patches/old/mm-swap-implement-dynamic-allocation-of-swap-table.patch

From: Kairui Song <kasong@tencent.com>
Subject: mm, swap: implement dynamic allocation of swap table
Date: Wed, 17 Sep 2025 00:00:59 +0800

Now swap table is cluster based, which means free clusters can free its
table since no one should modify it.

There could be speculative readers, like swap cache look up, protect them
by making them RCU protected.  All swap table should be filled with null
entries before free, so such readers will either see a NULL pointer or a
null filled table being lazy freed.

On allocation, allocate the table when a cluster is used by any order.

This way, we can reduce the memory usage of large swap device
significantly.

This idea to dynamically release unused swap cluster data was initially
suggested by Chris Li while proposing the cluster swap allocator and it
suits the swap table idea very well.

Link: https://lkml.kernel.org/r/20250916160100.31545-15-ryncsn@gmail.com
Co-developed-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Kairui Song <kasong@tencent.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/swap.h       |    2 
 mm/swap_state.c |    9 --
 mm/swap_table.h |   37 +++++++-
 mm/swapfile.c   |  197 ++++++++++++++++++++++++++++++++++++----------
 4 files changed, 194 insertions(+), 51 deletions(-)

--- a/mm/swapfile.c~mm-swap-implement-dynamic-allocation-of-swap-table
+++ a/mm/swapfile.c
@@ -59,6 +59,9 @@ static void swap_entries_free(struct swa
 static void swap_range_alloc(struct swap_info_struct *si,
 			     unsigned int nr_entries);
 static bool folio_swapcache_freeable(struct folio *folio);
+static void move_cluster(struct swap_info_struct *si,
+			 struct swap_cluster_info *ci, struct list_head *list,
+			 enum swap_cluster_flags new_flags);
 
 static DEFINE_SPINLOCK(swap_lock);
 static unsigned int nr_swapfiles;
@@ -105,6 +108,8 @@ static DEFINE_SPINLOCK(swap_avail_lock);
 
 struct swap_info_struct *swap_info[MAX_SWAPFILES];
 
+static struct kmem_cache *swap_table_cachep;
+
 static DEFINE_MUTEX(swapon_mutex);
 
 static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
@@ -401,10 +406,17 @@ static inline bool cluster_is_discard(st
 	return info->flags == CLUSTER_FLAG_DISCARD;
 }
 
+static inline bool cluster_table_is_alloced(struct swap_cluster_info *ci)
+{
+	return rcu_dereference_protected(ci->table, lockdep_is_held(&ci->lock));
+}
+
 static inline bool cluster_is_usable(struct swap_cluster_info *ci, int order)
 {
 	if (unlikely(ci->flags > CLUSTER_FLAG_USABLE))
 		return false;
+	if (!cluster_table_is_alloced(ci))
+		return false;
 	if (!order)
 		return true;
 	return cluster_is_empty(ci) || order == ci->order;
@@ -422,32 +434,90 @@ static inline unsigned int cluster_offse
 	return cluster_index(si, ci) * SWAPFILE_CLUSTER;
 }
 
-static int swap_cluster_alloc_table(struct swap_cluster_info *ci)
+static void swap_cluster_free_table(struct swap_cluster_info *ci)
 {
-	WARN_ON(ci->table);
-	ci->table = kzalloc(sizeof(unsigned long) * SWAPFILE_CLUSTER, GFP_KERNEL);
-	if (!ci->table)
-		return -ENOMEM;
-	return 0;
+	unsigned int ci_off;
+	struct swap_table *table;
+
+	/* Only empty cluster's table is allow to be freed  */
+	lockdep_assert_held(&ci->lock);
+	VM_WARN_ON_ONCE(!cluster_is_empty(ci));
+	for (ci_off = 0; ci_off < SWAPFILE_CLUSTER; ci_off++)
+		VM_WARN_ON_ONCE(!swp_tb_is_null(__swap_table_get(ci, ci_off)));
+	table = (void *)rcu_dereference_protected(ci->table, true);
+	rcu_assign_pointer(ci->table, NULL);
+
+	kmem_cache_free(swap_table_cachep, table);
 }
 
-static void swap_cluster_free_table(struct swap_cluster_info *ci)
+/*
+ * Allocate swap table for one cluster. Attempt an atomic allocation first,
+ * then fallback to sleeping allocation.
+ */
+static struct swap_cluster_info *
+swap_cluster_alloc_table(struct swap_info_struct *si,
+			 struct swap_cluster_info *ci)
 {
-	unsigned int ci_off;
-	unsigned long swp_tb;
+	struct swap_table *table;
 
-	if (!ci->table)
-		return;
+	/*
+	 * Only cluster isolation from the allocator does table allocation.
+	 * Swap allocator uses percpu clusters and holds the local lock.
+	 */
+	lockdep_assert_held(&ci->lock);
+	lockdep_assert_held(&this_cpu_ptr(&percpu_swap_cluster)->lock);
+
+	/* The cluster must be free and was just isolated from the free list. */
+	VM_WARN_ON_ONCE(ci->flags || !cluster_is_empty(ci));
+
+	table = kmem_cache_zalloc(swap_table_cachep,
+				  __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN);
+	if (table) {
+		rcu_assign_pointer(ci->table, table);
+		return ci;
+	}
+
+	/*
+	 * Try a sleep allocation. Each isolated free cluster may cause
+	 * a sleep allocation, but there is a limited number of them, so
+	 * the potential recursive allocation is limited.
+	 */
+	spin_unlock(&ci->lock);
+	if (!(si->flags & SWP_SOLIDSTATE))
+		spin_unlock(&si->global_cluster_lock);
+	local_unlock(&percpu_swap_cluster.lock);
+
+	table = kmem_cache_zalloc(swap_table_cachep,
+				  __GFP_HIGH | __GFP_NOMEMALLOC | GFP_KERNEL);
+
+	/*
+	 * Back to atomic context. We might have migrated to a new CPU with a
+	 * usable percpu cluster. But just keep using the isolated cluster to
+	 * make things easier. Migration indicates a slight change of workload
+	 * so using a new free cluster might not be a bad idea, and the worst
+	 * could happen with ignoring the percpu cluster is fragmentation,
+	 * which is acceptable since this fallback and race is rare.
+	 */
+	local_lock(&percpu_swap_cluster.lock);
+	if (!(si->flags & SWP_SOLIDSTATE))
+		spin_lock(&si->global_cluster_lock);
+	spin_lock(&ci->lock);
+
+	/* Nothing except this helper should touch a dangling empty cluster. */
+	if (WARN_ON_ONCE(cluster_table_is_alloced(ci))) {
+		if (table)
+			kmem_cache_free(swap_table_cachep, table);
+		return ci;
+	}
 
-	for (ci_off = 0; ci_off < SWAPFILE_CLUSTER; ci_off++) {
-		swp_tb = __swap_table_get(ci, ci_off);
-		if (!swp_tb_is_null(swp_tb))
-			pr_err_once("swap: unclean swap space on swapoff: 0x%lx",
-				    swp_tb);
+	if (!table) {
+		move_cluster(si, ci, &si->free_clusters, CLUSTER_FLAG_FREE);
+		spin_unlock(&ci->lock);
+		return NULL;
 	}
 
-	kfree(ci->table);
-	ci->table = NULL;
+	rcu_assign_pointer(ci->table, table);
+	return ci;
 }
 
 static void move_cluster(struct swap_info_struct *si,
@@ -479,7 +549,7 @@ static void swap_cluster_schedule_discar
 
 static void __free_cluster(struct swap_info_struct *si, struct swap_cluster_info *ci)
 {
-	lockdep_assert_held(&ci->lock);
+	swap_cluster_free_table(ci);
 	move_cluster(si, ci, &si->free_clusters, CLUSTER_FLAG_FREE);
 	ci->order = 0;
 }
@@ -494,15 +564,11 @@ static void __free_cluster(struct swap_i
  * this returns NULL for an non-empty list.
  */
 static struct swap_cluster_info *isolate_lock_cluster(
-		struct swap_info_struct *si, struct list_head *list)
+		struct swap_info_struct *si, struct list_head *list, int order)
 {
-	struct swap_cluster_info *ci, *ret = NULL;
+	struct swap_cluster_info *ci, *found = NULL;
 
 	spin_lock(&si->lock);
-
-	if (unlikely(!(si->flags & SWP_WRITEOK)))
-		goto out;
-
 	list_for_each_entry(ci, list, list) {
 		if (!spin_trylock(&ci->lock))
 			continue;
@@ -514,13 +580,19 @@ static struct swap_cluster_info *isolate
 
 		list_del(&ci->list);
 		ci->flags = CLUSTER_FLAG_NONE;
-		ret = ci;
+		found = ci;
 		break;
 	}
-out:
 	spin_unlock(&si->lock);
 
-	return ret;
+	if (found && !cluster_table_is_alloced(found)) {
+		/* Only an empty free cluster's swap table can be freed. */
+		VM_WARN_ON_ONCE(list != &si->free_clusters);
+		VM_WARN_ON_ONCE(!cluster_is_empty(found));
+		return swap_cluster_alloc_table(si, found);
+	}
+
+	return found;
 }
 
 /*
@@ -653,17 +725,27 @@ static void relocate_cluster(struct swap
  * added to free cluster list and its usage counter will be increased by 1.
  * Only used for initialization.
  */
-static void inc_cluster_info_page(struct swap_info_struct *si,
+static int inc_cluster_info_page(struct swap_info_struct *si,
 	struct swap_cluster_info *cluster_info, unsigned long page_nr)
 {
 	unsigned long idx = page_nr / SWAPFILE_CLUSTER;
+	struct swap_table *table;
 	struct swap_cluster_info *ci;
 
 	ci = cluster_info + idx;
+	if (!ci->table) {
+		table = kmem_cache_zalloc(swap_table_cachep, GFP_KERNEL);
+		if (!table)
+			return -ENOMEM;
+		rcu_assign_pointer(ci->table, table);
+	}
+
 	ci->count++;
 
 	VM_BUG_ON(ci->count > SWAPFILE_CLUSTER);
 	VM_BUG_ON(ci->flags);
+
+	return 0;
 }
 
 static bool cluster_reclaim_range(struct swap_info_struct *si,
@@ -845,7 +927,7 @@ static unsigned int alloc_swap_scan_list
 	unsigned int found = SWAP_ENTRY_INVALID;
 
 	do {
-		struct swap_cluster_info *ci = isolate_lock_cluster(si, list);
+		struct swap_cluster_info *ci = isolate_lock_cluster(si, list, order);
 		unsigned long offset;
 
 		if (!ci)
@@ -870,7 +952,7 @@ static void swap_reclaim_full_clusters(s
 	if (force)
 		to_scan = swap_usage_in_pages(si) / SWAPFILE_CLUSTER;
 
-	while ((ci = isolate_lock_cluster(si, &si->full_clusters))) {
+	while ((ci = isolate_lock_cluster(si, &si->full_clusters, 0))) {
 		offset = cluster_offset(si, ci);
 		end = min(si->max, offset + SWAPFILE_CLUSTER);
 		to_scan--;
@@ -1018,6 +1100,7 @@ new_cluster:
 done:
 	if (!(si->flags & SWP_SOLIDSTATE))
 		spin_unlock(&si->global_cluster_lock);
+
 	return found;
 }
 
@@ -1885,7 +1968,13 @@ swp_entry_t get_swap_page_of_type(int ty
 	/* This is called for allocating swap entry, not cache */
 	if (get_swap_device_info(si)) {
 		if (si->flags & SWP_WRITEOK) {
+			/*
+			 * Grab the local lock to be complaint
+			 * with swap table allocation.
+			 */
+			local_lock(&percpu_swap_cluster.lock);
 			offset = cluster_alloc_swap_entry(si, 0, 1);
+			local_unlock(&percpu_swap_cluster.lock);
 			if (offset) {
 				entry = swp_entry(si->type, offset);
 				atomic_long_dec(&nr_swap_pages);
@@ -2679,12 +2768,21 @@ static void wait_for_allocation(struct s
 static void free_cluster_info(struct swap_cluster_info *cluster_info,
 			      unsigned long maxpages)
 {
+	struct swap_cluster_info *ci;
 	int i, nr_clusters = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
 
 	if (!cluster_info)
 		return;
-	for (i = 0; i < nr_clusters; i++)
-		swap_cluster_free_table(&cluster_info[i]);
+	for (i = 0; i < nr_clusters; i++) {
+		ci = cluster_info + i;
+		/* Cluster with bad marks count will have a remaining table */
+		spin_lock(&ci->lock);
+		if (rcu_dereference_protected(ci->table, true)) {
+			ci->count = 0;
+			swap_cluster_free_table(ci);
+		}
+		spin_unlock(&ci->lock);
+	}
 	kvfree(cluster_info);
 }
 
@@ -2720,6 +2818,7 @@ SYSCALL_DEFINE1(swapoff, const char __us
 	struct address_space *mapping;
 	struct inode *inode;
 	struct filename *pathname;
+	unsigned int maxpages;
 	int err, found = 0;
 
 	if (!capable(CAP_SYS_ADMIN))
@@ -2826,8 +2925,8 @@ SYSCALL_DEFINE1(swapoff, const char __us
 	p->swap_map = NULL;
 	zeromap = p->zeromap;
 	p->zeromap = NULL;
+	maxpages = p->max;
 	cluster_info = p->cluster_info;
-	free_cluster_info(cluster_info, p->max);
 	p->max = 0;
 	p->cluster_info = NULL;
 	spin_unlock(&p->lock);
@@ -2839,6 +2938,7 @@ SYSCALL_DEFINE1(swapoff, const char __us
 	p->global_cluster = NULL;
 	vfree(swap_map);
 	kvfree(zeromap);
+	free_cluster_info(cluster_info, maxpages);
 	/* Destroy swap account information */
 	swap_cgroup_swapoff(p->type);
 
@@ -3217,11 +3317,8 @@ static struct swap_cluster_info *setup_c
 	if (!cluster_info)
 		goto err;
 
-	for (i = 0; i < nr_clusters; i++) {
+	for (i = 0; i < nr_clusters; i++)
 		spin_lock_init(&cluster_info[i].lock);
-		if (swap_cluster_alloc_table(&cluster_info[i]))
-			goto err_free;
-	}
 
 	if (!(si->flags & SWP_SOLIDSTATE)) {
 		si->global_cluster = kmalloc(sizeof(*si->global_cluster),
@@ -3240,16 +3337,23 @@ static struct swap_cluster_info *setup_c
 	 * See setup_swap_map(): header page, bad pages,
 	 * and the EOF part of the last cluster.
 	 */
-	inc_cluster_info_page(si, cluster_info, 0);
+	err = inc_cluster_info_page(si, cluster_info, 0);
+	if (err)
+		goto err;
 	for (i = 0; i < swap_header->info.nr_badpages; i++) {
 		unsigned int page_nr = swap_header->info.badpages[i];
 
 		if (page_nr >= maxpages)
 			continue;
-		inc_cluster_info_page(si, cluster_info, page_nr);
+		err = inc_cluster_info_page(si, cluster_info, page_nr);
+		if (err)
+			goto err;
+	}
+	for (i = maxpages; i < round_up(maxpages, SWAPFILE_CLUSTER); i++) {
+		err = inc_cluster_info_page(si, cluster_info, i);
+		if (err)
+			goto err;
 	}
-	for (i = maxpages; i < round_up(maxpages, SWAPFILE_CLUSTER); i++)
-		inc_cluster_info_page(si, cluster_info, i);
 
 	INIT_LIST_HEAD(&si->free_clusters);
 	INIT_LIST_HEAD(&si->full_clusters);
@@ -3963,6 +4067,15 @@ static int __init swapfile_init(void)
 
 	swapfile_maximum_size = arch_max_swapfile_size();
 
+	/*
+	 * Once a cluster is freed, it's swap table content is read
+	 * only, and all swap cache readers (swap_cache_*) verifies
+	 * the content before use. So it's safe to use RCU slab here.
+	 */
+	swap_table_cachep = kmem_cache_create("swap_table",
+			    sizeof(struct swap_table),
+			    0, SLAB_PANIC | SLAB_TYPESAFE_BY_RCU, NULL);
+
 #ifdef CONFIG_MIGRATION
 	if (swapfile_maximum_size >= (1UL << SWP_MIG_TOTAL_BITS))
 		swap_migration_ad_supported = true;
--- a/mm/swap.h~mm-swap-implement-dynamic-allocation-of-swap-table
+++ a/mm/swap.h
@@ -36,7 +36,7 @@ struct swap_cluster_info {
 	u16 count;
 	u8 flags;
 	u8 order;
-	atomic_long_t *table;	/* Swap table entries, see mm/swap_table.h */
+	atomic_long_t __rcu *table;	/* Swap table entries, see mm/swap_table.h */
 	struct list_head list;
 };
 
--- a/mm/swap_state.c~mm-swap-implement-dynamic-allocation-of-swap-table
+++ a/mm/swap_state.c
@@ -91,8 +91,8 @@ struct folio *swap_cache_get_folio(swp_e
 	struct folio *folio;
 
 	for (;;) {
-		swp_tb = __swap_table_get(__swap_entry_to_cluster(entry),
-					  swp_cluster_offset(entry));
+		swp_tb = swap_table_get(__swap_entry_to_cluster(entry),
+					swp_cluster_offset(entry));
 		if (!swp_tb_is_folio(swp_tb))
 			return NULL;
 		folio = swp_tb_to_folio(swp_tb);
@@ -115,11 +115,10 @@ void *swap_cache_get_shadow(swp_entry_t
 {
 	unsigned long swp_tb;
 
-	swp_tb = __swap_table_get(__swap_entry_to_cluster(entry),
-				  swp_cluster_offset(entry));
+	swp_tb = swap_table_get(__swap_entry_to_cluster(entry),
+				swp_cluster_offset(entry));
 	if (swp_tb_is_shadow(swp_tb))
 		return swp_tb_to_shadow(swp_tb);
-
 	return NULL;
 }
 
--- a/mm/swap_table.h~mm-swap-implement-dynamic-allocation-of-swap-table
+++ a/mm/swap_table.h
@@ -2,8 +2,15 @@
 #ifndef _MM_SWAP_TABLE_H
 #define _MM_SWAP_TABLE_H
 
+#include <linux/rcupdate.h>
+#include <linux/atomic.h>
 #include "swap.h"
 
+/* A typical flat array in each cluster as swap table */
+struct swap_table {
+	atomic_long_t entries[SWAPFILE_CLUSTER];
+};
+
 /*
  * A swap table entry represents the status of a swap slot on a swap
  * (physical or virtual) device. The swap table in each cluster is a
@@ -76,22 +83,46 @@ static inline void *swp_tb_to_shadow(uns
 static inline void __swap_table_set(struct swap_cluster_info *ci,
 				    unsigned int off, unsigned long swp_tb)
 {
+	atomic_long_t *table = rcu_dereference_protected(ci->table, true);
+
+	lockdep_assert_held(&ci->lock);
 	VM_WARN_ON_ONCE(off >= SWAPFILE_CLUSTER);
-	atomic_long_set(&ci->table[off], swp_tb);
+	atomic_long_set(&table[off], swp_tb);
 }
 
 static inline unsigned long __swap_table_xchg(struct swap_cluster_info *ci,
 					      unsigned int off, unsigned long swp_tb)
 {
+	atomic_long_t *table = rcu_dereference_protected(ci->table, true);
+
+	lockdep_assert_held(&ci->lock);
 	VM_WARN_ON_ONCE(off >= SWAPFILE_CLUSTER);
 	/* Ordering is guaranteed by cluster lock, relax */
-	return atomic_long_xchg_relaxed(&ci->table[off], swp_tb);
+	return atomic_long_xchg_relaxed(&table[off], swp_tb);
 }
 
 static inline unsigned long __swap_table_get(struct swap_cluster_info *ci,
 					     unsigned int off)
 {
+	atomic_long_t *table;
+
 	VM_WARN_ON_ONCE(off >= SWAPFILE_CLUSTER);
-	return atomic_long_read(&ci->table[off]);
+	table = rcu_dereference_check(ci->table, lockdep_is_held(&ci->lock));
+
+	return atomic_long_read(&table[off]);
+}
+
+static inline unsigned long swap_table_get(struct swap_cluster_info *ci,
+					unsigned int off)
+{
+	atomic_long_t *table;
+	unsigned long swp_tb;
+
+	rcu_read_lock();
+	table = rcu_dereference(ci->table);
+	swp_tb = table ? atomic_long_read(&table[off]) : null_to_swp_tb();
+	rcu_read_unlock();
+
+	return swp_tb;
 }
 #endif
_