[PATCH RFC 04/10] mm/hugetlb: Make userfaultfd_huge_must_wait() RCU-safe

[Peter Xu]

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@xxxxxxxxxx>
---
 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