linux-kernel - [PATCH RFC 17/30] shmem/userfaultfd: Pass over uffd-wp special swap pte when fork()

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Date:   Fri, 15 Jan 2021 12:08:54 -0500
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, peterx@...hat.com,
        Jerome Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: [PATCH RFC 17/30] shmem/userfaultfd: Pass over uffd-wp special swap pte when fork()

It should be handled similarly like other uffd-wp wr-protected ptes: we should
pass it over when the dst_vma has VM_UFFD_WP armed, otherwise drop it.

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/memory.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index f87b5a8a098e..59d56f57ba2c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -703,8 +703,21 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 	unsigned long vm_flags = dst_vma->vm_flags;
 	pte_t pte = *src_pte;
 	struct page *page;
-	swp_entry_t entry = pte_to_swp_entry(pte);
+	swp_entry_t entry;
+
+	if (unlikely(is_swap_special_pte(pte))) {
+		/*
+		 * uffd-wp special swap pte is the only possibility for now.
+		 * If dst vma is registered with uffd-wp, copy it over.
+		 * Otherwise, ignore this pte as if it's a none pte would work.
+		 */
+		WARN_ON_ONCE(!pte_swp_uffd_wp_special(pte));
+		if (userfaultfd_wp(dst_vma))
+			set_pte_at(dst_mm, addr, dst_pte, pte);
+		return 0;
+	}
 
+	entry = pte_to_swp_entry(pte);
 	if (likely(!non_swap_entry(entry))) {
 		if (swap_duplicate(entry) < 0)
 			return entry.val;
-- 
2.26.2