linux-kernel - Re: [PATCH v2 1/1] libnvdimm/e820: Add a new parameter to configure many regions per e820 entry

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Message-ID: <CA+CK2bAUfXQ_CSKs4MaaNNcgPx6MRjE6Jk85tKGYUOQBG8PFNg@mail.gmail.com>
Date: Wed, 28 May 2025 10:53:45 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Ira Weiny <ira.weiny@...el.com>, Michal Clapinski <mclapinski@...gle.com>, 
	Dan Williams <dan.j.williams@...el.com>, Vishal Verma <vishal.l.verma@...el.com>, 
	Dave Jiang <dave.jiang@...el.com>, Jonathan Corbet <corbet@....net>, nvdimm@...ts.linux.dev, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] libnvdimm/e820: Add a new parameter to configure
 many regions per e820 entry

On Tue, May 27, 2025 at 9:26 AM Mike Rapoport <rppt@...nel.org> wrote:
>
> On Mon, Apr 21, 2025 at 10:55:25AM -0400, Pasha Tatashin wrote:
> > On Sun, Apr 20, 2025 at 10:06 PM Ira Weiny <ira.weiny@...el.com> wrote:
> > >
> > > Michal Clapinski wrote:
> > > > Currently, the user has to specify each memory region to be used with
> > > > nvdimm via the memmap parameter. Due to the character limit of the
> > > > command line, this makes it impossible to have a lot of pmem devices.
> > > > This new parameter solves this issue by allowing users to divide
> > > > one e820 entry into many nvdimm regions.
> > > >
> > > > This change is needed for the hypervisor live update. VMs' memory will
> > > > be backed by those emulated pmem devices. To support various VM shapes
> > > > I want to create devdax devices at 1GB granularity similar to hugetlb.
> > >
> > > Why is it not sufficient to create a region out of a single memmap range
> > > and create multiple 1G dax devices within that single range?
> >
> > This method implies using the ndctl tool to create regions and convert
> > them to dax devices from userspace. This does not work for our use
> > case. We must have these 1 GB regions available during boot because we
> > do not want to lose memory for a devdax label. I.e., if fsdax is
> > created during boot (i.e. default pmem format), it does not have a
> > label. However, if it is created from userspace, we create a label
> > with partition properties, UUID, etc. Here, we need to use kernel
>
> Doesn't ndctl refuse to alter namespaces on "legacy" (i.e. memmap=)
> regions?

Hi Mike

ndctl works with legacy namespaces just fine. We can convert them to
devdax/fsdax/raw pmem, create remove label, etc.

Pasha

>
> > parameters to specify the properties of the pmem devices during boot
> > so they can persist across reboots without losing any memory to
> > labels.
> >
> > Pasha
>
> --
> Sincerely yours,
> Mike.