Re: [PATCH v17 03/10] x86: kdump: use macro CRASH_ADDR_LOW_MAX in functions reserve_crashkernel()

Borislav Petkov
2021-12-16T14:48:23Z

On Thu, Dec 16, 2021 at 08:08:30PM +0800, Leizhen (ThunderTown) wrote:
> If the memory of 'crash_base' is successfully allocated at (1), because the last
> parameter CRASH_ADDR_LOW_MAX is the upper bound, so we can sure that
> "crash_base < CRASH_ADDR_LOW_MAX". So that, reserve_crashkernel_low() will not be
> invoked at (3). That's why I said (1ULL << 32) is inaccurate and enlarge the CRASH_ADDR_LOW
> upper limit.

No, this is actually wrong - that check *must* be 4G. See:

eb6db83d1059 ("x86/setup: Do not reserve crashkernel high memory if low reservation failed")

It is even documented:

crashkernel=size[KMG],low
[KNL, X86-64] range under 4G. When crashkernel=X,high
is passed, kernel could allocate physical memory region
above 4G, that cause second kernel crash on system
that require some amount of low memory, e.g. swiotlb
requires at least 64M+32K low memory, also enough extra
low memory is needed to make sure DMA buffers for 32-bit
devices won't run out.

so you need to do a low allocation for DMA *when* the reserved memory is
above 4G. *NOT* above 512M. But that works due to the obscure situation,
as Baoquan stated, that reserve_crashkernel_low() returns 0 on 32-bit.

So all this is telling us is that that function needs serious cleanup.

--
Regards/Gruss,
Boris.