[RFC PATCH] x86_64/mm/boot: Fix kernel_ident_mapping_init() failure for kexec

[Xunlei Pang]

On 03/24/2017 at 08:04 PM, Kirill A. Shutemov wrote:
> On Mon, Mar 20, 2017 at 02:11:31PM +0800, Xunlei Pang wrote:
>> I found that the kdump is broken on linux-4.11.0-rc2+
> That's actually tip tree or linux-next. The problematic change is not in
> Linus' tree.
>
>> , probably
>> due to the 5level-paging feature that "#define p4d_present(p4d) 1",
>> as a result in ident_p4d_init(), it will go into ident_pud_init()
>> directly without allocating the new pud.
>>
>> Looks like this patch can make it work again.
> Okay, that's bisectability issue. Uncovered by splitting my patchset into
> parts.
>
> Could you check if applying "Part 2" of 5-level paging changes[1] would
> help you?

I confirmed that it works after applying your following patches:
  x86: Convert the rest of the code to support p4d_t
  x86/xen: Change __xen_pgd_walk() and xen_cleanmfnmap() to support p4d
  x86/kasan: Prepare clear_pgds() to switch to <asm-generic/pgtable-nop4d.h>
  x86/mm/pat: Add 5-level paging support
  x86/efi: Add 5-level paging support
  x86/kexec: Add 5-level paging support

Regards,
Xunlei

>
> Making the code work with both <asm-generic/5level-fixup.h> and
> <asm-generic/pgtable-nop4d.h> would make it even uglier. Not sure if it
> makes sense to address it on its own if second part fixes the situation.
>
> [1] http://lkml.kernel.org/r/20170317185515.8636-1-kirill.shutemov@linux.intel.com
>