linux-kernel - [PATCH RFC 04/10] mm/hugetlb: Make userfaultfd_huge_must

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Date:   Sun, 30 Oct 2022 17:29:23 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        James Houghton <jthoughton@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, peterx@...hat.com,
        Rik van Riel <riel@...riel.com>
Subject: [PATCH RFC 04/10] mm/hugetlb: Make userfaultfd_huge_must_wait() RCU-safe

RCU makes sure the pte_t* won't go away from under us.  Please refer to the
comment above huge_pte_offset() for more information.

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 fs/userfaultfd.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 07c81ab3fd4d..4e813e68e4f8 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -243,6 +243,9 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
 
 	mmap_assert_locked(mm);
 
+	/* For huge_pte_offset() */
+	rcu_read_lock();
+
 	ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
 
 	if (!ptep)
@@ -261,6 +264,7 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
 	if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
 		ret = true;
 out:
+	rcu_read_unlock();
 	return ret;
 }
 #else
-- 
2.37.3