linux-kernel - Re: [PATCH 1/1] nvdimm: allow exposing RAM carveouts as NVDIMM DIMM devices

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Message-ID: <aNuts7yMTJdtmDXZ@kernel.org>
Date: Tue, 30 Sep 2025 12:15:15 +0200
From: Mike Rapoport <rppt@...nel.org>
To: dan.j.williams@...el.com
Cc: Michał Cłapiński <mclapinski@...gle.com>,
	Ira Weiny <ira.weiny@...el.com>, Dave Jiang <dave.jiang@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>, jane.chu@...cle.com,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Tyler Hicks <code@...icks.com>, linux-kernel@...r.kernel.org,
	nvdimm@...ts.linux.dev
Subject: Re: [PATCH 1/1] nvdimm: allow exposing RAM carveouts as NVDIMM DIMM
 devices

On Fri, Sep 26, 2025 at 11:45:19AM -0700, dan.j.williams@...el.com wrote:
> Michał Cłapiński wrote:
> [..]
> > > As Mike says you would lose 128K at the end, but that indeed becomes
> > > losing that 1GB given alignment constraints.
> > >
> > > However, I think that could be solved by just separately vmalloc'ing the
> > > label space for this. Then instead of kernel parameters to sub-divide a
> > > region, you just have an initramfs script to do the same.
> > >
> > > Does that meet your needs?
> > 
> > Sorry, I'm having trouble imagining this.
> > If I wanted 500 1GB chunks, I would request a region of 500GB+space
> > for the label? Or is that a label and info-blocks?
> 
> You would specify an memmap= range of 500GB+128K*.
> 
> Force attach that range to Mike's RAMDAX driver.
> 
> [ modprobe -r nd_e820, don't build nd_820, or modprobe policy blocks nd_e820 ]
> echo ramdax > /sys/bus/platform/devices/e820_pmem/driver_override
> echo e820_pmem > /sys/bus/platform/drivers/ramdax
> 
> * forget what I said about vmalloc() previously, not needed
> 
> > Then on each boot the kernel would check if there is an actual
> > label/info-blocks in that space and if yes, it would recreate my
> > devices (including the fsdax/devdax type)?
> 
> Right, if that range is persistent the kernel would automatically parse
> the label space each boot and divide up the 500GB region space into
> namespaces.
> 
> 128K of label spaces gives you 509 potential namespaces.

I was also thinking that the label area can be put either in the end or in
the beginning of the memmap= range, so that if you specify memmap=<1G
aligned address - 128K> the actual space will be 1G aligned.

-- 
Sincerely yours,
Mike.