[RFC PATCH 06/10] mm/swap: switch to use multi index entries

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

From: Kairui Song <ryncsn@gmail.com>

Since now all explicit shadow clearing is gone and all swapin / swapout
path is all using swap cache, switch swap cache to use multi index so
swapping out of THP will be faster, also using less memory.

Test result of sequential swapin/out of 30G zero page on ZRAM:

               Before (us)        After (us)
Swapout:       33648529           33713283
Swapin:        40667696           40954646
Swapout (THP): 7658664            6921176  (+9.7%)
Swapin (THP) : 40602278           40891953

And after swapping out 30G with THP, the radix node usage dropped by a
lot:

Before: radix_tree_node 73728K
After:  radix_tree_node  7056K (-94%)

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/filemap.c     | 27 +++++++++++++++++
 mm/huge_memory.c | 77 +++++++++++++++++++-----------------------------
 mm/internal.h    |  2 ++
 mm/swap_state.c  | 54 ++++++++++-----------------------
 4 files changed, 75 insertions(+), 85 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 0ccdc9e92764..5e8e3fd26b8d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -919,6 +919,33 @@ static int __filemap_lock_store(struct xa_state *xas, struct folio *folio,
 	return xas_error(xas);
 }
 
+int __filemap_add_swapcache(struct address_space *mapping, struct folio *folio,
+			    pgoff_t index, gfp_t gfp, void **shadowp)
+{
+	XA_STATE_ORDER(xas, &mapping->i_pages, index, folio_order(folio));
+	long nr;
+	int ret;
+
+	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
+	VM_BUG_ON_FOLIO(!folio_test_swapcache(folio), folio);
+	mapping_set_update(&xas, mapping);
+
+	nr = folio_nr_pages(folio);
+	folio_ref_add(folio, nr);
+
+	ret = __filemap_lock_store(&xas, folio, index, gfp, shadowp);
+	if (likely(!ret)) {
+		mapping->nrpages += nr;
+		__node_stat_mod_folio(folio, NR_FILE_PAGES, nr);
+		__lruvec_stat_mod_folio(folio, NR_SWAPCACHE, nr);
+		xas_unlock_irq(&xas);
+	} else {
+		folio_put_refs(folio, nr);
+	}
+
+	return ret;
+}
+
 noinline int __filemap_add_folio(struct address_space *mapping,
 		struct folio *folio, pgoff_t index, gfp_t gfp, void **shadowp)
 {
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9859aa4f7553..4fd2f74b94a9 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2886,14 +2886,12 @@ static void __split_huge_page_tail(struct folio *folio, int tail,
 	lru_add_page_tail(head, page_tail, lruvec, list);
 }
 
-static void __split_huge_page(struct page *page, struct list_head *list,
-		pgoff_t end, unsigned int new_order)
+static void __split_huge_page(struct address_space *mapping, struct page *page,
+			      struct list_head *list, pgoff_t end, unsigned int new_order)
 {
 	struct folio *folio = page_folio(page);
 	struct page *head = &folio->page;
 	struct lruvec *lruvec;
-	struct address_space *swap_cache = NULL;
-	unsigned long offset = 0;
 	int i, nr_dropped = 0;
 	unsigned int new_nr = 1 << new_order;
 	int order = folio_order(folio);
@@ -2902,12 +2900,6 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 	/* complete memcg works before add pages to LRU */
 	split_page_memcg(head, order, new_order);
 
-	if (folio_test_anon(folio) && folio_test_swapcache(folio)) {
-		offset = swp_offset(folio->swap);
-		swap_cache = swap_address_space(folio->swap);
-		xa_lock(&swap_cache->i_pages);
-	}
-
 	/* lock lru list/PageCompound, ref frozen by page_ref_freeze */
 	lruvec = folio_lruvec_lock(folio);
 
@@ -2919,18 +2911,18 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 		if (head[i].index >= end) {
 			struct folio *tail = page_folio(head + i);
 
-			if (shmem_mapping(folio->mapping))
+			if (shmem_mapping(mapping))
 				nr_dropped++;
 			else if (folio_test_clear_dirty(tail))
 				folio_account_cleaned(tail,
-					inode_to_wb(folio->mapping->host));
+					inode_to_wb(mapping->host));
 			__filemap_remove_folio(tail, NULL);
 			folio_put(tail);
 		} else if (!PageAnon(page)) {
-			__xa_store(&folio->mapping->i_pages, head[i].index,
+			__xa_store(&mapping->i_pages, head[i].index,
 					head + i, 0);
-		} else if (swap_cache) {
-			__xa_store(&swap_cache->i_pages, offset + i,
+		} else if (folio_test_swapcache(folio)) {
+			__xa_store(&mapping->i_pages, swp_offset(folio->swap) + i,
 					head + i, 0);
 		}
 	}
@@ -2948,23 +2940,17 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 	split_page_owner(head, order, new_order);
 
 	/* See comment in __split_huge_page_tail() */
-	if (folio_test_anon(folio)) {
+	if (mapping) {
 		/* Additional pin to swap cache */
-		if (folio_test_swapcache(folio)) {
-			folio_ref_add(folio, 1 + new_nr);
-			xa_unlock(&swap_cache->i_pages);
-		} else {
-			folio_ref_inc(folio);
-		}
-	} else {
-		/* Additional pin to page cache */
 		folio_ref_add(folio, 1 + new_nr);
-		xa_unlock(&folio->mapping->i_pages);
+		xa_unlock(&mapping->i_pages);
+	} else {
+		folio_ref_inc(folio);
 	}
 	local_irq_enable();
 
 	if (nr_dropped)
-		shmem_uncharge(folio->mapping->host, nr_dropped);
+		shmem_uncharge(mapping->host, nr_dropped);
 	remap_page(folio, nr);
 
 	if (folio_test_swapcache(folio))
@@ -3043,11 +3029,12 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
 	/* reset xarray order to new order after split */
 	XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
+	struct address_space *mapping = folio_mapping(folio);;
 	struct anon_vma *anon_vma = NULL;
-	struct address_space *mapping = NULL;
 	int extra_pins, ret;
 	pgoff_t end;
 	bool is_hzp;
+	gfp_t gfp;
 
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
@@ -3079,7 +3066,6 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		}
 	}
 
-
 	is_hzp = is_huge_zero_page(&folio->page);
 	if (is_hzp) {
 		pr_warn_ratelimited("Called split_huge_page for huge zero page\n");
@@ -3089,6 +3075,17 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	if (folio_test_writeback(folio))
 		return -EBUSY;
 
+	if (mapping) {
+		gfp = current_gfp_context(mapping_gfp_mask(mapping) &
+					  GFP_RECLAIM_MASK);
+
+		xas_split_alloc(&xas, folio, folio_order(folio), gfp);
+		if (xas_error(&xas)) {
+			ret = xas_error(&xas);
+			goto out;
+		}
+	}
+
 	if (folio_test_anon(folio)) {
 		/*
 		 * The caller does not necessarily hold an mmap_lock that would
@@ -3104,33 +3101,19 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 			goto out;
 		}
 		end = -1;
-		mapping = NULL;
 		anon_vma_lock_write(anon_vma);
 	} else {
-		gfp_t gfp;
-
-		mapping = folio->mapping;
-
 		/* Truncated ? */
 		if (!mapping) {
 			ret = -EBUSY;
 			goto out;
 		}
 
-		gfp = current_gfp_context(mapping_gfp_mask(mapping) &
-							GFP_RECLAIM_MASK);
-
 		if (!filemap_release_folio(folio, gfp)) {
 			ret = -EBUSY;
 			goto out;
 		}
 
-		xas_split_alloc(&xas, folio, folio_order(folio), gfp);
-		if (xas_error(&xas)) {
-			ret = xas_error(&xas);
-			goto out;
-		}
-
 		anon_vma = NULL;
 		i_mmap_lock_read(mapping);
 
@@ -3189,7 +3172,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 			int nr = folio_nr_pages(folio);
 
 			xas_split(&xas, folio, folio_order(folio));
-			if (folio_test_pmd_mappable(folio) &&
+
+			if (!folio_test_anon(folio) &&
+			    folio_test_pmd_mappable(folio) &&
 			    new_order < HPAGE_PMD_ORDER) {
 				if (folio_test_swapbacked(folio)) {
 					__lruvec_stat_mod_folio(folio,
@@ -3202,7 +3187,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 			}
 		}
 
-		__split_huge_page(page, list, end, new_order);
+		__split_huge_page(mapping, page, list, end, new_order);
 		ret = 0;
 	} else {
 		spin_unlock(&ds_queue->split_queue_lock);
@@ -3218,9 +3203,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	if (anon_vma) {
 		anon_vma_unlock_write(anon_vma);
 		put_anon_vma(anon_vma);
-	}
-	if (mapping)
+	} else {
 		i_mmap_unlock_read(mapping);
+	}
 out:
 	xas_destroy(&xas);
 	count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
diff --git a/mm/internal.h b/mm/internal.h
index 7e486f2c502c..b2bbfd3c2b50 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1059,6 +1059,8 @@ struct migration_target_control {
  */
 size_t splice_folio_into_pipe(struct pipe_inode_info *pipe,
 			      struct folio *folio, loff_t fpos, size_t size);
+int __filemap_add_swapcache(struct address_space *mapping, struct folio *folio,
+			    pgoff_t index, gfp_t gfp, void **shadowp);
 
 /*
  * mm/vmalloc.c
diff --git a/mm/swap_state.c b/mm/swap_state.c
index b84e7b0ea4a5..caf69696f47c 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -90,48 +90,22 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
 {
 	struct address_space *address_space = swap_address_space(entry);
 	pgoff_t idx = swp_offset(entry);
-	XA_STATE_ORDER(xas, &address_space->i_pages, idx, folio_order(folio));
-	unsigned long i, nr = folio_nr_pages(folio);
-	void *old;
-
-	xas_set_update(&xas, workingset_update_node);
+	int ret;
 
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 	VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
 	VM_BUG_ON_FOLIO(!folio_test_swapbacked(folio), folio);
 
-	folio_ref_add(folio, nr);
 	folio_set_swapcache(folio);
 	folio->swap = entry;
 
-	do {
-		xas_lock_irq(&xas);
-		xas_create_range(&xas);
-		if (xas_error(&xas))
-			goto unlock;
-		for (i = 0; i < nr; i++) {
-			VM_BUG_ON_FOLIO(xas.xa_index != idx + i, folio);
-			if (shadowp) {
-				old = xas_load(&xas);
-				if (xa_is_value(old))
-					*shadowp = old;
-			}
-			xas_store(&xas, folio);
-			xas_next(&xas);
-		}
-		address_space->nrpages += nr;
-		__node_stat_mod_folio(folio, NR_FILE_PAGES, nr);
-		__lruvec_stat_mod_folio(folio, NR_SWAPCACHE, nr);
-unlock:
-		xas_unlock_irq(&xas);
-	} while (xas_nomem(&xas, gfp));
-
-	if (!xas_error(&xas))
-		return 0;
+	ret = __filemap_add_swapcache(address_space, folio, idx, gfp, shadowp);
+	if (ret) {
+		folio_clear_swapcache(folio);
+		folio->swap.val = 0;
+	}
 
-	folio_clear_swapcache(folio);
-	folio_ref_sub(folio, nr);
-	return xas_error(&xas);
+	return ret;
 }
 
 /*
@@ -142,7 +116,6 @@ void __delete_from_swap_cache(struct folio *folio,
 			swp_entry_t entry, void *shadow)
 {
 	struct address_space *address_space = swap_address_space(entry);
-	int i;
 	long nr = folio_nr_pages(folio);
 	pgoff_t idx = swp_offset(entry);
 	XA_STATE(xas, &address_space->i_pages, idx);
@@ -153,11 +126,9 @@ void __delete_from_swap_cache(struct folio *folio,
 	VM_BUG_ON_FOLIO(!folio_test_swapcache(folio), folio);
 	VM_BUG_ON_FOLIO(folio_test_writeback(folio), folio);
 
-	for (i = 0; i < nr; i++) {
-		void *entry = xas_store(&xas, shadow);
-		VM_BUG_ON_PAGE(entry != folio, entry);
-		xas_next(&xas);
-	}
+	xas_set_order(&xas, idx, folio_order(folio));
+	xas_store(&xas, shadow);
+
 	folio->swap.val = 0;
 	folio_clear_swapcache(folio);
 	address_space->nrpages -= nr;
@@ -252,6 +223,11 @@ void clear_shadow_from_swap_cache(swp_entry_t entry)
 
 	xas_set_update(&xas, workingset_update_node);
 
+	/*
+	 * On unmap, it may delete a larger order shadow here. It's mostly
+	 * fine since not entirely mapped folios are spiltted on swap out
+	 * and leaves shadows with order 0.
+	 */
 	xa_lock_irq(&address_space->i_pages);
 	if (xa_is_value(xas_load(&xas)))
 		xas_store(&xas, NULL);
-- 
2.43.0