[PATCH v3 04/14] mm/rmap: Break COW PTE in rmap walking

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

Some of the features (unmap, migrate, device exclusive, mkclean, etc)
might modify the pte entry via rmap. Add a new page vma mapped walk
flag, PVMW_BREAK_COW_PTE, to indicate the rmap walking to break COW PTE.

Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
---
 include/linux/rmap.h |  2 ++
 mm/migrate.c         |  3 ++-
 mm/page_vma_mapped.c |  2 ++
 mm/rmap.c            | 12 +++++++-----
 mm/vmscan.c          |  7 ++++++-
 5 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index bd3504d11b155..d0f07e5519736 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -368,6 +368,8 @@ int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
 #define PVMW_SYNC		(1 << 0)
 /* Look for migration entries rather than present PTEs */
 #define PVMW_MIGRATION		(1 << 1)
+/* Break COW-ed PTE during walking */
+#define PVMW_BREAK_COW_PTE	(1 << 2)
 
 struct page_vma_mapped_walk {
 	unsigned long pfn;
diff --git a/mm/migrate.c b/mm/migrate.c
index dff333593a8ae..a4be7e04c9b09 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -174,7 +174,8 @@ void putback_movable_pages(struct list_head *l)
 static bool remove_migration_pte(struct folio *folio,
 		struct vm_area_struct *vma, unsigned long addr, void *old)
 {
-	DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
+	DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr,
+			      PVMW_SYNC | PVMW_MIGRATION | PVMW_BREAK_COW_PTE);
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		rmap_t rmap_flags = RMAP_NONE;
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 93e13fc17d3cb..5dfc9236dc505 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -251,6 +251,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 			step_forward(pvmw, PMD_SIZE);
 			continue;
 		}
+		if (pvmw->flags & PVMW_BREAK_COW_PTE)
+			break_cow_pte(vma, pvmw->pmd, pvmw->address);
 		if (!map_pte(pvmw))
 			goto next_pte;
 this_pte:
diff --git a/mm/rmap.c b/mm/rmap.c
index 2ec925e5fa6a9..b1b7dcbd498be 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -807,7 +807,8 @@ static bool folio_referenced_one(struct folio *folio,
 		struct vm_area_struct *vma, unsigned long address, void *arg)
 {
 	struct folio_referenced_arg *pra = arg;
-	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
+	/* it will clear the entry, so we should break COW PTE. */
+	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE);
 	int referenced = 0;
 
 	while (page_vma_mapped_walk(&pvmw)) {
@@ -1012,7 +1013,8 @@ static int page_vma_mkclean_one(struct page_vma_mapped_walk *pvmw)
 static bool page_mkclean_one(struct folio *folio, struct vm_area_struct *vma,
 			     unsigned long address, void *arg)
 {
-	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_SYNC);
+	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address,
+			      PVMW_SYNC | PVMW_BREAK_COW_PTE);
 	int *cleaned = arg;
 
 	*cleaned += page_vma_mkclean_one(&pvmw);
@@ -1471,7 +1473,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 		     unsigned long address, void *arg)
 {
 	struct mm_struct *mm = vma->vm_mm;
-	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
+	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE);
 	pte_t pteval;
 	struct page *subpage;
 	bool anon_exclusive, ret = true;
@@ -1842,7 +1844,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 		     unsigned long address, void *arg)
 {
 	struct mm_struct *mm = vma->vm_mm;
-	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
+	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE);
 	pte_t pteval;
 	struct page *subpage;
 	bool anon_exclusive, ret = true;
@@ -2195,7 +2197,7 @@ static bool page_make_device_exclusive_one(struct folio *folio,
 		struct vm_area_struct *vma, unsigned long address, void *priv)
 {
 	struct mm_struct *mm = vma->vm_mm;
-	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
+	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE);
 	struct make_exclusive_args *args = priv;
 	pte_t pteval;
 	struct page *subpage;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 026199c047e0e..980d2056adfd1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1781,6 +1781,10 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 			}
 		}
 
+		/*
+		 * Break COW PTE since checking the reference
+		 * of folio might modify the PTE.
+		 */
 		if (!ignore_references)
 			references = folio_check_references(folio, sc);
 
@@ -1864,7 +1868,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 
 		/*
 		 * The folio is mapped into the page tables of one or more
-		 * processes. Try to unmap it here.
+		 * processes. Try to unmap it here. Also, since it will write
+		 * to the page tables, break COW PTE if they are.
 		 */
 		if (folio_mapped(folio)) {
 			enum ttu_flags flags = TTU_BATCH_FLUSH;
-- 
2.37.3