mm/hugetlb: make follow_hugetlb_page() safe to pmd unshare

Since follow_hugetlb_page() walks the pgtable, it needs the vma lock to
make sure the pgtable page will not be freed concurrently.

Link: https://lkml.kernel.org/r/20221216155223.2043727-1-peterx@redhat.comSigned-off-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb.c

@@ -6454,6 +6454,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			break;
 		}
 
+		hugetlb_vma_lock_read(vma);
 		/*
 		 * Some archs (sparc64, sh*) have multiple pte_ts to
 		 * each hugepage.  We have to make sure we get the
@@ -6478,6 +6479,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		    !hugetlbfs_pagecache_present(h, vma, vaddr)) {
 			if (pte)
 				spin_unlock(ptl);
+			hugetlb_vma_unlock_read(vma);
 			remainder = 0;
 			break;
 		}
@@ -6499,6 +6501,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 			if (pte)
 				spin_unlock(ptl);
+			hugetlb_vma_unlock_read(vma);
+
 			if (flags & FOLL_WRITE)
 				fault_flags |= FAULT_FLAG_WRITE;
 			else if (unshare)
@@ -6561,6 +6565,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			remainder -= pages_per_huge_page(h);
 			i += pages_per_huge_page(h);
 			spin_unlock(ptl);
+			hugetlb_vma_unlock_read(vma);
 			continue;
 		}
 
@@ -6590,6 +6595,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			if (WARN_ON_ONCE(!try_grab_folio(pages[i], refs,
 							 flags))) {
 				spin_unlock(ptl);
+				hugetlb_vma_unlock_read(vma);
 				remainder = 0;
 				err = -ENOMEM;
 				break;
@@ -6601,6 +6607,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		i += refs;
 
 		spin_unlock(ptl);
+		hugetlb_vma_unlock_read(vma);
 	}
 	*nr_pages = remainder;
 	/*