linux-kernel - [PATCH 7/9] mm, swap: remove contention workaround for swap cache

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Message-ID: <20250822192023.13477-8-ryncsn@gmail.com>
Date: Sat, 23 Aug 2025 03:20:21 +0800
From: Kairui Song <ryncsn@...il.com>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>,
	Hugh Dickins <hughd@...gle.com>,
	Chris Li <chrisl@...nel.org>,
	Barry Song <baohua@...nel.org>,
	Baoquan He <bhe@...hat.com>,
	Nhat Pham <nphamcs@...il.com>,
	Kemeng Shi <shikemeng@...weicloud.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Ying Huang <ying.huang@...ux.alibaba.com>,
	Johannes Weiner <hannes@...xchg.org>,
	David Hildenbrand <david@...hat.com>,
	Yosry Ahmed <yosryahmed@...gle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Zi Yan <ziy@...dia.com>,
	linux-kernel@...r.kernel.org,
	Kairui Song <kasong@...cent.com>,
	kernel test robot <oliver.sang@...el.com>
Subject: [PATCH 7/9] mm, swap: remove contention workaround for swap cache

From: Kairui Song <kasong@...cent.com>

Swap cluster setup will try to shuffle the clusters on initialization.
It was helpful to avoid contention for the swap cache space. The cluster
size (2M) was much smaller than each swap cache space (64M), so shuffling
the cluster means the allocator will try to allocate swap slots that are
in different swap cache spaces for each CPU, reducing the chance of two
CPUs using the same swap cache space, and hence reducing the contention.

Now, swap cache is managed by swap clusters, this shuffle is pointless.
Just remove it, and clean up related macros.

This should also improve the HDD swap performance as shuffling IO is a
bad idea for HDD, and now the shuffling is gone.

Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202504241621.f27743ec-lkp@intel.com
Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/swap.h     |  4 ----
 mm/swapfile.c | 32 ++++++++------------------------
 mm/zswap.c    |  7 +++++--
 3 files changed, 13 insertions(+), 30 deletions(-)

diff --git a/mm/swap.h b/mm/swap.h
index 4af42bc2cd72..ce3ec62cc05e 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -153,10 +153,6 @@ int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug);
 void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug);
 
 /* linux/mm/swap_state.c */
-/* One swap address space for each 64M swap space */
-#define SWAP_ADDRESS_SPACE_SHIFT	14
-#define SWAP_ADDRESS_SPACE_PAGES	(1 << SWAP_ADDRESS_SPACE_SHIFT)
-#define SWAP_ADDRESS_SPACE_MASK		(SWAP_ADDRESS_SPACE_PAGES - 1)
 extern struct address_space swap_space __ro_after_init;
 static inline struct address_space *swap_address_space(swp_entry_t entry)
 {
diff --git a/mm/swapfile.c b/mm/swapfile.c
index df68b5e242a6..0c8001c99f30 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3203,21 +3203,14 @@ static int setup_swap_map(struct swap_info_struct *si,
 	return 0;
 }
 
-#define SWAP_CLUSTER_INFO_COLS						\
-	DIV_ROUND_UP(L1_CACHE_BYTES, sizeof(struct swap_cluster_info))
-#define SWAP_CLUSTER_SPACE_COLS						\
-	DIV_ROUND_UP(SWAP_ADDRESS_SPACE_PAGES, SWAPFILE_CLUSTER)
-#define SWAP_CLUSTER_COLS						\
-	max_t(unsigned int, SWAP_CLUSTER_INFO_COLS, SWAP_CLUSTER_SPACE_COLS)
-
 static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
 						union swap_header *swap_header,
 						unsigned long maxpages)
 {
 	unsigned long nr_clusters = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
 	struct swap_cluster_info *cluster_info;
-	unsigned long i, j, idx;
 	int err = -ENOMEM;
+	unsigned long i;
 
 	cluster_info = kvcalloc(nr_clusters, sizeof(*cluster_info), GFP_KERNEL);
 	if (!cluster_info)
@@ -3266,22 +3259,13 @@ static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
 		INIT_LIST_HEAD(&si->frag_clusters[i]);
 	}
 
-	/*
-	 * Reduce false cache line sharing between cluster_info and
-	 * sharing same address space.
-	 */
-	for (j = 0; j < SWAP_CLUSTER_COLS; j++) {
-		for (i = 0; i < DIV_ROUND_UP(nr_clusters, SWAP_CLUSTER_COLS); i++) {
-			struct swap_cluster_info *ci;
-			idx = i * SWAP_CLUSTER_COLS + j;
-			ci = cluster_info + idx;
-			if (idx >= nr_clusters)
-				continue;
-			if (ci->count) {
-				ci->flags = CLUSTER_FLAG_NONFULL;
-				list_add_tail(&ci->list, &si->nonfull_clusters[0]);
-				continue;
-			}
+	for (i = 0; i < nr_clusters; i++) {
+		struct swap_cluster_info *ci = &cluster_info[i];
+
+		if (ci->count) {
+			ci->flags = CLUSTER_FLAG_NONFULL;
+			list_add_tail(&ci->list, &si->nonfull_clusters[0]);
+		} else {
 			ci->flags = CLUSTER_FLAG_FREE;
 			list_add_tail(&ci->list, &si->free_clusters);
 		}
diff --git a/mm/zswap.c b/mm/zswap.c
index c869859eec77..c0a9be14a725 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -237,10 +237,13 @@ static bool zswap_has_pool;
 * helpers and fwd declarations
 **********************************/
 
+/* One swap address space for each 64M swap space */
+#define ZSWAP_ADDRESS_SPACE_SHIFT 14
+#define ZSWAP_ADDRESS_SPACE_PAGES (1 << ZSWAP_ADDRESS_SPACE_SHIFT)
 static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
 {
 	return &zswap_trees[swp_type(swp)][swp_offset(swp)
-		>> SWAP_ADDRESS_SPACE_SHIFT];
+		>> ZSWAP_ADDRESS_SPACE_SHIFT];
 }
 
 #define zswap_pool_debug(msg, p)				\
@@ -1771,7 +1774,7 @@ int zswap_swapon(int type, unsigned long nr_pages)
 	struct xarray *trees, *tree;
 	unsigned int nr, i;
 
-	nr = DIV_ROUND_UP(nr_pages, SWAP_ADDRESS_SPACE_PAGES);
+	nr = DIV_ROUND_UP(nr_pages, ZSWAP_ADDRESS_SPACE_PAGES);
 	trees = kvcalloc(nr, sizeof(*tree), GFP_KERNEL);
 	if (!trees) {
 		pr_err("alloc failed, zswap disabled for swap type %d\n", type);
-- 
2.51.0