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Date:   Fri, 22 Mar 2019 16:22:16 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     Borislav Petkov <bp@...en8.de>, Joerg Roedel <jroedel@...e.de>,
        Baoquan He <bhe@...hat.com>,
        Jerry Hoemann <jerry.hoemann@....com>, x86@...nel.org,
        Randy Dunlap <rdunlap@...radead.org>,
        kexec@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yinghai Lu <yinghai@...nel.org>, vgoyal@...hat.com,
        iommu@...ts.linux-foundation.org, konrad.wilk@...cle.com
Subject: Re: [PATCHv7] x86/kdump: bugfix, make the behavior of crashkernel=X
 consistent with kaslr

Hi Pingfan, 

Thanks for the effort,
On 03/01/19 at 11:19am, Pingfan Liu wrote:
> On Fri, Mar 1, 2019 at 11:04 AM Pingfan Liu <kernelfans@...il.com> wrote:
> >
> > Hi Borislav,
> >
> > Do you think the following patch is good at present?
> > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> > index 81f9d23..9213073 100644
> > --- a/arch/x86/kernel/setup.c
> > +++ b/arch/x86/kernel/setup.c
> > @@ -460,7 +460,7 @@ static void __init
> > memblock_x86_reserve_range_setup_data(void)
> >  # 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_LOW_MAX    (1 << 32)
> >  # define CRASH_ADDR_HIGH_MAX   MAXMEM
> >  #endif
> >
> Or patch lools like:
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 3d872a5..ed0def5 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -459,7 +459,7 @@ static void __init
> memblock_x86_reserve_range_setup_data(void)
>  # 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_LOW_MAX    (1 << 32)
>  # define CRASH_ADDR_HIGH_MAX   MAXMEM
>  #endif
> 
> @@ -551,6 +551,15 @@ static void __init reserve_crashkernel(void)
>                                                     high ? CRASH_ADDR_HIGH_MAX
>                                                          : CRASH_ADDR_LOW_MAX,
>                                                     crash_size, CRASH_ALIGN);
> +#ifdef CONFIG_X86_64
> +               /*
> +                * crashkernel=X reserve below 4G fails? Try MAXMEM
> +                */
> +               if (!high && !crash_base)
> +                       crash_base = memblock_find_in_range(CRASH_ALIGN,
> +                                               CRASH_ADDR_HIGH_MAX,
> +                                               crash_size, CRASH_ALIGN);
> +#endif
> 
> which tries 0-4G, the fall back to 4G above

This way looks good to me, I will do some testing with old kexec-tools,
Once testing done I can take up this again and repost later with some documentation
update.  Also will split to 2 patches  one to drop the old limitation,
another for the fallback.

Thanks
Dave

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