[PATCH RFC 09/10] mm/hugetlb: Make hugetlb_fault() RCU-safe

From: Peter Xu <peterx@redhat.com>
To: linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc: James Houghton <jthoughton@google.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	David Hildenbrand <david@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Rik van Riel <riel@surriel.com>,
	peterx@redhat.com, Andrew Morton <akpm@linux-foundation.org>,
	Muchun Song <songmuchun@bytedance.com>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Nadav Amit <nadav.amit@gmail.com>
Subject: [PATCH RFC 09/10] mm/hugetlb: Make hugetlb_fault() RCU-safe
Date: Sun, 30 Oct 2022 17:30:43 -0400	[thread overview]
Message-ID: <20221030213043.335669-1-peterx@redhat.com> (raw)
In-Reply-To: <20221030212929.335473-1-peterx@redhat.com>

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>
---
 mm/hugetlb.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5dc87e4e6780..6d336d286394 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5822,6 +5822,8 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	int need_wait_lock = 0;
 	unsigned long haddr = address & huge_page_mask(h);
 
+	/* For huge_pte_offset() */
+	rcu_read_lock();
 	ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
 	if (ptep) {
 		/*
@@ -5830,13 +5832,15 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 		 * not actually modifying content here.
 		 */
 		entry = huge_ptep_get(ptep);
+		rcu_read_unlock();
 		if (unlikely(is_hugetlb_entry_migration(entry))) {
 			migration_entry_wait_huge(vma, ptep);
 			return 0;
 		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
 			return VM_FAULT_HWPOISON_LARGE |
 				VM_FAULT_SET_HINDEX(hstate_index(h));
-	}
+	} else
+		rcu_read_unlock();
 
 	/*
 	 * Serialize hugepage allocation and instantiation, so that we don't
-- 
2.37.3

