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

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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




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