Re: [PATCH 1/2] X86/kdump: move crashkernel=X to reserve under 4G by default

Ingo Molnar
2019-04-21T18:24:43Z

* Dave Young <dyoung@redhat.com> wrote:

> The kdump crashkernel low reservation is limited to under 896M even for
> X86_64. This obscure and miserable limitation exists for old kexec-tools
> compatibility, but the reason is not documented anywhere.
>
> Some more tests/investigations about the background:
> a) Previously old kexec-tools can only load purgatory to memory under 2G,
> Eric remove that limitation in 2012 in kexec-tools:
> Commit b4f9f8599679 ("kexec x86_64: Make purgatory relocatable anywhere
> in the 64bit address space.")
>
> b) back in 2013 Yinghai removed all the limitations in new kexec-tools,
> bzImage64 can be loaded to anywhere.
> Commit 82c3dd2280d2 ("kexec, x86_64: Load bzImage64 above 4G")
>
> c) test results with old kexec-tools with old and latest kernels.
> 1. old kexec-tools can not build with modern toolchain anymore,
> I built it in a RHEL6 vm
> 2. 2.0.0 kexec-tools does not work with latest kernel even with
> memory under 896M and give an error:
> "ELF core (kcore) parse failed", it needs below kexec-tools fix
> Commit ed15ba1b9977 ("build_mem_phdrs(): check if p_paddr is invalid")
> 3. even with patched kexec-tools which fixes 2), it still needs some
> other fixes to work correctly for kaslr enabled kernels.
>
> So the situation is:
> * old kexec-tools is already broken with latest kernels
> * we can not keep this limitations forever just for compatibility of very
> old kexec-tools.
> * If one must use old tools then he/she can choose crashkernel=X@Y
> * people have reported bugs crashkernel=384M failed because kaslr makes
> the 0-896M space sparse,
> * crashkernel can reserve in low or high area, it is natural to understand
> low as memory under 4G
>
> Hence drop the 896M limitation, and change crashkernel low reservation to
> reserve under 4G by default.
>
> Signed-off-by: Dave Young <dyoung@redhat.com>
> ---
> arch/x86/kernel/setup.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> --- linux-x86.orig/arch/x86/kernel/setup.c
> +++ linux-x86/arch/x86/kernel/setup.c
> @@ -71,6 +71,7 @@
> #include <linux/tboot.h>
> #include <linux/jiffies.h>
> #include <linux/mem_encrypt.h>
> +#include <linux/sizes.h>
>
> #include <linux/usb/xhci-dbgp.h>
> #include <video/edid.h>
> @@ -448,18 +449,17 @@ static void __init memblock_x86_reserve_
> #ifdef CONFIG_KEXEC_CORE
>
> /* 16M alignment for crash kernel regions */
> -#define CRASH_ALIGN (16 << 20)
> +#define CRASH_ALIGN SZ_16M
>
> /*
> * Keep the crash kernel below this limit. On 32 bits earlier kernels
> * would limit the kernel to the low 512 MiB due to mapping restrictions.
> - * On 64bit, old kexec-tools need to under 896MiB.
> */
> #ifdef CONFIG_X86_32
> -# define CRASH_ADDR_LOW_MAX (512 << 20)
> -# define CRASH_ADDR_HIGH_MAX (512 << 20)
> +# define CRASH_ADDR_LOW_MAX SZ_512M
> +# define CRASH_ADDR_HIGH_MAX SZ_512M
> #else
> -# define CRASH_ADDR_LOW_MAX (896UL << 20)
> +# define CRASH_ADDR_LOW_MAX SZ_4G
> # define CRASH_ADDR_HIGH_MAX MAXMEM
> #endif

Reviewed-by: Ingo Molnar <mingo@kernel.org>

Thanks,

Ingo