[PATCH 5/7] mm: thp: split huge page to any lower order pages.

[Zi Yan]

From: Zi Yan <ziy@xxxxxxxxxx>

To split a THP to any lower order pages, we need to reform THPs on
subpages at given order and add page refcount based on the new page
order. Also we need to reinitialize page_deferred_list after removing
the page from the split_queue, otherwise a subsequent split will see
list corruption when checking the page_deferred_list again.

It has many uses, like minimizing the number of pages after
truncating a pagecache THP. For anonymous THPs, we can only split them
to order-0 like before until we add support for any size anonymous THPs.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 include/linux/huge_mm.h |   8 +++
 mm/huge_memory.c        | 119 +++++++++++++++++++++++++++++-----------
 mm/swap.c               |   1 -
 3 files changed, 96 insertions(+), 32 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 7723deda33e2..0c856f805617 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -182,6 +182,8 @@ bool is_transparent_hugepage(struct page *page);
 
 bool can_split_huge_page(struct page *page, int *pextra_pins);
 int split_huge_page_to_list(struct page *page, struct list_head *list);
+int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
+		unsigned int new_order);
 static inline int split_huge_page(struct page *page)
 {
 	return split_huge_page_to_list(page, NULL);
@@ -385,6 +387,12 @@ split_huge_page_to_list(struct page *page, struct list_head *list)
 {
 	return 0;
 }
+static inline int
+split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
+		unsigned int new_order)
+{
+	return 0;
+}
 static inline int split_huge_page(struct page *page)
 {
 	return 0;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index aae7405a0989..cc70f70862d8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2325,12 +2325,14 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
 
 static void unmap_page(struct page *page)
 {
-	enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
-		TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
+	enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_RMAP_LOCKED;
 	bool unmap_success;
 
 	VM_BUG_ON_PAGE(!PageHead(page), page);
 
+	if (thp_order(page) >= HPAGE_PMD_ORDER)
+		ttu_flags |= TTU_SPLIT_HUGE_PMD;
+
 	if (PageAnon(page))
 		ttu_flags |= TTU_SPLIT_FREEZE;
 
@@ -2338,21 +2340,23 @@ static void unmap_page(struct page *page)
 	VM_BUG_ON_PAGE(!unmap_success, page);
 }
 
-static void remap_page(struct page *page, unsigned int nr)
+static void remap_page(struct page *page, unsigned int nr, unsigned int new_nr)
 {
-	int i;
-	if (PageTransHuge(page)) {
+	unsigned int i;
+
+	if (thp_nr_pages(page) == nr) {
 		remove_migration_ptes(page, page, true);
 	} else {
-		for (i = 0; i < nr; i++)
+		for (i = 0; i < nr; i += new_nr)
 			remove_migration_ptes(page + i, page + i, true);
 	}
 }
 
 static void __split_huge_page_tail(struct page *head, int tail,
-		struct lruvec *lruvec, struct list_head *list)
+		struct lruvec *lruvec, struct list_head *list, unsigned int new_order)
 {
 	struct page *page_tail = head + tail;
+	unsigned long compound_head_flag = new_order ? (1L << PG_head) : 0;
 
 	VM_BUG_ON_PAGE(atomic_read(&page_tail->_mapcount) != -1, page_tail);
 
@@ -2376,6 +2380,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
 #ifdef CONFIG_64BIT
 			 (1L << PG_arch_2) |
 #endif
+			 compound_head_flag |
 			 (1L << PG_dirty)));
 
 	/* ->mapping in first tail page is compound_mapcount */
@@ -2384,7 +2389,10 @@ static void __split_huge_page_tail(struct page *head, int tail,
 	page_tail->mapping = head->mapping;
 	page_tail->index = head->index + tail;
 
-	/* Page flags must be visible before we make the page non-compound. */
+	/*
+	 * Page flags must be visible before we make the page non-compound or
+	 * a compound page in new_order.
+	 */
 	smp_wmb();
 
 	/*
@@ -2394,10 +2402,15 @@ static void __split_huge_page_tail(struct page *head, int tail,
 	 * which needs correct compound_head().
 	 */
 	clear_compound_head(page_tail);
+	if (new_order) {
+		prep_compound_page(page_tail, new_order);
+		thp_prep(page_tail);
+	}
 
 	/* Finally unfreeze refcount. Additional reference from page cache. */
-	page_ref_unfreeze(page_tail, 1 + (!PageAnon(head) ||
-					  PageSwapCache(head)));
+	page_ref_unfreeze(page_tail, 1 + ((!PageAnon(head) ||
+					   PageSwapCache(head)) ?
+						thp_nr_pages(page_tail) : 0));
 
 	if (page_is_young(head))
 		set_page_young(page_tail);
@@ -2415,7 +2428,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
 }
 
 static void __split_huge_page(struct page *page, struct list_head *list,
-		pgoff_t end, unsigned long flags)
+		pgoff_t end, unsigned long flags, unsigned int new_order)
 {
 	struct page *head = compound_head(page);
 	pg_data_t *pgdat = page_pgdat(head);
@@ -2424,12 +2437,13 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 	unsigned long offset = 0;
 	unsigned int order = thp_order(head);
 	unsigned int nr = thp_nr_pages(head);
+	unsigned int new_nr = 1 << new_order;
 	int i;
 
 	lruvec = mem_cgroup_page_lruvec(head, pgdat);
 
 	/* complete memcg works before add pages to LRU */
-	mem_cgroup_split_huge_fixup(head, 0);
+	mem_cgroup_split_huge_fixup(head, new_order);
 
 	if (PageAnon(head) && PageSwapCache(head)) {
 		swp_entry_t entry = { .val = page_private(head) };
@@ -2439,46 +2453,54 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 		xa_lock(&swap_cache->i_pages);
 	}
 
-	for (i = nr - 1; i >= 1; i--) {
-		__split_huge_page_tail(head, i, lruvec, list);
+	for (i = nr - new_nr; i >= new_nr; i -= new_nr) {
+		__split_huge_page_tail(head, i, lruvec, list, new_order);
 		/* Some pages can be beyond i_size: drop them from page cache */
 		if (head[i].index >= end) {
 			ClearPageDirty(head + i);
 			__delete_from_page_cache(head + i, NULL);
 			if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
-				shmem_uncharge(head->mapping->host, 1);
+				shmem_uncharge(head->mapping->host, new_nr);
 			put_page(head + i);
 		} else if (!PageAnon(page)) {
 			__xa_store(&head->mapping->i_pages, head[i].index,
 					head + i, 0);
 		} else if (swap_cache) {
+			/*
+			 * split anonymous THPs (including swapped out ones) to
+			 * non-zero order not supported
+			 */
+			VM_BUG_ON(new_order);
 			__xa_store(&swap_cache->i_pages, offset + i,
 					head + i, 0);
 		}
 	}
 
-	ClearPageCompound(head);
+	if (!new_order)
+		ClearPageCompound(head);
+	else
+		set_compound_order(head, new_order);
 
-	split_page_owner(head, order, 0);
+	split_page_owner(head, order, new_order);
 
 	/* See comment in __split_huge_page_tail() */
 	if (PageAnon(head)) {
 		/* Additional pin to swap cache */
 		if (PageSwapCache(head)) {
-			page_ref_add(head, 2);
+			page_ref_add(head, 1 + new_nr);
 			xa_unlock(&swap_cache->i_pages);
 		} else {
 			page_ref_inc(head);
 		}
 	} else {
 		/* Additional pin to page cache */
-		page_ref_add(head, 2);
+		page_ref_add(head, 1 + new_nr);
 		xa_unlock(&head->mapping->i_pages);
 	}
 
 	spin_unlock_irqrestore(&pgdat->lru_lock, flags);
 
-	remap_page(head, nr);
+	remap_page(head, nr, new_nr);
 
 	if (PageSwapCache(head)) {
 		swp_entry_t entry = { .val = page_private(head) };
@@ -2486,7 +2508,14 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 		split_swap_cluster(entry);
 	}
 
-	for (i = 0; i < nr; i++) {
+	/*
+	 * set page to its compound_head when split to THPs, so that GUP pin and
+	 * PG_locked are transferred to the right after-split page
+	 */
+	if (new_order)
+		page = compound_head(page);
+
+	for (i = 0; i < nr; i += new_nr) {
 		struct page *subpage = head + i;
 		if (subpage == page)
 			continue;
@@ -2604,37 +2633,61 @@ bool can_split_huge_page(struct page *page, int *pextra_pins)
  * This function splits huge page into normal pages. @page can point to any
  * subpage of huge page to split. Split doesn't change the position of @page.
  *
+ * See split_huge_page_to_list_to_order() for more details.
+ *
+ * Returns 0 if the hugepage is split successfully.
+ * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under
+ * us.
+ */
+int split_huge_page_to_list(struct page *page, struct list_head *list)
+{
+	return split_huge_page_to_list_to_order(page, list, 0);
+}
+
+/*
+ * This function splits huge page into pages in @new_order. @page can point to
+ * any subpage of huge page to split. Split doesn't change the position of
+ * @page.
+ *
  * Only caller must hold pin on the @page, otherwise split fails with -EBUSY.
  * The huge page must be locked.
  *
  * If @list is null, tail pages will be added to LRU list, otherwise, to @list.
  *
- * Both head page and tail pages will inherit mapping, flags, and so on from
- * the hugepage.
+ * Pages in new_order will inherit mapping, flags, and so on from the hugepage.
  *
- * GUP pin and PG_locked transferred to @page. Rest subpages can be freed if
- * they are not mapped.
+ * GUP pin and PG_locked transferred to @page or the compound page @page belongs
+ * to. Rest subpages can be freed if they are not mapped.
  *
  * Returns 0 if the hugepage is split successfully.
  * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under
  * us.
  */
-int split_huge_page_to_list(struct page *page, struct list_head *list)
+int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
+				     unsigned int new_order)
 {
 	struct page *head = compound_head(page);
 	struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
 	struct deferred_split *ds_queue = get_deferred_split_queue(head);
-	XA_STATE(xas, &head->mapping->i_pages, head->index);
+	/* reset xarray order to new order after split */
+	XA_STATE_ORDER(xas, &head->mapping->i_pages, head->index, new_order);
 	struct anon_vma *anon_vma = NULL;
 	struct address_space *mapping = NULL;
 	int count, mapcount, extra_pins, ret;
 	unsigned long flags;
 	pgoff_t end;
 
+	VM_BUG_ON(thp_order(head) <= new_order);
 	VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
 	VM_BUG_ON_PAGE(!PageLocked(head), head);
 	VM_BUG_ON_PAGE(!PageCompound(head), head);
 
+	/* Cannot split THP to order-1 (no order-1 THPs) */
+	VM_BUG_ON(new_order == 1);
+
+	/* Split anonymous THP to non-zero order not support */
+	VM_BUG_ON(PageAnon(head) && new_order);
+
 	if (PageWriteback(head))
 		return -EBUSY;
 
@@ -2720,18 +2773,22 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 	if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
 		if (!list_empty(page_deferred_list(head))) {
 			ds_queue->split_queue_len--;
-			list_del(page_deferred_list(head));
+			list_del_init(page_deferred_list(head));
 		}
 		spin_unlock(&ds_queue->split_queue_lock);
 		if (mapping) {
 			if (PageSwapBacked(head))
 				__dec_lruvec_page_state(head, NR_SHMEM_THPS);
-			else
+			else if (!new_order)
+				/*
+				 * Decrease THP stats only if split to normal
+				 * pages
+				 */
 				__mod_lruvec_page_state(head, NR_FILE_THPS,
 						-thp_nr_pages(head));
 		}
 
-		__split_huge_page(page, list, end, flags);
+		__split_huge_page(page, list, end, flags, new_order);
 		ret = 0;
 	} else {
 		if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
@@ -2746,7 +2803,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 fail:		if (mapping)
 			xas_unlock(&xas);
 		spin_unlock_irqrestore(&pgdata->lru_lock, flags);
-		remap_page(head, thp_nr_pages(head));
+		remap_page(head, thp_nr_pages(head), 1);
 		ret = -EBUSY;
 	}
 
diff --git a/mm/swap.c b/mm/swap.c
index b667870c8a0b..8f53ab593438 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -983,7 +983,6 @@ void lru_add_page_tail(struct page *page, struct page *page_tail,
 		       struct lruvec *lruvec, struct list_head *list)
 {
 	VM_BUG_ON_PAGE(!PageHead(page), page);
-	VM_BUG_ON_PAGE(PageCompound(page_tail), page);
 	VM_BUG_ON_PAGE(PageLRU(page_tail), page);
 	lockdep_assert_held(&lruvec_pgdat(lruvec)->lru_lock);
 
-- 
2.28.0