[PATCH v3 0/5] Implement writeback for zsmalloc

[PATCH v3 0/5] Implement writeback for zsmalloc

[Nhat Pham]

Changelog:
v3:
  * Set pool->ops = NULL when pool->zpool_ops is null (patch 4).
  * Stop holding pool's lock when calling lock_zspage() (patch 5).
    (suggested by Sergey Senozhatsky)
  * Stop holding pool's lock when checking pool->ops and retries.
    (patch 5) (suggested by Sergey Senozhatsky)
  * Fix formatting issues (.shrink, extra spaces in casting removed).
    (patch 5) (suggested by Sergey Senozhatsky)

v2:
  * Add missing CONFIG_ZPOOL ifdefs (patch 5)
    (detected by kernel test robot).

Unlike other zswap's allocators such as zbud or z3fold, zsmalloc
currently lacks the writeback mechanism. This means that when the zswap
pool is full, it will simply reject further allocations, and the pages
will be written directly to swap.

This series of patches implements writeback for zsmalloc. When the zswap
pool becomes full, zsmalloc will attempt to evict all the compressed
objects in the least-recently used zspages.

There are 5 patches in this series:

Johannes Weiner (1):
  zswap: fix writeback lock ordering for zsmalloc

Nhat Pham (4):
  zsmalloc: Consolidate zs_pool's migrate_lock and size_class's locks
  zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order
  zsmalloc: Add ops fields to zs_pool to store evict handlers
  zsmalloc: Implement writeback mechanism for zsmalloc

 mm/zsmalloc.c | 346 +++++++++++++++++++++++++++++++++++++++++---------
 mm/zswap.c    |  37 +++---
 2 files changed, 303 insertions(+), 80 deletions(-)

--
2.30.2

[PATCH v3 1/5] zswap: fix writeback lock ordering for zsmalloc

[Nhat Pham]

From: Johannes Weiner <hannes@cmpxchg.org>

zswap's customary lock order is tree->lock before pool->lock, because
the tree->lock protects the entries' refcount, and the free callbacks in
the backends acquire their respective pool locks to dispatch the backing
object. zsmalloc's map callback takes the pool lock, so zswap must not
grab the tree->lock while a handle is mapped. This currently only
happens during writeback, which isn't implemented for zsmalloc. In
preparation for it, move the tree->lock section out of the mapped entry
section

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/zswap.c | 37 ++++++++++++++++++++-----------------
 1 file changed, 20 insertions(+), 17 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 2d48fd59cc7a..2d69c1d678fe 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -958,7 +958,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 	};

 	if (!zpool_can_sleep_mapped(pool)) {
-		tmp = kmalloc(PAGE_SIZE, GFP_ATOMIC);
+		tmp = kmalloc(PAGE_SIZE, GFP_KERNEL);
 		if (!tmp)
 			return -ENOMEM;
 	}
@@ -968,6 +968,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 	swpentry = zhdr->swpentry; /* here */
 	tree = zswap_trees[swp_type(swpentry)];
 	offset = swp_offset(swpentry);
+	zpool_unmap_handle(pool, handle);

 	/* find and ref zswap entry */
 	spin_lock(&tree->lock);
@@ -975,20 +976,12 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 	if (!entry) {
 		/* entry was invalidated */
 		spin_unlock(&tree->lock);
-		zpool_unmap_handle(pool, handle);
 		kfree(tmp);
 		return 0;
 	}
 	spin_unlock(&tree->lock);
 	BUG_ON(offset != entry->offset);

-	src = (u8 *)zhdr + sizeof(struct zswap_header);
-	if (!zpool_can_sleep_mapped(pool)) {
-		memcpy(tmp, src, entry->length);
-		src = tmp;
-		zpool_unmap_handle(pool, handle);
-	}
-
 	/* try to allocate swap cache page */
 	switch (zswap_get_swap_cache_page(swpentry, &page)) {
 	case ZSWAP_SWAPCACHE_FAIL: /* no memory or invalidate happened */
@@ -1006,6 +999,14 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 		acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
 		dlen = PAGE_SIZE;

+		zhdr = zpool_map_handle(pool, handle, ZPOOL_MM_RO);
+		src = (u8 *)zhdr + sizeof(struct zswap_header);
+		if (!zpool_can_sleep_mapped(pool)) {
+			memcpy(tmp, src, entry->length);
+			src = tmp;
+			zpool_unmap_handle(pool, handle);
+		}
+
 		mutex_lock(acomp_ctx->mutex);
 		sg_init_one(&input, src, entry->length);
 		sg_init_table(&output, 1);
@@ -1015,6 +1016,11 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 		dlen = acomp_ctx->req->dlen;
 		mutex_unlock(acomp_ctx->mutex);

+		if (!zpool_can_sleep_mapped(pool))
+			kfree(tmp);
+		else
+			zpool_unmap_handle(pool, handle);
+
 		BUG_ON(ret);
 		BUG_ON(dlen != PAGE_SIZE);

@@ -1045,7 +1051,11 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 		zswap_entry_put(tree, entry);
 	spin_unlock(&tree->lock);

-	goto end;
+	return ret;
+
+fail:
+	if (!zpool_can_sleep_mapped(pool))
+		kfree(tmp);

 	/*
 	* if we get here due to ZSWAP_SWAPCACHE_EXIST
@@ -1054,17 +1064,10 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 	* if we free the entry in the following put
 	* it is also okay to return !0
 	*/
-fail:
 	spin_lock(&tree->lock);
 	zswap_entry_put(tree, entry);
 	spin_unlock(&tree->lock);

-end:
-	if (zpool_can_sleep_mapped(pool))
-		zpool_unmap_handle(pool, handle);
-	else
-		kfree(tmp);
-
 	return ret;
 }

--
2.30.2

[PATCH v3 2/5] zsmalloc: Consolidate zs_pool's migrate_lock and size_class's locks

[Nhat Pham]

Currently, zsmalloc has a hierarchy of locks, which includes a
pool-level migrate_lock, and a lock for each size class. We have to
obtain both locks in the hotpath in most cases anyway, except for
zs_malloc. This exception will no longer exist when we introduce a LRU
into the zs_pool for the new writeback functionality - we will need to
obtain a pool-level lock to synchronize LRU handling even in zs_malloc.

In preparation for zsmalloc writeback, consolidate these locks into a
single pool-level lock, which drastically reduces the complexity of
synchronization in zsmalloc.

We have also benchmarked the lock consolidation to see the performance
effect of this change on zram.

First, we ran a synthetic FS workload on a server machine with 36 cores
(same machine for all runs), using

fs_mark  -d  ../zram1mnt  -s  100000  -n  2500  -t  32  -k

before and after for btrfs and ext4 on zram (FS usage is 80%).

Here is the result (unit is file/second):

With lock consolidation (btrfs):
Average: 13520.2, Median: 13531.0, Stddev: 137.5961482019028

Without lock consolidation (btrfs):
Average: 13487.2, Median: 13575.0, Stddev: 309.08283679298665

With lock consolidation (ext4):
Average: 16824.4, Median: 16839.0, Stddev: 89.97388510006668

Without lock consolidation (ext4)
Average: 16958.0, Median: 16986.0, Stddev: 194.7370021336469

As you can see, we observe a 0.3% regression for btrfs, and a 0.9%
regression for ext4. This is a small, barely measurable difference in my
opinion.

For a more realistic scenario, we also tries building the kernel on zram.
Here is the time it takes (in seconds):

With lock consolidation (btrfs):
real
Average: 319.6, Median: 320.0, Stddev: 0.8944271909999159
user
Average: 6894.2, Median: 6895.0, Stddev: 25.528415540334656
sys
Average: 521.4, Median: 522.0, Stddev: 1.51657508881031

Without lock consolidation (btrfs):
real
Average: 319.8, Median: 320.0, Stddev: 0.8366600265340756
user
Average: 6896.6, Median: 6899.0, Stddev: 16.04057355583023
sys
Average: 520.6, Median: 521.0, Stddev: 1.140175425099138

With lock consolidation (ext4):
real
Average: 320.0, Median: 319.0, Stddev: 1.4142135623730951
user
Average: 6896.8, Median: 6878.0, Stddev: 28.621670111997307
sys
Average: 521.2, Median: 521.0, Stddev: 1.7888543819998317

Without lock consolidation (ext4)
real
Average: 319.6, Median: 319.0, Stddev: 0.8944271909999159
user
Average: 6886.2, Median: 6887.0, Stddev: 16.93221781102523
sys
Average: 520.4, Median: 520.0, Stddev: 1.140175425099138

The difference is entirely within the noise of a typical run on zram. This
hardly justifies the complexity of maintaining both the pool lock and
the class lock. In fact, for writeback, we would need to introduce yet
another lock to prevent data races on the pool's LRU, further
complicating the lock handling logic. IMHO, it is just better to
collapse all of these into a single pool-level lock.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/zsmalloc.c | 87 ++++++++++++++++++++++-----------------------------
 1 file changed, 37 insertions(+), 50 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index d03941cace2c..326faa751f0a 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -33,8 +33,7 @@
 /*
  * lock ordering:
  *	page_lock
- *	pool->migrate_lock
- *	class->lock
+ *	pool->lock
  *	zspage->lock
  */

@@ -192,7 +191,6 @@ static const int fullness_threshold_frac = 4;
 static size_t huge_class_size;

 struct size_class {
-	spinlock_t lock;
 	struct list_head fullness_list[NR_ZS_FULLNESS];
 	/*
 	 * Size of objects stored in this class. Must be multiple
@@ -247,8 +245,7 @@ struct zs_pool {
 #ifdef CONFIG_COMPACTION
 	struct work_struct free_work;
 #endif
-	/* protect page/zspage migration */
-	rwlock_t migrate_lock;
+	spinlock_t lock;
 };

 struct zspage {
@@ -355,7 +352,7 @@ static void cache_free_zspage(struct zs_pool *pool, struct zspage *zspage)
 	kmem_cache_free(pool->zspage_cachep, zspage);
 }

-/* class->lock(which owns the handle) synchronizes races */
+/* pool->lock(which owns the handle) synchronizes races */
 static void record_obj(unsigned long handle, unsigned long obj)
 {
 	*(unsigned long *)handle = obj;
@@ -452,7 +449,7 @@ static __maybe_unused int is_first_page(struct page *page)
 	return PagePrivate(page);
 }

-/* Protected by class->lock */
+/* Protected by pool->lock */
 static inline int get_zspage_inuse(struct zspage *zspage)
 {
 	return zspage->inuse;
@@ -597,13 +594,13 @@ static int zs_stats_size_show(struct seq_file *s, void *v)
 		if (class->index != i)
 			continue;

-		spin_lock(&class->lock);
+		spin_lock(&pool->lock);
 		class_almost_full = zs_stat_get(class, CLASS_ALMOST_FULL);
 		class_almost_empty = zs_stat_get(class, CLASS_ALMOST_EMPTY);
 		obj_allocated = zs_stat_get(class, OBJ_ALLOCATED);
 		obj_used = zs_stat_get(class, OBJ_USED);
 		freeable = zs_can_compact(class);
-		spin_unlock(&class->lock);
+		spin_unlock(&pool->lock);

 		objs_per_zspage = class->objs_per_zspage;
 		pages_used = obj_allocated / objs_per_zspage *
@@ -916,7 +913,7 @@ static void __free_zspage(struct zs_pool *pool, struct size_class *class,

 	get_zspage_mapping(zspage, &class_idx, &fg);

-	assert_spin_locked(&class->lock);
+	assert_spin_locked(&pool->lock);

 	VM_BUG_ON(get_zspage_inuse(zspage));
 	VM_BUG_ON(fg != ZS_EMPTY);
@@ -1247,19 +1244,19 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
 	BUG_ON(in_interrupt());

 	/* It guarantees it can get zspage from handle safely */
-	read_lock(&pool->migrate_lock);
+	spin_lock(&pool->lock);
 	obj = handle_to_obj(handle);
 	obj_to_location(obj, &page, &obj_idx);
 	zspage = get_zspage(page);

 	/*
-	 * migration cannot move any zpages in this zspage. Here, class->lock
+	 * migration cannot move any zpages in this zspage. Here, pool->lock
 	 * is too heavy since callers would take some time until they calls
 	 * zs_unmap_object API so delegate the locking from class to zspage
 	 * which is smaller granularity.
 	 */
 	migrate_read_lock(zspage);
-	read_unlock(&pool->migrate_lock);
+	spin_unlock(&pool->lock);

 	class = zspage_class(pool, zspage);
 	off = (class->size * obj_idx) & ~PAGE_MASK;
@@ -1412,8 +1409,8 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
 	size += ZS_HANDLE_SIZE;
 	class = pool->size_class[get_size_class_index(size)];

-	/* class->lock effectively protects the zpage migration */
-	spin_lock(&class->lock);
+	/* pool->lock effectively protects the zpage migration */
+	spin_lock(&pool->lock);
 	zspage = find_get_zspage(class);
 	if (likely(zspage)) {
 		obj = obj_malloc(pool, zspage, handle);
@@ -1421,12 +1418,12 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
 		fix_fullness_group(class, zspage);
 		record_obj(handle, obj);
 		class_stat_inc(class, OBJ_USED, 1);
-		spin_unlock(&class->lock);
+		spin_unlock(&pool->lock);

 		return handle;
 	}

-	spin_unlock(&class->lock);
+	spin_unlock(&pool->lock);

 	zspage = alloc_zspage(pool, class, gfp);
 	if (!zspage) {
@@ -1434,7 +1431,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
 		return (unsigned long)ERR_PTR(-ENOMEM);
 	}

-	spin_lock(&class->lock);
+	spin_lock(&pool->lock);
 	obj = obj_malloc(pool, zspage, handle);
 	newfg = get_fullness_group(class, zspage);
 	insert_zspage(class, zspage, newfg);
@@ -1447,7 +1444,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)

 	/* We completely set up zspage so mark them as movable */
 	SetZsPageMovable(pool, zspage);
-	spin_unlock(&class->lock);
+	spin_unlock(&pool->lock);

 	return handle;
 }
@@ -1491,16 +1488,14 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
 		return;

 	/*
-	 * The pool->migrate_lock protects the race with zpage's migration
+	 * The pool->lock protects the race with zpage's migration
 	 * so it's safe to get the page from handle.
 	 */
-	read_lock(&pool->migrate_lock);
+	spin_lock(&pool->lock);
 	obj = handle_to_obj(handle);
 	obj_to_page(obj, &f_page);
 	zspage = get_zspage(f_page);
 	class = zspage_class(pool, zspage);
-	spin_lock(&class->lock);
-	read_unlock(&pool->migrate_lock);

 	obj_free(class->size, obj);
 	class_stat_dec(class, OBJ_USED, 1);
@@ -1510,7 +1505,7 @@ void zs_free(struct zs_pool *pool, unsigned long handle)

 	free_zspage(pool, class, zspage);
 out:
-	spin_unlock(&class->lock);
+	spin_unlock(&pool->lock);
 	cache_free_handle(pool, handle);
 }
 EXPORT_SYMBOL_GPL(zs_free);
@@ -1867,16 +1862,12 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 	pool = zspage->pool;

 	/*
-	 * The pool migrate_lock protects the race between zpage migration
+	 * The pool's lock protects the race between zpage migration
 	 * and zs_free.
 	 */
-	write_lock(&pool->migrate_lock);
+	spin_lock(&pool->lock);
 	class = zspage_class(pool, zspage);

-	/*
-	 * the class lock protects zpage alloc/free in the zspage.
-	 */
-	spin_lock(&class->lock);
 	/* the migrate_write_lock protects zpage access via zs_map_object */
 	migrate_write_lock(zspage);

@@ -1906,10 +1897,9 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 	replace_sub_page(class, zspage, newpage, page);
 	/*
 	 * Since we complete the data copy and set up new zspage structure,
-	 * it's okay to release migration_lock.
+	 * it's okay to release the pool's lock.
 	 */
-	write_unlock(&pool->migrate_lock);
-	spin_unlock(&class->lock);
+	spin_unlock(&pool->lock);
 	dec_zspage_isolation(zspage);
 	migrate_write_unlock(zspage);

@@ -1964,9 +1954,9 @@ static void async_free_zspage(struct work_struct *work)
 		if (class->index != i)
 			continue;

-		spin_lock(&class->lock);
+		spin_lock(&pool->lock);
 		list_splice_init(&class->fullness_list[ZS_EMPTY], &free_pages);
-		spin_unlock(&class->lock);
+		spin_unlock(&pool->lock);
 	}

 	list_for_each_entry_safe(zspage, tmp, &free_pages, list) {
@@ -1976,9 +1966,9 @@ static void async_free_zspage(struct work_struct *work)
 		get_zspage_mapping(zspage, &class_idx, &fullness);
 		VM_BUG_ON(fullness != ZS_EMPTY);
 		class = pool->size_class[class_idx];
-		spin_lock(&class->lock);
+		spin_lock(&pool->lock);
 		__free_zspage(pool, class, zspage);
-		spin_unlock(&class->lock);
+		spin_unlock(&pool->lock);
 	}
 };

@@ -2039,10 +2029,11 @@ static unsigned long __zs_compact(struct zs_pool *pool,
 	struct zspage *dst_zspage = NULL;
 	unsigned long pages_freed = 0;

-	/* protect the race between zpage migration and zs_free */
-	write_lock(&pool->migrate_lock);
-	/* protect zpage allocation/free */
-	spin_lock(&class->lock);
+	/*
+	 * protect the race between zpage migration and zs_free
+	 * as well as zpage allocation/free
+	 */
+	spin_lock(&pool->lock);
 	while ((src_zspage = isolate_zspage(class, true))) {
 		/* protect someone accessing the zspage(i.e., zs_map_object) */
 		migrate_write_lock(src_zspage);
@@ -2067,7 +2058,7 @@ static unsigned long __zs_compact(struct zs_pool *pool,
 			putback_zspage(class, dst_zspage);
 			migrate_write_unlock(dst_zspage);
 			dst_zspage = NULL;
-			if (rwlock_is_contended(&pool->migrate_lock))
+			if (spin_is_contended(&pool->lock))
 				break;
 		}

@@ -2084,11 +2075,9 @@ static unsigned long __zs_compact(struct zs_pool *pool,
 			pages_freed += class->pages_per_zspage;
 		} else
 			migrate_write_unlock(src_zspage);
-		spin_unlock(&class->lock);
-		write_unlock(&pool->migrate_lock);
+		spin_unlock(&pool->lock);
 		cond_resched();
-		write_lock(&pool->migrate_lock);
-		spin_lock(&class->lock);
+		spin_lock(&pool->lock);
 	}

 	if (src_zspage) {
@@ -2096,8 +2085,7 @@ static unsigned long __zs_compact(struct zs_pool *pool,
 		migrate_write_unlock(src_zspage);
 	}

-	spin_unlock(&class->lock);
-	write_unlock(&pool->migrate_lock);
+	spin_unlock(&pool->lock);

 	return pages_freed;
 }
@@ -2200,7 +2188,7 @@ struct zs_pool *zs_create_pool(const char *name)
 		return NULL;

 	init_deferred_free(pool);
-	rwlock_init(&pool->migrate_lock);
+	spin_lock_init(&pool->lock);

 	pool->name = kstrdup(name, GFP_KERNEL);
 	if (!pool->name)
@@ -2271,7 +2259,6 @@ struct zs_pool *zs_create_pool(const char *name)
 		class->index = i;
 		class->pages_per_zspage = pages_per_zspage;
 		class->objs_per_zspage = objs_per_zspage;
-		spin_lock_init(&class->lock);
 		pool->size_class[i] = class;
 		for (fullness = ZS_EMPTY; fullness < NR_ZS_FULLNESS;
 							fullness++)
--
2.30.2

[PATCH v3 3/5] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Nhat Pham]

This helps determines the coldest zspages as candidates for writeback.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/zsmalloc.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 326faa751f0a..600c40121544 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -239,6 +239,9 @@ struct zs_pool {
 	/* Compact classes */
 	struct shrinker shrinker;

+	/* List tracking the zspages in LRU order by most recently added object */
+	struct list_head lru;
+
 #ifdef CONFIG_ZSMALLOC_STAT
 	struct dentry *stat_dentry;
 #endif
@@ -260,6 +263,10 @@ struct zspage {
 	unsigned int freeobj;
 	struct page *first_page;
 	struct list_head list; /* fullness list */
+
+	/* links the zspage to the lru list in the pool */
+	struct list_head lru;
+
 	struct zs_pool *pool;
 #ifdef CONFIG_COMPACTION
 	rwlock_t lock;
@@ -352,6 +359,16 @@ static void cache_free_zspage(struct zs_pool *pool, struct zspage *zspage)
 	kmem_cache_free(pool->zspage_cachep, zspage);
 }

+/* Moves the zspage to the front of the zspool's LRU */
+static void move_to_front(struct zs_pool *pool, struct zspage *zspage)
+{
+	assert_spin_locked(&pool->lock);
+
+	if (!list_empty(&zspage->lru))
+		list_del(&zspage->lru);
+	list_add(&zspage->lru, &pool->lru);
+}
+
 /* pool->lock(which owns the handle) synchronizes races */
 static void record_obj(unsigned long handle, unsigned long obj)
 {
@@ -953,6 +970,7 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
 	}

 	remove_zspage(class, zspage, ZS_EMPTY);
+	list_del(&zspage->lru);
 	__free_zspage(pool, class, zspage);
 }

@@ -998,6 +1016,8 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
 		off %= PAGE_SIZE;
 	}

+	INIT_LIST_HEAD(&zspage->lru);
+
 	set_freeobj(zspage, 0);
 }

@@ -1418,6 +1438,8 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
 		fix_fullness_group(class, zspage);
 		record_obj(handle, obj);
 		class_stat_inc(class, OBJ_USED, 1);
+		/* Move the zspage to front of pool's LRU */
+		move_to_front(pool, zspage);
 		spin_unlock(&pool->lock);

 		return handle;
@@ -1444,6 +1466,8 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)

 	/* We completely set up zspage so mark them as movable */
 	SetZsPageMovable(pool, zspage);
+	/* Move the zspage to front of pool's LRU */
+	move_to_front(pool, zspage);
 	spin_unlock(&pool->lock);

 	return handle;
@@ -1967,6 +1991,7 @@ static void async_free_zspage(struct work_struct *work)
 		VM_BUG_ON(fullness != ZS_EMPTY);
 		class = pool->size_class[class_idx];
 		spin_lock(&pool->lock);
+		list_del(&zspage->lru);
 		__free_zspage(pool, class, zspage);
 		spin_unlock(&pool->lock);
 	}
@@ -2278,6 +2303,8 @@ struct zs_pool *zs_create_pool(const char *name)
 	 */
 	zs_register_shrinker(pool);

+	INIT_LIST_HEAD(&pool->lru);
+
 	return pool;

 err:
--
2.30.2

[PATCH v3 4/5] zsmalloc: Add ops fields to zs_pool to store evict handlers

[Nhat Pham]

This adds fields to zs_pool to store evict handlers for writeback,
analogous to the zbud allocator.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/zsmalloc.c | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 600c40121544..ac86cffa62cd 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -225,6 +225,12 @@ struct link_free {
 	};
 };

+struct zs_pool;
+
+struct zs_ops {
+	int (*evict)(struct zs_pool *pool, unsigned long handle);
+};
+
 struct zs_pool {
 	const char *name;

@@ -242,6 +248,12 @@ struct zs_pool {
 	/* List tracking the zspages in LRU order by most recently added object */
 	struct list_head lru;

+#ifdef CONFIG_ZPOOL
+	const struct zs_ops *ops;
+	struct zpool *zpool;
+	const struct zpool_ops *zpool_ops;
+#endif
+
 #ifdef CONFIG_ZSMALLOC_STAT
 	struct dentry *stat_dentry;
 #endif
@@ -379,6 +391,18 @@ static void record_obj(unsigned long handle, unsigned long obj)

 #ifdef CONFIG_ZPOOL

+static int zs_zpool_evict(struct zs_pool *pool, unsigned long handle)
+{
+	if (pool->zpool && pool->zpool_ops && pool->zpool_ops->evict)
+		return pool->zpool_ops->evict(pool->zpool, handle);
+	else
+		return -ENOENT;
+}
+
+static const struct zs_ops zs_zpool_ops = {
+	.evict =	zs_zpool_evict
+};
+
 static void *zs_zpool_create(const char *name, gfp_t gfp,
 			     const struct zpool_ops *zpool_ops,
 			     struct zpool *zpool)
@@ -388,7 +412,19 @@ static void *zs_zpool_create(const char *name, gfp_t gfp,
 	 * different contexts and its caller must provide a valid
 	 * gfp mask.
 	 */
-	return zs_create_pool(name);
+	struct zs_pool *pool = zs_create_pool(name);
+
+	if (pool) {
+		pool->zpool = zpool;
+		pool->zpool_ops = zpool_ops;
+
+		if (zpool_ops)
+			pool->ops = &zs_zpool_ops;
+		else
+			pool->ops = NULL;
+	}
+
+	return pool;
 }

 static void zs_zpool_destroy(void *pool)
--
2.30.2

[PATCH v3 5/5] zsmalloc: Implement writeback mechanism for zsmalloc

[Nhat Pham]

This commit adds the writeback mechanism for zsmalloc, analogous to the
zbud allocator. Zsmalloc will attempt to determine the coldest zspage
(i.e least recently used) in the pool, and attempt to write back all the
stored compressed objects via the pool's evict handler.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/zsmalloc.c | 200 ++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 185 insertions(+), 15 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index ac86cffa62cd..3868ad3cd038 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -279,10 +279,13 @@ struct zspage {
 	/* links the zspage to the lru list in the pool */
 	struct list_head lru;

+	bool under_reclaim;
+
+	/* list of unfreed handles whose objects have been reclaimed */
+	unsigned long *deferred_handles;
+
 	struct zs_pool *pool;
-#ifdef CONFIG_COMPACTION
 	rwlock_t lock;
-#endif
 };

 struct mapping_area {
@@ -303,10 +306,11 @@ static bool ZsHugePage(struct zspage *zspage)
 	return zspage->huge;
 }

-#ifdef CONFIG_COMPACTION
 static void migrate_lock_init(struct zspage *zspage);
 static void migrate_read_lock(struct zspage *zspage);
 static void migrate_read_unlock(struct zspage *zspage);
+
+#ifdef CONFIG_COMPACTION
 static void migrate_write_lock(struct zspage *zspage);
 static void migrate_write_lock_nested(struct zspage *zspage);
 static void migrate_write_unlock(struct zspage *zspage);
@@ -314,9 +318,6 @@ static void kick_deferred_free(struct zs_pool *pool);
 static void init_deferred_free(struct zs_pool *pool);
 static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage);
 #else
-static void migrate_lock_init(struct zspage *zspage) {}
-static void migrate_read_lock(struct zspage *zspage) {}
-static void migrate_read_unlock(struct zspage *zspage) {}
 static void migrate_write_lock(struct zspage *zspage) {}
 static void migrate_write_lock_nested(struct zspage *zspage) {}
 static void migrate_write_unlock(struct zspage *zspage) {}
@@ -446,6 +447,27 @@ static void zs_zpool_free(void *pool, unsigned long handle)
 	zs_free(pool, handle);
 }

+static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries);
+
+static int zs_zpool_shrink(void *pool, unsigned int pages,
+			unsigned int *reclaimed)
+{
+	unsigned int total = 0;
+	int ret = -EINVAL;
+
+	while (total < pages) {
+		ret = zs_reclaim_page(pool, 8);
+		if (ret < 0)
+			break;
+		total++;
+	}
+
+	if (reclaimed)
+		*reclaimed = total;
+
+	return ret;
+}
+
 static void *zs_zpool_map(void *pool, unsigned long handle,
 			enum zpool_mapmode mm)
 {
@@ -484,6 +506,7 @@ static struct zpool_driver zs_zpool_driver = {
 	.malloc_support_movable = true,
 	.malloc =		  zs_zpool_malloc,
 	.free =			  zs_zpool_free,
+	.shrink =		  zs_zpool_shrink,
 	.map =			  zs_zpool_map,
 	.unmap =		  zs_zpool_unmap,
 	.total_size =		  zs_zpool_total_size,
@@ -957,6 +980,21 @@ static int trylock_zspage(struct zspage *zspage)
 	return 0;
 }

+/*
+ * Free all the deferred handles whose objects are freed in zs_free.
+ */
+static void free_handles(struct zs_pool *pool, struct zspage *zspage)
+{
+	unsigned long handle = (unsigned long)zspage->deferred_handles;
+
+	while (handle) {
+		unsigned long nxt_handle = handle_to_obj(handle);
+
+		cache_free_handle(pool, handle);
+		handle = nxt_handle;
+	}
+}
+
 static void __free_zspage(struct zs_pool *pool, struct size_class *class,
 				struct zspage *zspage)
 {
@@ -971,6 +1009,9 @@ static void __free_zspage(struct zs_pool *pool, struct size_class *class,
 	VM_BUG_ON(get_zspage_inuse(zspage));
 	VM_BUG_ON(fg != ZS_EMPTY);

+	/* Free all deferred handles from zs_free */
+	free_handles(pool, zspage);
+
 	next = page = get_first_page(zspage);
 	do {
 		VM_BUG_ON_PAGE(!PageLocked(page), page);
@@ -1053,6 +1094,8 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
 	}

 	INIT_LIST_HEAD(&zspage->lru);
+	zspage->under_reclaim = false;
+	zspage->deferred_handles = NULL;

 	set_freeobj(zspage, 0);
 }
@@ -1474,11 +1517,8 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
 		fix_fullness_group(class, zspage);
 		record_obj(handle, obj);
 		class_stat_inc(class, OBJ_USED, 1);
-		/* Move the zspage to front of pool's LRU */
-		move_to_front(pool, zspage);
-		spin_unlock(&pool->lock);

-		return handle;
+		goto out;
 	}

 	spin_unlock(&pool->lock);
@@ -1502,6 +1542,8 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)

 	/* We completely set up zspage so mark them as movable */
 	SetZsPageMovable(pool, zspage);
+
+out:
 	/* Move the zspage to front of pool's LRU */
 	move_to_front(pool, zspage);
 	spin_unlock(&pool->lock);
@@ -1559,12 +1601,24 @@ void zs_free(struct zs_pool *pool, unsigned long handle)

 	obj_free(class->size, obj);
 	class_stat_dec(class, OBJ_USED, 1);
+
+	if (zspage->under_reclaim) {
+		/*
+		 * Reclaim needs the handles during writeback. It'll free
+		 * them along with the zspage when it's done with them.
+		 *
+		 * Record current deferred handle at the memory location
+		 * whose address is given by handle.
+		 */
+		record_obj(handle, (unsigned long)zspage->deferred_handles);
+		zspage->deferred_handles = (unsigned long *)handle;
+		spin_unlock(&pool->lock);
+		return;
+	}
 	fullness = fix_fullness_group(class, zspage);
-	if (fullness != ZS_EMPTY)
-		goto out;
+	if (fullness == ZS_EMPTY)
+		free_zspage(pool, class, zspage);

-	free_zspage(pool, class, zspage);
-out:
 	spin_unlock(&pool->lock);
 	cache_free_handle(pool, handle);
 }
@@ -1764,7 +1818,7 @@ static enum fullness_group putback_zspage(struct size_class *class,
 	return fullness;
 }

-#ifdef CONFIG_COMPACTION
+#if defined(CONFIG_ZPOOL) || defined(CONFIG_COMPACTION)
 /*
  * To prevent zspage destroy during migration, zspage freeing should
  * hold locks of all pages in the zspage.
@@ -1806,6 +1860,24 @@ static void lock_zspage(struct zspage *zspage)
 	}
 	migrate_read_unlock(zspage);
 }
+#endif /* defined(CONFIG_ZPOOL) || defined(CONFIG_COMPACTION) */
+
+#ifdef CONFIG_ZPOOL
+/*
+ * Unlocks all the pages of the zspage.
+ *
+ * pool->lock must be held before this function is called
+ * to prevent the underlying pages from migrating.
+ */
+static void unlock_zspage(struct zspage *zspage)
+{
+	struct page *page = get_first_page(zspage);
+
+	do {
+		unlock_page(page);
+	} while ((page = get_next_page(page)) != NULL);
+}
+#endif /* CONFIG_ZPOOL */

 static void migrate_lock_init(struct zspage *zspage)
 {
@@ -1822,6 +1894,7 @@ static void migrate_read_unlock(struct zspage *zspage) __releases(&zspage->lock)
 	read_unlock(&zspage->lock);
 }

+#ifdef CONFIG_COMPACTION
 static void migrate_write_lock(struct zspage *zspage)
 {
 	write_lock(&zspage->lock);
@@ -2382,6 +2455,103 @@ void zs_destroy_pool(struct zs_pool *pool)
 }
 EXPORT_SYMBOL_GPL(zs_destroy_pool);

+#ifdef CONFIG_ZPOOL
+static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries)
+{
+	int i, obj_idx, ret = 0;
+	unsigned long handle;
+	struct zspage *zspage;
+	struct page *page;
+	enum fullness_group fullness;
+
+	if (retries == 0 || !pool->ops || !pool->ops->evict)
+		return -EINVAL;
+
+	/* Lock LRU and fullness list */
+	spin_lock(&pool->lock);
+	if (list_empty(&pool->lru)) {
+		spin_unlock(&pool->lock);
+		return -EINVAL;
+	}
+
+	for (i = 0; i < retries; i++) {
+		struct size_class *class;
+
+		zspage = list_last_entry(&pool->lru, struct zspage, lru);
+		list_del(&zspage->lru);
+
+		/* zs_free may free objects, but not the zspage and handles */
+		zspage->under_reclaim = true;
+
+		class = zspage_class(pool, zspage);
+		fullness = get_fullness_group(class, zspage);
+
+		/* Lock out object allocations and object compaction */
+		remove_zspage(class, zspage, fullness);
+
+		spin_unlock(&pool->lock);
+
+		/* Lock backing pages into place */
+		lock_zspage(zspage);
+
+		obj_idx = 0;
+		page = zspage->first_page;
+		while (1) {
+			handle = find_alloced_obj(class, page, &obj_idx);
+			if (!handle) {
+				page = get_next_page(page);
+				if (!page)
+					break;
+				obj_idx = 0;
+				continue;
+			}
+
+			/*
+			 * This will write the object and call
+			 * zs_free.
+			 *
+			 * zs_free will free the object, but the
+			 * under_reclaim flag prevents it from freeing
+			 * the zspage altogether. This is necessary so
+			 * that we can continue working with the
+			 * zspage potentially after the last object
+			 * has been freed.
+			 */
+			ret = pool->ops->evict(pool, handle);
+			if (ret)
+				goto next;
+
+			obj_idx++;
+		}
+
+next:
+		/* For freeing the zspage, or putting it back in the pool and LRU list. */
+		spin_lock(&pool->lock);
+		zspage->under_reclaim = false;
+
+		if (!get_zspage_inuse(zspage)) {
+			/*
+			 * Fullness went stale as zs_free() won't touch it
+			 * while the page is removed from the pool. Fix it
+			 * up for the check in __free_zspage().
+			 */
+			zspage->fullness = ZS_EMPTY;
+
+			__free_zspage(pool, class, zspage);
+			spin_unlock(&pool->lock);
+			return 0;
+		}
+
+		putback_zspage(class, zspage);
+		list_add(&zspage->lru, &pool->lru);
+		unlock_zspage(zspage);
+	}
+
+	spin_unlock(&pool->lock);
+	return -EAGAIN;
+}
+#endif /* CONFIG_ZPOOL */
+
 static int __init zs_init(void)
 {
 	int ret;
--
2.30.2

Re: [PATCH v3 0/5] Implement writeback for zsmalloc

[Johannes Weiner]

On Tue, Nov 08, 2022 at 11:32:02AM -0800, Nhat Pham wrote:
> Changelog:
> v3:
>   * Set pool->ops = NULL when pool->zpool_ops is null (patch 4).
>   * Stop holding pool's lock when calling lock_zspage() (patch 5).
>     (suggested by Sergey Senozhatsky)
>   * Stop holding pool's lock when checking pool->ops and retries.
>     (patch 5) (suggested by Sergey Senozhatsky)
>   * Fix formatting issues (.shrink, extra spaces in casting removed).
>     (patch 5) (suggested by Sergey Senozhatsky)
> 
> v2:
>   * Add missing CONFIG_ZPOOL ifdefs (patch 5)
>     (detected by kernel test robot).
> 
> Unlike other zswap's allocators such as zbud or z3fold, zsmalloc
> currently lacks the writeback mechanism. This means that when the zswap
> pool is full, it will simply reject further allocations, and the pages
> will be written directly to swap.
> 
> This series of patches implements writeback for zsmalloc. When the zswap
> pool becomes full, zsmalloc will attempt to evict all the compressed
> objects in the least-recently used zspages.
> 
> There are 5 patches in this series:

For the series:

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Re: [PATCH v3 0/5] Implement writeback for zsmalloc

[Andrew Morton]

On Tue,  8 Nov 2022 11:32:02 -0800 Nhat Pham <nphamcs@gmail.com> wrote:

> This series of patches implements writeback for zsmalloc. 

There's quite a bit of churn in zsmalloc at present.  So for the sake
of clarity I have dropped all zsmalloc patches except for "zsmalloc:
replace IS_ERR() with IS_ERR_VALUE()".

Please coordinate with Sergey and Minchan on getting all this pending
work finalized and reviewed.

Re: [PATCH v3 3/5] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Minchan Kim]

On Tue, Nov 08, 2022 at 11:32:05AM -0800, Nhat Pham wrote:
> This helps determines the coldest zspages as candidates for writeback.
> 
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> ---
>  mm/zsmalloc.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 326faa751f0a..600c40121544 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -239,6 +239,9 @@ struct zs_pool {
>  	/* Compact classes */
>  	struct shrinker shrinker;
> 
> +	/* List tracking the zspages in LRU order by most recently added object */
> +	struct list_head lru;
> +
>  #ifdef CONFIG_ZSMALLOC_STAT
>  	struct dentry *stat_dentry;
>  #endif
> @@ -260,6 +263,10 @@ struct zspage {
>  	unsigned int freeobj;
>  	struct page *first_page;
>  	struct list_head list; /* fullness list */
> +
> +	/* links the zspage to the lru list in the pool */
> +	struct list_head lru;

Please put the LRU logic under config ZSMALLOC_LRU since we don't need
the additional logic to others.

Re: [PATCH v3 3/5] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Nhat Pham]

> Please put the LRU logic under config ZSMALLOC_LRU since we don't need the
> additional logic to others.

I think the existing CONFIG_ZPOOL would be a good option for this purpose. It
should disable the LRU behavior for non-zswap use case (zram for e.g). The
eviction logic is also currently defined under this. What do you think,
Minchan?

Re: [PATCH v3 3/5] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Minchan Kim]

On Thu, Nov 10, 2022 at 09:18:31AM -0800, Nhat Pham wrote:
> > Please put the LRU logic under config ZSMALLOC_LRU since we don't need the
> > additional logic to others.
> 
> I think the existing CONFIG_ZPOOL would be a good option for this purpose. It
> should disable the LRU behavior for non-zswap use case (zram for e.g). The
> eviction logic is also currently defined under this. What do you think,
> Minchan?

That sounds good.

Sergey and I are working to change zsmalloc zspage size.
https://lore.kernel.org/linux-mm/20221031054108.541190-1-senozhatsky@chromium.org/

Could you send a new version once we settle those change down
in Andrew's tree to minimize conflict?
(Feel free to join the review/discussion if you are also interested ;-))

Thanks.

Re: [PATCH v3 3/5] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Johannes Weiner]

On Thu, Nov 10, 2022 at 02:48:32PM -0800, Minchan Kim wrote:
> On Thu, Nov 10, 2022 at 09:18:31AM -0800, Nhat Pham wrote:
> > > Please put the LRU logic under config ZSMALLOC_LRU since we don't need the
> > > additional logic to others.
> > 
> > I think the existing CONFIG_ZPOOL would be a good option for this purpose. It
> > should disable the LRU behavior for non-zswap use case (zram for e.g). The
> > eviction logic is also currently defined under this. What do you think,
> > Minchan?
> 
> That sounds good.
> 
> Sergey and I are working to change zsmalloc zspage size.
> https://lore.kernel.org/linux-mm/20221031054108.541190-1-senozhatsky@chromium.org/
> 
> Could you send a new version once we settle those change down
> in Andrew's tree to minimize conflict?
> (Feel free to join the review/discussion if you are also interested ;-))

I've been reading through that thread, and it doesn't look like it'll
be ready for the upcoming merge window. (I've tried to contribute
something useful to it, but it's a fairly difficult tuning problem,
and I don't know if a sysfs knob is the best answer, either...)

Would you have any objections to putting Nhat's patches here into 6.2?

It doesn't sound like there was any more feedback (except the trivial
ifdef around the LRU), and the patches are otherwise ready to go.

Re: [PATCH v3 3/5] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Minchan Kim]

On Wed, Nov 16, 2022 at 05:02:57PM -0500, Johannes Weiner wrote:
> On Thu, Nov 10, 2022 at 02:48:32PM -0800, Minchan Kim wrote:
> > On Thu, Nov 10, 2022 at 09:18:31AM -0800, Nhat Pham wrote:
> > > > Please put the LRU logic under config ZSMALLOC_LRU since we don't need the
> > > > additional logic to others.
> > > 
> > > I think the existing CONFIG_ZPOOL would be a good option for this purpose. It
> > > should disable the LRU behavior for non-zswap use case (zram for e.g). The
> > > eviction logic is also currently defined under this. What do you think,
> > > Minchan?
> > 
> > That sounds good.
> > 
> > Sergey and I are working to change zsmalloc zspage size.
> > https://lore.kernel.org/linux-mm/20221031054108.541190-1-senozhatsky@chromium.org/
> > 
> > Could you send a new version once we settle those change down
> > in Andrew's tree to minimize conflict?
> > (Feel free to join the review/discussion if you are also interested ;-))
> 
> I've been reading through that thread, and it doesn't look like it'll
> be ready for the upcoming merge window. (I've tried to contribute

Depending on the discussion status :)

> something useful to it, but it's a fairly difficult tuning problem,
> and I don't know if a sysfs knob is the best answer, either...)

That's the point.

> 
> Would you have any objections to putting Nhat's patches here into 6.2?

I don't want to block due to other issues so no objection from my side.

> 
> It doesn't sound like there was any more feedback (except the trivial
> ifdef around the LRU), and the patches are otherwise ready to go.

In fact, I didn't start the review yet so please post it unless
Sergey objects it.

Thank you.

Re: [PATCH v3 3/5] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Sergey Senozhatsky]

On (22/11/16 15:49), Minchan Kim wrote:
> > It doesn't sound like there was any more feedback (except the trivial
> > ifdef around the LRU), and the patches are otherwise ready to go.
> 
> In fact, I didn't start the review yet so please post it unless
> Sergey objects it.

I didn't start review yet, but if you have a new version then
we can take a look.