linux-kernel - Re: [RFC PATCH] ARM: Support allocating crashkernel above 4G for LPAE

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Message-ID: <ZrDRCP9tyvXLfAYs@arm.com>
Date: Mon, 5 Aug 2024 14:18:00 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Jinjie Ruan <ruanjinjie@...wei.com>, bhe@...hat.com,
	akpm@...ux-foundation.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] ARM: Support allocating crashkernel above 4G for LPAE

On Fri, Aug 02, 2024 at 12:01:43PM +0100, Russell King wrote:
> On Fri, Aug 02, 2024 at 05:25:10PM +0800, Jinjie Ruan wrote:
> > As ARM LPAE feature support accessing memory beyond the 4G limit, define
> > HAVE_ARCH_CRASHKERNEL_RESERVATION_HIGH macro to support reserving crash

At least in 6.11-rc1, there's no trace of such macro anywhere. So not
sure this patch has any effect (I haven't checked linux-next though).

> > memory above 4G for ARM32 LPAE.
> > 
> > No test because there is no LPAE ARM32 hardware.
> 
> Why are you submitting patches for features you can't test?
> 
> I'm not going to apply this without it being properly tested, because I
> don't believe that this will work in the generic case.
> 
> If the crash kernel is located in memory outside of the lower 4GiB of
> address space, and there is no alias within physical address space
> for that memory, then there is *no* *way* for such a kernel to boot.
> 
> So, right now I believe this patch to be *fundamentally* wrong.

Indeed. Even on arm64, we keep some crashkernel reservations in the
lower parts of the memory for ZONE_DMA allocations.

On arch/arm with LPAE, we could do something similar like forcing some
lowmem reservation and allowing explicit allocation in the higher ranges
with crashkernel=<size>,high. We should, of course, force the kdump
image placement in the lower memory. The user kexec tools must be taught
to interpret this information and provide a DT accordingly to the crash
kernel.

-- 
Catalin