[v6,6/6] zswap: shrinks zswap pool based on memory pressure

diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst
index 45b98390e938..522ae22ccb84 100644
--- a/Documentation/admin-guide/mm/zswap.rst
+++ b/Documentation/admin-guide/mm/zswap.rst
@@ -153,6 +153,13 @@  attribute, e. g.::
 
 Setting this parameter to 100 will disable the hysteresis.
 
+When there is a sizable amount of cold memory residing in the zswap pool, it
+can be advantageous to proactively write these cold pages to swap and reclaim
+the memory for other use cases. By default, the zswap shrinker is disabled.
+User can enable it as follows:
+
+  echo Y > /sys/module/zswap/parameters/shrinker_enabled
+
 A debugfs interface is provided for various statistic about pool size, number
 of pages stored, same-value filled pages and various counters for the reasons
 pages are rejected.
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 7b1816450bfc..b23bc5390240 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -22,6 +22,7 @@ 
 #include <linux/mm_types.h>
 #include <linux/page-flags.h>
 #include <linux/local_lock.h>
+#include <linux/zswap.h>
 #include <asm/page.h>
 
 /* Free memory management - zoned buddy allocator.  */
@@ -641,6 +642,7 @@  struct lruvec {
 #ifdef CONFIG_MEMCG
 	struct pglist_data *pgdat;
 #endif
+	struct zswap_lruvec_state zswap_lruvec_state;
 };
 
 /* Isolate for asynchronous migration */
diff --git a/include/linux/zswap.h b/include/linux/zswap.h
index e571e393669b..cbd373ba88d2 100644
--- a/include/linux/zswap.h
+++ b/include/linux/zswap.h
@@ -5,20 +5,40 @@ 
 #include <linux/types.h>
 #include <linux/mm_types.h>
 
+struct lruvec;
+
 extern u64 zswap_pool_total_size;
 extern atomic_t zswap_stored_pages;
 
 #ifdef CONFIG_ZSWAP
 
+struct zswap_lruvec_state {
+	/*
+	 * Number of pages in zswap that should be protected from the shrinker.
+	 * This number is an estimate of the following counts:
+	 *
+	 * a) Recent page faults.
+	 * b) Recent insertion to the zswap LRU. This includes new zswap stores,
+	 *    as well as recent zswap LRU rotations.
+	 *
+	 * These pages are likely to be warm, and might incur IO if the are written
+	 * to swap.
+	 */
+	atomic_long_t nr_zswap_protected;
+};
+
 bool zswap_store(struct folio *folio);
 bool zswap_load(struct folio *folio);
 void zswap_invalidate(int type, pgoff_t offset);
 void zswap_swapon(int type);
 void zswap_swapoff(int type);
 void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg);
-
+void zswap_lruvec_state_init(struct lruvec *lruvec);
+void zswap_lruvec_swapin(struct page *page);
 #else
 
+struct zswap_lruvec_state {};
+
 static inline bool zswap_store(struct folio *folio)
 {
 	return false;
@@ -33,7 +53,8 @@  static inline void zswap_invalidate(int type, pgoff_t offset) {}
 static inline void zswap_swapon(int type) {}
 static inline void zswap_swapoff(int type) {}
 static inline void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg) {}
-
+static inline void zswap_lruvec_init(struct lruvec *lruvec) {}
+static inline void zswap_lruvec_swapin(struct page *page) {}
 #endif
 
 #endif /* _LINUX_ZSWAP_H */
diff --git a/mm/mmzone.c b/mm/mmzone.c
index b594d3f268fe..c01896eca736 100644
--- a/mm/mmzone.c
+++ b/mm/mmzone.c
@@ -78,6 +78,7 @@  void lruvec_init(struct lruvec *lruvec)
 
 	memset(lruvec, 0, sizeof(struct lruvec));
 	spin_lock_init(&lruvec->lru_lock);
+	zswap_lruvec_state_init(lruvec);
 
 	for_each_lru(lru)
 		INIT_LIST_HEAD(&lruvec->lists[lru]);
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 6c84236382f3..94ed2d508db0 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -687,6 +687,7 @@  struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
 					&page_allocated, false);
 	if (unlikely(page_allocated))
 		swap_readpage(page, false, NULL);
+	zswap_lruvec_swapin(page);
 	return page;
 }
 
@@ -862,6 +863,7 @@  static struct page *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
 					&page_allocated, false);
 	if (unlikely(page_allocated))
 		swap_readpage(page, false, NULL);
+	zswap_lruvec_swapin(page);
 	return page;
 }
 
diff --git a/mm/zswap.c b/mm/zswap.c
index 9f5142524d48..943090dfe793 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -148,6 +148,10 @@  module_param_named(exclusive_loads, zswap_exclusive_loads_enabled, bool, 0644);
 /* Number of zpools in zswap_pool (empirically determined for scalability) */
 #define ZSWAP_NR_ZPOOLS 32
 
+/* Enable/disable memory pressure-based shrinker. */
+static bool zswap_shrinker_enabled;
+module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
+
 /*********************************
 * data structures
 **********************************/
@@ -177,6 +181,8 @@  struct zswap_pool {
 	char tfm_name[CRYPTO_MAX_ALG_NAME];
 	struct list_lru list_lru;
 	struct mem_cgroup *next_shrink;
+	struct shrinker *shrinker;
+	atomic_t nr_stored;
 };
 
 /*
@@ -275,17 +281,26 @@  static bool zswap_can_accept(void)
 			DIV_ROUND_UP(zswap_pool_total_size, PAGE_SIZE);
 }
 
+static u64 get_zswap_pool_size(struct zswap_pool *pool)
+{
+	u64 pool_size = 0;
+	int i;
+
+	for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
+		pool_size += zpool_get_total_size(pool->zpools[i]);
+
+	return pool_size;
+}
+
 static void zswap_update_total_size(void)
 {
 	struct zswap_pool *pool;
 	u64 total = 0;
-	int i;
 
 	rcu_read_lock();
 
 	list_for_each_entry_rcu(pool, &zswap_pools, list)
-		for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
-			total += zpool_get_total_size(pool->zpools[i]);
+		total += get_zswap_pool_size(pool);
 
 	rcu_read_unlock();
 
@@ -338,13 +353,34 @@  static void zswap_entry_cache_free(struct zswap_entry *entry)
 	kmem_cache_free(zswap_entry_cache, entry);
 }
 
+/*********************************
+* zswap lruvec functions
+**********************************/
+void zswap_lruvec_state_init(struct lruvec *lruvec)
+{
+	atomic_long_set(&lruvec->zswap_lruvec_state.nr_zswap_protected, 0);
+}
+
+void zswap_lruvec_swapin(struct page *page)
+{
+	struct lruvec *lruvec;
+
+	if (page) {
+		lruvec = folio_lruvec(page_folio(page));
+		atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected);
+	}
+}
+
 /*********************************
 * lru functions
 **********************************/
 static void zswap_lru_add(struct list_lru *list_lru, struct zswap_entry *entry)
 {
+	atomic_long_t *nr_zswap_protected;
+	unsigned long lru_size, old, new;
 	int nid = entry_to_nid(entry);
 	struct mem_cgroup *memcg;
+	struct lruvec *lruvec;
 
 	/*
 	 * Note that it is safe to use rcu_read_lock() here, even in the face of
@@ -362,6 +398,19 @@  static void zswap_lru_add(struct list_lru *list_lru, struct zswap_entry *entry)
 	memcg = mem_cgroup_from_entry(entry);
 	/* will always succeed */
 	list_lru_add(list_lru, &entry->lru, nid, memcg);
+
+	/* Update the protection area */
+	lru_size = list_lru_count_one(list_lru, nid, memcg);
+	lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(nid));
+	nr_zswap_protected = &lruvec->zswap_lruvec_state.nr_zswap_protected;
+	old = atomic_long_inc_return(nr_zswap_protected);
+	/*
+	 * Decay to avoid overflow and adapt to changing workloads.
+	 * This is based on LRU reclaim cost decaying heuristics.
+	 */
+	do {
+		new = old > lru_size / 4 ? old / 2 : old;
+	} while (!atomic_long_try_cmpxchg(nr_zswap_protected, &old, new));
 	rcu_read_unlock();
 }
 
@@ -383,6 +432,7 @@  static void zswap_lru_putback(struct list_lru *list_lru,
 	int nid = entry_to_nid(entry);
 	spinlock_t *lock = &list_lru->node[nid].lock;
 	struct mem_cgroup *memcg;
+	struct lruvec *lruvec;
 
 	rcu_read_lock();
 	memcg = mem_cgroup_from_entry(entry);
@@ -390,6 +440,10 @@  static void zswap_lru_putback(struct list_lru *list_lru,
 	/* we cannot use list_lru_add here, because it increments node's lru count */
 	list_lru_putback(list_lru, &entry->lru, nid, memcg);
 	spin_unlock(lock);
+
+	lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry_to_nid(entry)));
+	/* increment the protection area to account for the LRU rotation. */
+	atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected);
 	rcu_read_unlock();
 }
 
@@ -479,6 +533,7 @@  static void zswap_free_entry(struct zswap_entry *entry)
 	else {
 		zswap_lru_del(&entry->pool->list_lru, entry);
 		zpool_free(zswap_find_zpool(entry), entry->handle);
+		atomic_dec(&entry->pool->nr_stored);
 		zswap_pool_put(entry->pool);
 	}
 	zswap_entry_cache_free(entry);
@@ -520,6 +575,95 @@  static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
 	return entry;
 }
 
+/*********************************
+* shrinker functions
+**********************************/
+static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,
+				       spinlock_t *lock, void *arg);
+
+static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
+		struct shrink_control *sc)
+{
+	struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
+	unsigned long shrink_ret, nr_protected, lru_size;
+	struct zswap_pool *pool = shrinker->private_data;
+	bool encountered_page_in_swapcache = false;
+
+	nr_protected =
+		atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
+	lru_size = list_lru_shrink_count(&pool->list_lru, sc);
+
+	/*
+	 * Abort if the shrinker is disabled or if we are shrinking into the
+	 * protected region.
+	 */
+	if (!zswap_shrinker_enabled || nr_protected >= lru_size - sc->nr_to_scan) {
+		sc->nr_scanned = 0;
+		return SHRINK_STOP;
+	}
+
+	shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
+		&encountered_page_in_swapcache);
+
+	if (encountered_page_in_swapcache)
+		return SHRINK_STOP;
+
+	return shrink_ret ? shrink_ret : SHRINK_STOP;
+}
+
+static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
+		struct shrink_control *sc)
+{
+	struct zswap_pool *pool = shrinker->private_data;
+	struct mem_cgroup *memcg = sc->memcg;
+	struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
+	unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
+
+#ifdef CONFIG_MEMCG_KMEM
+	cgroup_rstat_flush(memcg->css.cgroup);
+	nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
+	nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
+#else
+	/* use pool stats instead of memcg stats */
+	nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
+	nr_stored = atomic_read(&pool->nr_stored);
+#endif
+
+	if (!zswap_shrinker_enabled || !nr_stored)
+		return 0;
+
+	nr_protected =
+		atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
+	nr_freeable = list_lru_shrink_count(&pool->list_lru, sc);
+	/*
+	 * Subtract the lru size by an estimate of the number of pages
+	 * that should be protected.
+	 */
+	nr_freeable = nr_freeable > nr_protected ? nr_freeable - nr_protected : 0;
+
+	/*
+	 * Scale the number of freeable pages by the memory saving factor.
+	 * This ensures that the better zswap compresses memory, the fewer
+	 * pages we will evict to swap (as it will otherwise incur IO for
+	 * relatively small memory saving).
+	 */
+	return mult_frac(nr_freeable, nr_backing, nr_stored);
+}
+
+static void zswap_alloc_shrinker(struct zswap_pool *pool)
+{
+	pool->shrinker =
+		shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
+	if (!pool->shrinker)
+		return;
+
+	pool->shrinker->private_data = pool;
+	pool->shrinker->scan_objects = zswap_shrinker_scan;
+	pool->shrinker->count_objects = zswap_shrinker_count;
+	pool->shrinker->batch = 0;
+	pool->shrinker->seeks = DEFAULT_SEEKS;
+}
+
 /*********************************
 * per-cpu code
 **********************************/
@@ -715,6 +859,7 @@  static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
 				       spinlock_t *lock, void *arg)
 {
 	struct zswap_entry *entry = container_of(item, struct zswap_entry, lru);
+	bool *encountered_page_in_swapcache = (bool *)arg;
 	struct zswap_tree *tree;
 	pgoff_t swpoffset;
 	enum lru_status ret = LRU_REMOVED_RETRY;
@@ -750,6 +895,17 @@  static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
 		zswap_reject_reclaim_fail++;
 		zswap_lru_putback(&entry->pool->list_lru, entry);
 		ret = LRU_RETRY;
+
+		/*
+		 * Encountering a page already in swap cache is a sign that we are shrinking
+		 * into the warmer region. We should terminate shrinking (if we're in the dynamic
+		 * shrinker context).
+		 */
+		if (writeback_result == -EEXIST && encountered_page_in_swapcache) {
+			ret = LRU_SKIP;
+			*encountered_page_in_swapcache = true;
+		}
+
 		goto put_unlock;
 	}
 	zswap_written_back_pages++;
@@ -889,6 +1045,11 @@  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
 				       &pool->node);
 	if (ret)
 		goto error;
+
+	zswap_alloc_shrinker(pool);
+	if (!pool->shrinker)
+		goto error;
+
 	pr_debug("using %s compressor\n", pool->tfm_name);
 
 	/* being the current pool takes 1 ref; this func expects the
@@ -896,13 +1057,19 @@  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
 	 */
 	kref_init(&pool->kref);
 	INIT_LIST_HEAD(&pool->list);
-	list_lru_init_memcg(&pool->list_lru, NULL);
+	if (list_lru_init_memcg(&pool->list_lru, pool->shrinker))
+		goto lru_fail;
+	shrinker_register(pool->shrinker);
 	INIT_WORK(&pool->shrink_work, shrink_worker);
+	atomic_set(&pool->nr_stored, 0);
 
 	zswap_pool_debug("created", pool);
 
 	return pool;
 
+lru_fail:
+	list_lru_destroy(&pool->list_lru);
+	shrinker_free(pool->shrinker);
 error:
 	if (pool->acomp_ctx)
 		free_percpu(pool->acomp_ctx);
@@ -960,6 +1127,7 @@  static void zswap_pool_destroy(struct zswap_pool *pool)
 
 	zswap_pool_debug("destroying", pool);
 
+	shrinker_free(pool->shrinker);
 	cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
 	free_percpu(pool->acomp_ctx);
 	list_lru_destroy(&pool->list_lru);
@@ -1516,6 +1684,7 @@  bool zswap_store(struct folio *folio)
 	if (entry->length) {
 		INIT_LIST_HEAD(&entry->lru);
 		zswap_lru_add(&entry->pool->list_lru, entry);
+		atomic_inc(&entry->pool->nr_stored);
 	}
 	spin_unlock(&tree->lock);