1 files changed, 63 insertions, 9 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 6f9c62c746be..d039863e014b 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -204,13 +204,57 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
 	return true;
 }
 
+static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
+					  struct folio *folio,
+					  unsigned long idx)
+{
+	struct page *page = folio_page(folio, idx);
+	bool contains_data;
+	pte_t newpte;
+	void *addr;
+
+	VM_BUG_ON_PAGE(PageCompound(page), page);
+	VM_BUG_ON_PAGE(!PageAnon(page), page);
+	VM_BUG_ON_PAGE(!PageLocked(page), page);
+	VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);
+
+	if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) ||
+	    mm_forbids_zeropage(pvmw->vma->vm_mm))
+		return false;
+
+	/*
+	 * The pmd entry mapping the old thp was flushed and the pte mapping
+	 * this subpage has been non present. If the subpage is only zero-filled
+	 * then map it to the shared zeropage.
+	 */
+	addr = kmap_local_page(page);
+	contains_data = memchr_inv(addr, 0, PAGE_SIZE);
+	kunmap_local(addr);
+
+	if (contains_data)
+		return false;
+
+	newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
+					pvmw->vma->vm_page_prot));
+	set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
+
+	dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
+	return true;
+}
+
+struct rmap_walk_arg {
+	struct folio *folio;
+	bool map_unused_to_zeropage;
+};
+
 /*
  * Restore a potential migration pte to a working pte entry
  */
 static bool remove_migration_pte(struct folio *folio,
-		struct vm_area_struct *vma, unsigned long addr, void *old)
+		struct vm_area_struct *vma, unsigned long addr, void *arg)
 {
-	DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
+	struct rmap_walk_arg *rmap_walk_arg = arg;
+	DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		rmap_t rmap_flags = RMAP_NONE;
@@ -234,6 +278,9 @@ static bool remove_migration_pte(struct folio *folio,
 			continue;
 		}
 #endif
+		if (rmap_walk_arg->map_unused_to_zeropage &&
+		    try_to_map_unused_to_zeropage(&pvmw, folio, idx))
+			continue;
 
 		folio_get(folio);
 		pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
@@ -312,14 +359,21 @@ static bool remove_migration_pte(struct folio *folio,
  * Get rid of all migration entries and replace them by
  * references to the indicated page.
  */
-void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked)
+void remove_migration_ptes(struct folio *src, struct folio *dst, int flags)
 {
+	struct rmap_walk_arg rmap_walk_arg = {
+		.folio = src,
+		.map_unused_to_zeropage = flags & RMP_USE_SHARED_ZEROPAGE,
+	};
+
 	struct rmap_walk_control rwc = {
 		.rmap_one = remove_migration_pte,
-		.arg = src,
+		.arg = &rmap_walk_arg,
 	};
 
-	if (locked)
+	VM_BUG_ON_FOLIO((flags & RMP_USE_SHARED_ZEROPAGE) && (src != dst), src);
+
+	if (flags & RMP_LOCKED)
 		rmap_walk_locked(dst, &rwc);
 	else
 		rmap_walk(dst, &rwc);
@@ -934,7 +988,7 @@ static int writeout(struct address_space *mapping, struct folio *folio)
 	 * At this point we know that the migration attempt cannot
 	 * be successful.
 	 */
-	remove_migration_ptes(folio, folio, false);
+	remove_migration_ptes(folio, folio, 0);
 
 	rc = mapping->a_ops->writepage(&folio->page, &wbc);
 
@@ -1098,7 +1152,7 @@ static void migrate_folio_undo_src(struct folio *src,
 				   struct list_head *ret)
 {
 	if (page_was_mapped)
-		remove_migration_ptes(src, src, false);
+		remove_migration_ptes(src, src, 0);
 	/* Drop an anon_vma reference if we took one */
 	if (anon_vma)
 		put_anon_vma(anon_vma);
@@ -1336,7 +1390,7 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 		lru_add_drain();
 
 	if (old_page_state & PAGE_WAS_MAPPED)
-		remove_migration_ptes(src, dst, false);
+		remove_migration_ptes(src, dst, 0);
 
 out_unlock_both:
 	folio_unlock(dst);
@@ -1474,7 +1528,7 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
 
 	if (page_was_mapped)
 		remove_migration_ptes(src,
-			rc == MIGRATEPAGE_SUCCESS ? dst : src, false);
+			rc == MIGRATEPAGE_SUCCESS ? dst : src, 0);
 
 unlock_put_anon:
 	folio_unlock(dst);