[PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping

[Baolin Wang]

On 5/10/2022 12:41 AM, Peter Xu wrote:
> On Fri, May 06, 2022 at 12:07:13PM -0700, Mike Kravetz wrote:
>> On 5/3/22 03:03, Gerald Schaefer wrote:
>>> On Tue, 3 May 2022 10:19:46 +0800
>>> Baolin Wang <baolin.wang at linux.alibaba.com> wrote:
>>>> On 5/2/2022 10:02 PM, Gerald Schaefer wrote:
> 
> [...]
> 
>>>> Please see previous code, we'll use the original pte value to check if
>>>> it is uffd-wp armed, and if need to mark it dirty though the hugetlbfs
>>>> is set noop_dirty_folio().
>>>>
>>>> pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval);
>>>
>>> Uh, ok, that wouldn't work on s390, but we also don't have
>>> CONFIG_PTE_MARKER_UFFD_WP / HAVE_ARCH_USERFAULTFD_WP set, so
>>> I guess we will be fine (for now).
>>>
>>> Still, I find it a bit unsettling that pte_install_uffd_wp_if_needed()
>>> would work on a potential hugetlb *pte, directly de-referencing it
>>> instead of using huge_ptep_get().
>>>
>>> The !pte_none(*pte) check at the beginning would be broken in the
>>> hugetlb case for s390 (not sure about other archs, but I think s390
>>> might be the only exception strictly requiring huge_ptep_get()
>>> for de-referencing hugetlb *pte pointers).
> 
> We could have used is_vm_hugetlb_page(vma) within the helper so as to
> properly use either generic pte or hugetlb version of pte fetching.  We may
> want to conditionally do set_[huge_]pte_at() too at the end.
> 
> I could prepare a patch for that even if it's not really anything urgently
> needed. I assume that won't need to block this patchset since we need the
> pteval for pte_dirty() check anyway and uffd-wp definitely needs it too.

OK. Thanks Peter.