Re: [PATCH 1/6] mm/hugetlb: Fix uffd-wp during fork()

Mika Penttilä
2023-04-14T14:24:25Z

On 14.4.2023 17.09, Peter Xu wrote:
> On Fri, Apr 14, 2023 at 12:45:29PM +0300, Mika Penttilä wrote:
>>> } else if (unlikely(is_hugetlb_entry_migration(entry))) {
>>> swp_entry_t swp_entry = pte_to_swp_entry(entry);
>>> - bool uffd_wp = huge_pte_uffd_wp(entry);
>
> [1]
>
>>> if (!is_readable_migration_entry(swp_entry) && cow) {
>>> /*
>>> @@ -5049,11 +5050,12 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>>> swp_entry = make_readable_migration_entry(
>>> swp_offset(swp_entry));
>>> entry = swp_entry_to_pte(swp_entry);
>
> [2]
>
>>> - if (userfaultfd_wp(src_vma) && uffd_wp)
>>> - entry = huge_pte_mkuffd_wp(entry);
>>> + if (userfaultfd_wp(src_vma) &&
>>> + pte_swp_uffd_wp(entry))
>>> + entry = pte_swp_mkuffd_wp(entry);
>>
>>
>> This looks interesting with pte_swp_uffd_wp and pte_swp_mkuffd_wp ?
>
> Could you explain what do you mean?
>

Yes like you noticed also you called pte_swp_mkuffd_wp(entry) iff
pte_swp_uffd_wp(entry) which is of course a nop.

But the fixup not dropping the temp var should work.

> I think these helpers are the right ones to use, as afaict hugetlb
> migration should follow the same pte format with !hugetlb. However, I
> noticed I did it wrong when dropping the temp var - when at [1], "entry"
> still points to the src entry, but at [2] it's already pointing to the
> newly created one.. so I think I can't drop the var, a fixup should like:
>
> ===8<===
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 083aae35bff8..cd3a9d8f4b70 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5041,6 +5041,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> set_huge_pte_at(dst, addr, dst_pte, entry);
> } else if (unlikely(is_hugetlb_entry_migration(entry))) {
> swp_entry_t swp_entry = pte_to_swp_entry(entry);
> + bool uffd_wp = pte_swp_uffd_wp(entry);
>
> if (!is_readable_migration_entry(swp_entry) && cow) {
> /*
> @@ -5050,8 +5051,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> swp_entry = make_readable_migration_entry(
> swp_offset(swp_entry));
> entry = swp_entry_to_pte(swp_entry);
> - if (userfaultfd_wp(src_vma) &&
> - pte_swp_uffd_wp(entry))
> + if (userfaultfd_wp(src_vma) && uffd_wp)
> entry = pte_swp_mkuffd_wp(entry);
> set_huge_pte_at(src, addr, src_pte, entry);
> ===8<===
>
> Besides, did I miss something else?
>
> Thanks,
>

--Mika