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

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Date:   Fri, 14 Apr 2023 10:09:59 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Mika Penttilä <mika.penttila@...sol.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-stable <stable@...r.kernel.org>
Subject: Re: [PATCH 1/6] mm/hugetlb: Fix uffd-wp during fork()

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?

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,

-- 
Peter Xu