[RFC PATCH v2 6/9] mm, pgtable: Add COW_PTE_OWNER_EXCLUSIVE flag

[Chih-En Lin <shiyn.lin@gmail.com>]

For present COW logic (physical page), in some situations (e.g., pinned
page), we cannot share those pages. To make the COW PTE consistent with
current logic, introduce the COW_PTE_OWNER_EXCLUSIVE flag to avoid doing
COW to the PTE table during fork(). The following is a list of the
exclusive flag used.

- GUP pinnig with COW physical page will get in trouble. Currently, it
  will not do COW when GUP works. Follow the rule here.

Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
---
 include/linux/pgtable.h | 18 ++++++++++++++++++
 mm/gup.c                | 13 +++++++++++--
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 8b497d7d800ed..9b08a3361d490 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -656,6 +656,24 @@ static inline int cow_pte_count(pmd_t *pmd)
 	return page_count(pmd_page(*pmd));
 }
 
+/* Keep the first bit clear. See more detail in the comments of struct page. */
+#define COW_PTE_OWNER_EXCLUSIVE ((pmd_t *) 0x02UL)
+
+static inline void pmd_cow_pte_mkexclusive(pmd_t *pmd)
+{
+	set_cow_pte_owner(pmd, COW_PTE_OWNER_EXCLUSIVE);
+}
+
+static inline bool pmd_cow_pte_exclusive(pmd_t *pmd)
+{
+	return cow_pte_owner_is_same(pmd, COW_PTE_OWNER_EXCLUSIVE);
+}
+
+static inline void pmd_cow_pte_clear_mkexclusive(pmd_t *pmd)
+{
+	set_cow_pte_owner(pmd, NULL);
+}
+
 #ifndef pte_access_permitted
 #define pte_access_permitted(pte, write) \
 	(pte_present(pte) && (!(write) || pte_write(pte)))
diff --git a/mm/gup.c b/mm/gup.c
index 5abdaf4874605..4949c8d42a400 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -634,6 +634,11 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 		mark_page_accessed(page);
 	}
 out:
+	/*
+	 * We don't share the PTE when any other pinned page exists. And
+	 * let the exclusive flag stick around until the table is freed.
+	 */
+	pmd_cow_pte_mkexclusive(pmd);
 	pte_unmap_unlock(ptep, ptl);
 	return page;
 no_page:
@@ -932,6 +937,7 @@ static int get_gate_page(struct mm_struct *mm, unsigned long address,
 	pte = pte_offset_map(pmd, address);
 	if (pte_none(*pte))
 		goto unmap;
+	pmd_cow_pte_clear_mkexclusive(pmd);
 	*vma = get_gate_vma(mm);
 	if (!page)
 		goto out;
@@ -2764,8 +2770,11 @@ static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned lo
 			if (!gup_huge_pd(__hugepd(pmd_val(pmd)), addr,
 					 PMD_SHIFT, next, flags, pages, nr))
 				return 0;
-		} else if (!gup_pte_range(pmd, addr, next, flags, pages, nr))
-			return 0;
+		} else {
+			if (!gup_pte_range(pmd, addr, next, flags, pages, nr))
+				return 0;
+			pmd_cow_pte_mkexclusive(&pmd);
+		}
 	} while (pmdp++, addr = next, addr != end);
 
 	return 1;
-- 
2.37.3