Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

Tom Lendacky
2022-09-26T13:38:43Z

On 9/26/22 07:10, Kirill A. Shutemov wrote:
> On Sat, Sep 24, 2022 at 04:03:02AM +0300, Kirill A. Shutemov wrote:
>> On Thu, Sep 22, 2022 at 09:31:12AM -0500, Tom Lendacky wrote:
>>> On 9/8/22 14:28, Mike Rapoport wrote:
>>>> On Thu, Sep 08, 2022 at 09:23:07AM -0700, Dionna Amalie Glaze wrote:
>>>>>>
>>>>>> Looks like the first access to the memory map fails, although I think
>>>>>> it's not in INIT_LIST_HEAD() but rather in init_page_count().
>>>>>>
>>>>>> I'd start with making sure that page_alloc::memmap_alloc() actually returns
>>>>>> accepted memory. If you build kernel with CONFIG_DEBUG_VM=y the memory map
>>>>>> will poisoned in this function, so my guess is it'd crash there.
>>>>>>
>>>>>
>>>>> That's a wonderful hint, thank you! I did not run this test
>>>>> CONFIG_DEBUG_VM set, but you think it's possible it could still be
>>>>> here?
>>>>
>>>> It depends on how you configured your kernel. Say, defconfig does not set
>>>> it.
>>>>
>>>
>>> I also hit the issue at 256GB. My config is using CONFIG_SPARSEMEM_VMEMMAP
>>> and fails in memmap_init_range() when attempting to add the first PFN. It
>>> looks like the underlying page that is backing the vmemmap has not been
>>> accepted (I receive a #VC 0x404 => page not validated).
>>>
>>> Kirill, is this a path that you've looked at? It would appear that somewhere
>>> in the vmemmap_populate_hugepages() path, some memory acceptance needs to be
>>> done for the pages that are used to back vmemmap. I'm not very familiar with
>>> this code, so I'm not sure why everything works for a guest with 255GB of
>>> memory, but then fails for a guest with 256GB of memory.
>>
>> Hm. I don't have machine that large at hands at the moment. And I have not
>> looked at the codepath before.
>>
>> I will try to look into the issue.
>
> I'm not able to trigger the bug.
>
> With help of vm.overcommit_memory=1, I was managed boot TDX guest to shell
> with 256G and 1T of guest memory just fine.
>
> Any chance it is SEV-SNP specific?

There's always a chance. I'll do some more tracing and see what I can find
to try and be certain.

>
> Or maybe there some difference in kernel config? Could you share yours?

Yes, I'll send that to you off-list.

Thanks,
Tom

>