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

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