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From:   Peter Xu <peterx@redhat.com>
To:     linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        James Houghton <jthoughton@google.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        David Hildenbrand <david@redhat.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Mike Kravetz <mike.kravetz@oracle.com>, peterx@redhat.com,
        Rik van Riel <riel@surriel.com>
Subject: [PATCH RFC 06/10] mm/hugetlb: Make page_vma_mapped_walk() RCU-safe
Date:   Sun, 30 Oct 2022 17:29:25 -0400
Message-Id: <20221030212929.335473-7-peterx@redhat.com>
In-Reply-To: <20221030212929.335473-1-peterx@redhat.com>
References: <20221030212929.335473-1-peterx@redhat.com>
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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@redhat.com>
---
 include/linux/rmap.h | 3 +++
 mm/page_vma_mapped.c | 7 ++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index bd3504d11b15..d2c5e69a56f2 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -408,6 +408,9 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
 		pte_unmap(pvmw->pte);
 	if (pvmw->ptl)
 		spin_unlock(pvmw->ptl);
+	/* Hugetlb uses RCU lock for safe access of huge_pte_offset() */
+	if (is_vm_hugetlb_page(pvmw->vma))
+		rcu_read_unlock();
 }
 
 bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw);
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 93e13fc17d3c..513210a59d7b 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -169,10 +169,15 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 		if (pvmw->pte)
 			return not_found(pvmw);
 
+		/* For huge_pte_offset() */
+		rcu_read_lock();
+
 		/* when pud is not present, pte will be NULL */
 		pvmw->pte = huge_pte_offset(mm, pvmw->address, size);
-		if (!pvmw->pte)
+		if (!pvmw->pte) {
+			rcu_read_unlock();
 			return false;
+		}
 
 		pvmw->ptl = huge_pte_lock(hstate, mm, pvmw->pte);
 		if (!check_pte(pvmw))
-- 
2.37.3