Re: [RFC v2 PATCH] reserve_mem: add support for static memory

[Mike Rapoport]

Hi Shyam,

On Wed, Jun 24, 2026 at 06:22:33PM -0700, Shyam Saini wrote:
> On 21 Jun 2026 13:36, Mike Rapoport wrote:
> > On Thu, Jun 18, 2026 at 11:23:31PM -0700, Shyam Saini wrote:
> > > reserve_mem relies on dynamic memory allocation, this limits the
> > > usecase where memory is required to be preserved across the boots.
> > > Eg: ramoops memory reservation on ACPI platforms
> > >
> > > So add support to pass a pre-determined static address and reserve
> > > memory at a specified location. This enables use case like ramoops
> > > on ACPI platforms to reliably access ramoops region with previous
> > > boot logs.
> > > 
> > > Also skip the parsing of <align> when static address is passed.
> > > 
> > > Example syntax for static address
> > >  reserve_mem=4M@0x1E0000000:oops
> > 
> > reserve_mem is best effort by design because such hacks as well as memmap=
> > cannot guarantee this memory is actually free.
> > 
> > If you want to preserve ramoops reliably, use KHO with reserve_mem.
> > The first kernel will allocate memory, this memory will be preserved by KHO
> > and could be picked up by the second kernel.
> 
> ok, On ARM64 DTS systems, we can reserve ramoops memory in the device tree during
> the warm reboot.

The cc list actually implied x86 ;-)
Added arm64 folks now.

> For an equivalent ARM64 ACPI platform, what is the recommended way to reserve
> and preserve that memory across the boots? 

I don't think it exists, but a command line option (be it memmap= or
reserve_mem=) does not seem the right way to me.

Most of the arguments that were made against adding memmap= to arm64 [1]
apply here.

If kexec is an option, KHO provides a reliable way to preserve memory
across boots.

If kexec is not an option, we should look for a generic way to specify
something like DT's reserved_mem for ACPI/EFI systems.

[1] https://lkml.kernel.org/lkml/20201118063314.22940-1-song.bao.hua@xxxxxxxxxxxxx/T/

> Thanks,
> Shyam

-- 
Sincerely yours,
Mike.