[PATCH v2 6/6] mm/hugetlb: make detecting shared pte more reliable

[Miaohe Lin]

If the pagetables are shared, we shouldn't copy or take references. Since
src could have unshared and dst shares with another vma, huge_pte_none()
is thus used to determine whether dst_pte is shared. But this check isn't
reliable. A shared pte could have pte none in pagetable in fact. The page
count of ptep page should be checked here in order to reliably determine
whether pte is shared.

[Thanks Lukas for cleanup unused local variable dst_entry.]

Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Lukas Bulwahn <lukas.bulwahn@xxxxxxxxx>
---
 mm/hugetlb.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2dfd10599f98..8aa62765a055 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4763,7 +4763,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 			    struct vm_area_struct *dst_vma,
 			    struct vm_area_struct *src_vma)
 {
-	pte_t *src_pte, *dst_pte, entry, dst_entry;
+	pte_t *src_pte, *dst_pte, entry;
 	struct page *ptepage;
 	unsigned long addr;
 	bool cow = is_cow_mapping(src_vma->vm_flags);
@@ -4808,15 +4808,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 
 		/*
 		 * If the pagetables are shared don't copy or take references.
-		 * dst_pte == src_pte is the common case of src/dest sharing.
 		 *
+		 * dst_pte == src_pte is the common case of src/dest sharing.
 		 * However, src could have 'unshared' and dst shares with
-		 * another vma.  If dst_pte !none, this implies sharing.
-		 * Check here before taking page table lock, and once again
-		 * after taking the lock below.
+		 * another vma. So page_count of ptep page is checked instead
+		 * to reliably determine whether pte is shared.
 		 */
-		dst_entry = huge_ptep_get(dst_pte);
-		if ((dst_pte == src_pte) || !huge_pte_none(dst_entry)) {
+		if (page_count(virt_to_page(dst_pte)) > 1) {
 			addr |= last_addr_mask;
 			continue;
 		}
@@ -4825,13 +4823,10 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 		src_ptl = huge_pte_lockptr(h, src, src_pte);
 		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
 		entry = huge_ptep_get(src_pte);
-		dst_entry = huge_ptep_get(dst_pte);
 again:
-		if (huge_pte_none(entry) || !huge_pte_none(dst_entry)) {
+		if (huge_pte_none(entry)) {
 			/*
-			 * Skip if src entry none.  Also, skip in the
-			 * unlikely case dst entry !none as this implies
-			 * sharing with another vma.
+			 * Skip if src entry none.
 			 */
 			;
 		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry))) {
@@ -4910,7 +4905,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 					restore_reserve_on_error(h, dst_vma, addr,
 								new);
 					put_page(new);
-					/* dst_entry won't change as in child */
+					/* huge_ptep of dst_pte won't change as in child */
 					goto again;
 				}
 				hugetlb_install_page(dst_vma, dst_pte, addr, new);
-- 
2.23.0