linux-kernel - [PATCH RFC 07/10] mm/hugetlb: Make hugetlb_follow_page

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Date:   Sun, 30 Oct 2022 17:29:26 -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 07/10] mm/hugetlb: Make hugetlb_follow_page_mask() 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>
---
 mm/hugetlb.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9869c12e6460..85214095fb85 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6229,10 +6229,12 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
 	if (WARN_ON_ONCE(flags & FOLL_PIN))
 		return NULL;
 
+	/* For huge_pte_offset() */
+	rcu_read_lock();
 retry:
 	pte = huge_pte_offset(mm, haddr, huge_page_size(h));
 	if (!pte)
-		return NULL;
+		goto out_rcu;
 
 	ptl = huge_pte_lock(h, mm, pte);
 	entry = huge_ptep_get(pte);
@@ -6266,6 +6268,8 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
 	}
 out:
 	spin_unlock(ptl);
+out_rcu:
+	rcu_read_unlock();
 	return page;
 }
 
-- 
2.37.3