linux-kernel - Re: [PATCH v2 3/3] x86/kdump/64: Change the upper limit of crashkernel reservation

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Date:   Tue, 12 Mar 2019 12:48:38 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        kirill.shutemov@...ux.intel.com, mingo@...nel.org, bp@...en8.de,
        hpa@...or.com, dyoung@...hat.com, x86@...nel.org
Subject: Re: [PATCH v2 3/3] x86/kdump/64: Change the upper limit of
 crashkernel reservation

On Tue, Mar 12, 2019 at 08:50:04AM +0800, Baoquan He wrote:
> Restrict kdump to only reserve crashkernel below 64TB. Since the kdump
> jumping may be from 5-level to 4-level, and the kdump kernel is put
> above 64TB in 5-level kernel, then the jumping will fail. And the
> crashkernel reservation is done during the 1st kernel bootup, there's
> no way to detect the paging mode of kdump kernel at that time.
> 
> Hence change the upper limit of crashkernel reservation to 64TB
> on x86_64.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  arch/x86/kernel/setup.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 3d872a527cd9..d4d1366738a4 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -451,16 +451,26 @@ static void __init memblock_x86_reserve_range_setup_data(void)
>  #define CRASH_ALIGN		(16 << 20)
>  
>  /*
> - * 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.
> + * 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 be under 896MiB. The later
> + * supports to put kernel above 4G, up to system RAM top. Here
> + * kdump kernel need be restricted to be under 64TB, which is
> + * the upper limit of system RAM in 4-level paing mode. Since
> + * the kdump jumping could be from 5-level to 4-level, the jumping
> + * will fail if kernel is put above 64TB, and there's no way to
> + * detect the paging mode of the kernel which will be loaded for
> + * dumping during the 1st kernel bootup.
>   */
>  #ifdef CONFIG_X86_32
>  # define CRASH_ADDR_LOW_MAX	(512 << 20)
>  # define CRASH_ADDR_HIGH_MAX	(512 << 20)
>  #else
>  # define CRASH_ADDR_LOW_MAX	(896UL << 20)
> -# define CRASH_ADDR_HIGH_MAX	MAXMEM
> +# define CRASH_ADDR_HIGH_MAX	(1ULL < 46)

Maybe

# define CRASH_ADDR_HIGH_MAX (64UL << 40)

to match the notation for the rest of defines?


Otherwise the patchset look good to me. You can use my ack for all of
them:

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

>  #endif
>  
>  static int __init reserve_crashkernel_low(void)
> -- 
> 2.17.2
> 

-- 
 Kirill A. Shutemov