can create linear or
> striped logical volumes on persistent memory and these volumes still have
> the direct_access method, so they can be mapped with the function
> dax_direct_access().
>
> If we create logical volumes on persistent memory, the method
> dax_direct_access() won't return the whole device, it will return only a
> part. When dax_direct_access() returns the whole device, my driver just
> uses it without vmap. When dax_direct_access() return only a part of the
> device, my driver calls it repeatedly to get all the parts and then
> assembles the parts into a linear address space with vmap.

I know I proposed "call dax_direct_access() once" as a strawman for an
in-kernel driver user, but it's better to call it per access so you
can better stay in sync with base driver events like new media errors
and unplug / driver-unload. Either that, or at least have a plan how
to handle those events.

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