Re: [PATCH] mm: thp: Deny THP for guest_memfd and secretmem in file_thp_enabled()

[Ackerley Tng <ackerleytng@google.com>]

"David Hildenbrand (Arm)" <david@kernel.org> writes:

> On 2/11/26 01:58, Ackerley Tng wrote:
>> Ackerley Tng <ackerleytng@google.com> writes:
>>
>>> "David Hildenbrand (Arm)" <david@kernel.org> writes:
>>>
>>>>
>>>> Yes. If there is no guest_memfd we wouldn't need it.
>>>>
>>>
>>> Seems like on 5.15.199 there's a hugepage_vma_check(), which will return
>>> false since secretmem has vma->vm_ops defined [1], so secretmem VMAs are
>>> skipped.
>>>
>>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/mm/khugepaged.c?h=v5.15.199#n469
>>>
>>
>> On 6.1.162, secretmem VMAs are skipped since secretmem VMAs are not
>> anonymous [2].
>>
>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/mm/huge_memory.c?h=v6.1.162#n135
>>
>> Same for 6.6.123 [3].
>>
>> [3] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/mm/huge_memory.c?h=v6.6.123#n125
>>
>> It breaks in 6.12.69 [4].
>
>
> Do you have a reproducer? If so, which behavior does it trigger?
>
> I would assume that we would suddenly have secretmem pages (THP) that
> have a directmap. Or some page copy would crash the kernel.
>

Is there a good way to verify from userspace that the directmap hasn't
been restored? Should I use CONFIG_PTDUMP_DEBUGFS?

> --
> Cheers,
>
> David