linux-kernel - [PATCH RFC 18/30] hugetlb/userfaultfd: Hook page faults for uffd write protection

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Date:   Fri, 15 Jan 2021 12:08:55 -0500
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, peterx@...hat.com,
        Jerome Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: [PATCH RFC 18/30] hugetlb/userfaultfd: Hook page faults for uffd write protection

Hook up hugetlbfs_fault() with the capability to handle userfaultfd-wp faults.

We do this slightly earlier than hugetlb_cow() so that we can avoid taking some
extra locks that we definitely don't need.

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/hugetlb.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d029d938d26d..dcbbba53bd10 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4544,6 +4544,25 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	if (unlikely(!pte_same(entry, huge_ptep_get(ptep))))
 		goto out_ptl;
 
+	/* Handle userfault-wp first, before trying to lock more pages */
+	if (userfaultfd_pte_wp(vma, huge_ptep_get(ptep)) &&
+	    (flags & FAULT_FLAG_WRITE) && !huge_pte_write(entry)) {
+		struct vm_fault vmf = {
+			.vma = vma,
+			.address = haddr,
+			.flags = flags,
+		};
+
+		spin_unlock(ptl);
+		if (pagecache_page) {
+			unlock_page(pagecache_page);
+			put_page(pagecache_page);
+		}
+		mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+		i_mmap_unlock_read(mapping);
+		return handle_userfault(&vmf, VM_UFFD_WP);
+	}
+
 	/*
 	 * hugetlb_cow() requires page locks of pte_page(entry) and
 	 * pagecache_page, so here we need take the former one
-- 
2.26.2