[RFC PATCH v2 3/9] mm, pgtable: Add ownership to PTE table

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

Introduce the ownership to the PTE table. It uses the address of PMD
index to track the ownership to identify which process can update
their page table state from the COWed PTE table.

Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
---
 include/linux/mm.h       |  1 +
 include/linux/mm_types.h |  5 ++++-
 include/linux/pgtable.h  | 10 ++++++++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 21f8b27bd9fd3..965523dcca3b8 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2289,6 +2289,7 @@ static inline bool pgtable_pte_page_ctor(struct page *page)
 		return false;
 	__SetPageTable(page);
 	inc_lruvec_page_state(page, NR_PAGETABLE);
+	page->cow_pte_owner = NULL;
 	return true;
 }
 
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index cf97f3884fda2..42798b59cec4e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -152,7 +152,10 @@ struct page {
 			struct list_head deferred_list;
 		};
 		struct {	/* Page table pages */
-			unsigned long _pt_pad_1;	/* compound_head */
+			union {
+				unsigned long _pt_pad_1; /* compound_head */
+				pmd_t *cow_pte_owner; /* cow pte: pmd */
+			};
 			pgtable_t pmd_huge_pte; /* protected by page->ptl */
 			unsigned long _pt_pad_2;	/* mapping */
 			union {
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index d03d01aefe989..9dca787a3f4dd 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -615,6 +615,16 @@ static inline int pte_unused(pte_t pte)
 }
 #endif
 
+static inline void set_cow_pte_owner(pmd_t *pmd, pmd_t *owner)
+{
+	smp_store_release(&pmd_page(*pmd)->cow_pte_owner, owner);
+}
+
+static inline bool cow_pte_owner_is_same(pmd_t *pmd, pmd_t *owner)
+{
+	return smp_load_acquire(&pmd_page(*pmd)->cow_pte_owner) == owner;
+}
+
 #ifndef pte_access_permitted
 #define pte_access_permitted(pte, write) \
 	(pte_present(pte) && (!(write) || pte_write(pte)))
-- 
2.37.3