zswap: shrink zswap pool based on memory pressure

Currently, we only shrink the zswap pool when the user-defined limit is
hit.  This means that if we set the limit too high, cold data that are
unlikely to be used again will reside in the pool, wasting precious
memory.  It is hard to predict how much zswap space will be needed ahead
of time, as this depends on the workload (specifically, on factors such as
memory access patterns and compressibility of the memory pages).

This patch implements a memcg- and NUMA-aware shrinker for zswap, that is
initiated when there is memory pressure.  The shrinker does not have any
parameter that must be tuned by the user, and can be opted in or out on a
per-memcg basis.

Furthermore, to make it more robust for many workloads and prevent
overshrinking (i.e evicting warm pages that might be refaulted into
memory), we build in the following heuristics:

* Estimate the number of warm pages residing in zswap, and attempt to
  protect this region of the zswap LRU.
* Scale the number of freeable objects by an estimate of the memory
  saving factor. The better zswap compresses the data, the fewer pages
  we will evict to swap (as we will otherwise incur IO for relatively
  small memory saving).
* During reclaim, if the shrinker encounters a page that is also being
  brought into memory, the shrinker will cautiously terminate its
  shrinking action, as this is a sign that it is touching the warmer
  region of the zswap LRU.

As a proof of concept, we ran the following synthetic benchmark: build the
linux kernel in a memory-limited cgroup, and allocate some cold data in
tmpfs to see if the shrinker could write them out and improved the overall
performance.  Depending on the amount of cold data generated, we observe
from 14% to 35% reduction in kernel CPU time used in the kernel builds.

[nphamcs@gmail.com: check shrinker enablement early, use less costly stat flushing]
  Link: https://lkml.kernel.org/r/20231206194456.3234203-1-nphamcs@gmail.com
Link: https://lkml.kernel.org/r/20231130194023.4102148-7-nphamcs@gmail.com
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/zswap.rst

@@ -153,6 +153,16 @@ 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
+
+This can be enabled at the boot time if ``CONFIG_ZSWAP_SHRINKER_DEFAULT_ON`` is
+selected.
+
 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.

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 */

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_page_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_state_init(struct lruvec *lruvec) {}
+static inline void zswap_page_swapin(struct page *page) {}
 #endif
 
 #endif /* _LINUX_ZSWAP_H */

mm/Kconfig

@@ -61,6 +61,20 @@ config ZSWAP_EXCLUSIVE_LOADS_DEFAULT_ON
 	  The cost is that if the page was never dirtied and needs to be
 	  swapped out again, it will be re-compressed.
 
+config ZSWAP_SHRINKER_DEFAULT_ON
+	bool "Shrink the zswap pool on memory pressure"
+	depends on ZSWAP
+	default n
+	help
+	  If selected, the zswap shrinker will be enabled, and the pages
+	  stored in the zswap pool will become available for reclaim (i.e
+	  written back to the backing swap device) on memory pressure.
+
+	  This means that zswap writeback could happen even if the pool is
+	  not yet full, or the cgroup zswap limit has not been reached,
+	  reducing the chance that cold pages will reside in the zswap pool
+	  and consume memory indefinitely.
+
 choice
 	prompt "Default compressor"
 	depends on ZSWAP

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]);

mm/swap_state.c

@@ -687,6 +687,7 @@ skip:
 					&page_allocated, false);
 	if (unlikely(page_allocated))
 		swap_readpage(page, false, NULL);
+	zswap_page_swapin(page);
 	return page;
 }
 
@@ -862,6 +863,7 @@ skip:
 					&page_allocated, false);
 	if (unlikely(page_allocated))
 		swap_readpage(page, false, NULL);
+	zswap_page_swapin(page);
 	return page;
 }
 

mm/zswap.c

@@ -148,6 +148,11 @@ 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 = IS_ENABLED(
+		CONFIG_ZSWAP_SHRINKER_DEFAULT_ON);
+module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
+
 /*********************************
 * data structures
 **********************************/
@@ -177,6 +182,8 @@ 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 +282,26 @@ 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();
 
@@ -344,13 +360,34 @@ 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_page_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
@@ -368,6 +405,19 @@ 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();
 }
 
@@ -389,6 +439,7 @@ 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);
@@ -396,6 +447,10 @@ 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();
 }
 
@@ -485,6 +540,7 @@ 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);
@@ -526,6 +582,109 @@ 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;
+
+	if (!zswap_shrinker_enabled) {
+		sc->nr_scanned = 0;
+		return SHRINK_STOP;
+	}
+
+	nr_protected =
+		atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
+	lru_size = list_lru_shrink_count(&pool->list_lru, sc);
+
+	/*
+	 * Abort if we are shrinking into the protected region.
+	 *
+	 * This short-circuiting is necessary because if we have too many multiple
+	 * concurrent reclaimers getting the freeable zswap object counts at the
+	 * same time (before any of them made reasonable progress), the total
+	 * number of reclaimed objects might be more than the number of unprotected
+	 * objects (i.e the reclaimers will reclaim into the protected area of the
+	 * zswap LRU).
+	 */
+	if (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;
+
+	if (!zswap_shrinker_enabled)
+		return 0;
+
+#ifdef CONFIG_MEMCG_KMEM
+	mem_cgroup_flush_stats();
+	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 (!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
 **********************************/
@@ -721,6 +880,7 @@ 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;
@@ -756,6 +916,17 @@ 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++;
@@ -914,6 +1085,11 @@ 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
@@ -921,13 +1097,19 @@ 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);
@@ -985,6 +1167,7 @@ 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);
@@ -1536,6 +1719,7 @@ insert_entry:
 	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);