[PATCH v7 0/6] Implement writeback for zsmalloc

[PATCH v7 0/6] Implement writeback for zsmalloc

[Nhat Pham]

Changelog:
v7:
  * Rebase this series of patch on top of mm-unstable
    which removes some duplicate code (patch 1).
v6-fix:
  * Add a comment explaining LRU update logic in zs_map_object.
    (patch 4)
    (sugegested by Sergey Senozhatsky and Johannes Weiner).
  * Use get_first_page() and add cond_resched() in retry-loop.
    (patch 6) (sugegested by Sergey Senozhatsky).
v6:
  * Move the move-to-front logic into zs_map_object (patch 4)
    (suggested by Minchan Kim).
  * Small clean up for free_zspage at free_handles() call site
    (patch 6) (suggested by Minchan Kim).
v5:
  * Add a new patch that eliminates unused code in zpool and simplify
    the logic for storing evict handler in zbud/z3fold (patch 2)
  * Remove redudant fields in zs_pool (previously required by zpool)
    (patch 3)
  * Wrap under_reclaim and deferred handle freeing logic in CONFIG_ZPOOL
    (patch 6) (suggested by Minchan Kim)
  * Move a small piece of refactoring from patch 6 to patch 4.
v4:
  * Wrap the new LRU logic in CONFIG_ZPOOL (patch 3).
    (suggested by Minchan Kim)
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 6 patches in this series:

Johannes Weiner (2):
  zswap: fix writeback lock ordering for zsmalloc
  zpool: clean out dead code

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 zpool_ops field to zs_pool to store evict handlers
  zsmalloc: Implement writeback mechanism for zsmalloc

 mm/z3fold.c   |  36 +-----
 mm/zbud.c     |  32 +----
 mm/zpool.c    |  10 +-
 mm/zsmalloc.c | 342 +++++++++++++++++++++++++++++++++++++++++---------
 mm/zswap.c    |  35 +++---
 5 files changed, 311 insertions(+), 144 deletions(-)

--
2.30.2

[PATCH v7 1/6] 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 | 35 +++++++++++++++++++----------------
 1 file changed, 19 insertions(+), 16 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 3019f0bde194..f6c89049cf70 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -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 v7 2/6] zpool: clean out dead code

[Nhat Pham]

From: Johannes Weiner <hannes@cmpxchg.org>

There is a lot of provision for flexibility that isn't actually needed
or used. Zswap (the only zpool user) always passes zpool_ops with an
.evict method set. The backends who reclaim only do so for zswap, so
they can also directly call zpool_ops without indirection or checks.

Finally, there is no need to check the retries parameters and bail
with -EINVAL in the reclaim function, when that's called just a few
lines below with a hard-coded 8. There is no need to duplicate the
evictable and sleep_mapped attrs from the driver in zpool_ops.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 mm/z3fold.c | 36 +++++-------------------------------
 mm/zbud.c   | 32 +++++---------------------------
 mm/zpool.c  | 10 ++--------
 3 files changed, 12 insertions(+), 66 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index cf71da10d04e..a4de0c317ac7 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -68,9 +68,6 @@
  * Structures
 *****************/
 struct z3fold_pool;
-struct z3fold_ops {
-	int (*evict)(struct z3fold_pool *pool, unsigned long handle);
-};

 enum buddy {
 	HEADLESS = 0,
@@ -138,8 +135,6 @@ struct z3fold_header {
  * @stale:	list of pages marked for freeing
  * @pages_nr:	number of z3fold pages in the pool.
  * @c_handle:	cache for z3fold_buddy_slots allocation
- * @ops:	pointer to a structure of user defined operations specified at
- *		pool creation time.
  * @zpool:	zpool driver
  * @zpool_ops:	zpool operations structure with an evict callback
  * @compact_wq:	workqueue for page layout background optimization
@@ -158,7 +153,6 @@ struct z3fold_pool {
 	struct list_head stale;
 	atomic64_t pages_nr;
 	struct kmem_cache *c_handle;
-	const struct z3fold_ops *ops;
 	struct zpool *zpool;
 	const struct zpool_ops *zpool_ops;
 	struct workqueue_struct *compact_wq;
@@ -907,13 +901,11 @@ static inline struct z3fold_header *__z3fold_alloc(struct z3fold_pool *pool,
  * z3fold_create_pool() - create a new z3fold pool
  * @name:	pool name
  * @gfp:	gfp flags when allocating the z3fold pool structure
- * @ops:	user-defined operations for the z3fold pool
  *
  * Return: pointer to the new z3fold pool or NULL if the metadata allocation
  * failed.
  */
-static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
-		const struct z3fold_ops *ops)
+static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp)
 {
 	struct z3fold_pool *pool = NULL;
 	int i, cpu;
@@ -949,7 +941,6 @@ static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
 	if (!pool->release_wq)
 		goto out_wq;
 	INIT_WORK(&pool->work, free_pages_work);
-	pool->ops = ops;
 	return pool;

 out_wq:
@@ -1230,10 +1221,6 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 	slots.pool = (unsigned long)pool | (1 << HANDLES_NOFREE);

 	spin_lock(&pool->lock);
-	if (!pool->ops || !pool->ops->evict || retries == 0) {
-		spin_unlock(&pool->lock);
-		return -EINVAL;
-	}
 	for (i = 0; i < retries; i++) {
 		if (list_empty(&pool->lru)) {
 			spin_unlock(&pool->lock);
@@ -1319,17 +1306,17 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 		}
 		/* Issue the eviction callback(s) */
 		if (middle_handle) {
-			ret = pool->ops->evict(pool, middle_handle);
+			ret = pool->zpool_ops->evict(pool->zpool, middle_handle);
 			if (ret)
 				goto next;
 		}
 		if (first_handle) {
-			ret = pool->ops->evict(pool, first_handle);
+			ret = pool->zpool_ops->evict(pool->zpool, first_handle);
 			if (ret)
 				goto next;
 		}
 		if (last_handle) {
-			ret = pool->ops->evict(pool, last_handle);
+			ret = pool->zpool_ops->evict(pool->zpool, last_handle);
 			if (ret)
 				goto next;
 		}
@@ -1593,26 +1580,13 @@ static const struct movable_operations z3fold_mops = {
  * zpool
  ****************/

-static int z3fold_zpool_evict(struct z3fold_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 z3fold_ops z3fold_zpool_ops = {
-	.evict =	z3fold_zpool_evict
-};
-
 static void *z3fold_zpool_create(const char *name, gfp_t gfp,
 			       const struct zpool_ops *zpool_ops,
 			       struct zpool *zpool)
 {
 	struct z3fold_pool *pool;

-	pool = z3fold_create_pool(name, gfp,
-				zpool_ops ? &z3fold_zpool_ops : NULL);
+	pool = z3fold_create_pool(name, gfp);
 	if (pool) {
 		pool->zpool = zpool;
 		pool->zpool_ops = zpool_ops;
diff --git a/mm/zbud.c b/mm/zbud.c
index 6348932430b8..3acd26193920 100644
--- a/mm/zbud.c
+++ b/mm/zbud.c
@@ -74,10 +74,6 @@

 struct zbud_pool;

-struct zbud_ops {
-	int (*evict)(struct zbud_pool *pool, unsigned long handle);
-};
-
 /**
  * struct zbud_pool - stores metadata for each zbud pool
  * @lock:	protects all pool fields and first|last_chunk fields of any
@@ -90,8 +86,6 @@ struct zbud_ops {
  * @lru:	list tracking the zbud pages in LRU order by most recently
  *		added buddy.
  * @pages_nr:	number of zbud pages in the pool.
- * @ops:	pointer to a structure of user defined operations specified at
- *		pool creation time.
  * @zpool:	zpool driver
  * @zpool_ops:	zpool operations structure with an evict callback
  *
@@ -110,7 +104,6 @@ struct zbud_pool {
 	};
 	struct list_head lru;
 	u64 pages_nr;
-	const struct zbud_ops *ops;
 	struct zpool *zpool;
 	const struct zpool_ops *zpool_ops;
 };
@@ -212,12 +205,11 @@ static int num_free_chunks(struct zbud_header *zhdr)
 /**
  * zbud_create_pool() - create a new zbud pool
  * @gfp:	gfp flags when allocating the zbud pool structure
- * @ops:	user-defined operations for the zbud pool
  *
  * Return: pointer to the new zbud pool or NULL if the metadata allocation
  * failed.
  */
-static struct zbud_pool *zbud_create_pool(gfp_t gfp, const struct zbud_ops *ops)
+static struct zbud_pool *zbud_create_pool(gfp_t gfp)
 {
 	struct zbud_pool *pool;
 	int i;
@@ -231,7 +223,6 @@ static struct zbud_pool *zbud_create_pool(gfp_t gfp, const struct zbud_ops *ops)
 	INIT_LIST_HEAD(&pool->buddied);
 	INIT_LIST_HEAD(&pool->lru);
 	pool->pages_nr = 0;
-	pool->ops = ops;
 	return pool;
 }

@@ -419,8 +410,7 @@ static int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
 	unsigned long first_handle = 0, last_handle = 0;

 	spin_lock(&pool->lock);
-	if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) ||
-			retries == 0) {
+	if (list_empty(&pool->lru)) {
 		spin_unlock(&pool->lock);
 		return -EINVAL;
 	}
@@ -444,12 +434,12 @@ static int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)

 		/* Issue the eviction callback(s) */
 		if (first_handle) {
-			ret = pool->ops->evict(pool, first_handle);
+			ret = pool->zpool_ops->evict(pool->zpool, first_handle);
 			if (ret)
 				goto next;
 		}
 		if (last_handle) {
-			ret = pool->ops->evict(pool, last_handle);
+			ret = pool->zpool_ops->evict(pool->zpool, last_handle);
 			if (ret)
 				goto next;
 		}
@@ -524,25 +514,13 @@ static u64 zbud_get_pool_size(struct zbud_pool *pool)
  * zpool
  ****************/

-static int zbud_zpool_evict(struct zbud_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 zbud_ops zbud_zpool_ops = {
-	.evict =	zbud_zpool_evict
-};
-
 static void *zbud_zpool_create(const char *name, gfp_t gfp,
 			       const struct zpool_ops *zpool_ops,
 			       struct zpool *zpool)
 {
 	struct zbud_pool *pool;

-	pool = zbud_create_pool(gfp, zpool_ops ? &zbud_zpool_ops : NULL);
+	pool = zbud_create_pool(gfp);
 	if (pool) {
 		pool->zpool = zpool;
 		pool->zpool_ops = zpool_ops;
diff --git a/mm/zpool.c b/mm/zpool.c
index f46c0d5e766c..571f5c5031dd 100644
--- a/mm/zpool.c
+++ b/mm/zpool.c
@@ -21,9 +21,6 @@
 struct zpool {
 	struct zpool_driver *driver;
 	void *pool;
-	const struct zpool_ops *ops;
-	bool evictable;
-	bool can_sleep_mapped;
 };

 static LIST_HEAD(drivers_head);
@@ -177,9 +174,6 @@ struct zpool *zpool_create_pool(const char *type, const char *name, gfp_t gfp,

 	zpool->driver = driver;
 	zpool->pool = driver->create(name, gfp, ops, zpool);
-	zpool->ops = ops;
-	zpool->evictable = driver->shrink && ops && ops->evict;
-	zpool->can_sleep_mapped = driver->sleep_mapped;

 	if (!zpool->pool) {
 		pr_err("couldn't create %s pool\n", type);
@@ -380,7 +374,7 @@ u64 zpool_get_total_size(struct zpool *zpool)
  */
 bool zpool_evictable(struct zpool *zpool)
 {
-	return zpool->evictable;
+	return zpool->driver->shrink;
 }

 /**
@@ -398,7 +392,7 @@ bool zpool_evictable(struct zpool *zpool)
  */
 bool zpool_can_sleep_mapped(struct zpool *zpool)
 {
-	return zpool->can_sleep_mapped;
+	return zpool->driver->sleep_mapped;
 }

 MODULE_LICENSE("GPL");
--
2.30.2

[PATCH v7 3/6] 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>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/zsmalloc.c | 87 ++++++++++++++++++++++-----------------------------
 1 file changed, 37 insertions(+), 50 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 78feda34ad9a..5427a00a0518 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);
@@ -1268,19 +1265,19 @@ 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;
@@ -1433,8 +1430,8 @@ 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);
@@ -1442,12 +1439,12 @@ 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) {
@@ -1455,7 +1452,7 @@ 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);
@@ -1468,7 +1465,7 @@ 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;
 }
@@ -1512,16 +1509,14 @@ 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);
@@ -1531,7 +1526,7 @@ 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);
@@ -1888,16 +1883,12 @@ 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);

@@ -1927,10 +1918,9 @@ 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);

@@ -1985,9 +1975,9 @@ 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) {
@@ -1997,9 +1987,9 @@ 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);
 	}
 };

@@ -2060,10 +2050,11 @@ 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);
@@ -2088,7 +2079,7 @@ 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;
 		}

@@ -2105,11 +2096,9 @@ 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) {
@@ -2117,8 +2106,7 @@ 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;
 }
@@ -2221,7 +2209,7 @@ 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)
@@ -2292,7 +2280,6 @@ 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 v7 4/6] 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 | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 5427a00a0518..b1bc231d94a3 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -239,6 +239,11 @@ struct zs_pool {
 	/* Compact classes */
 	struct shrinker shrinker;

+#ifdef CONFIG_ZPOOL
+	/* List tracking the zspages in LRU order by most recently added object */
+	struct list_head lru;
+#endif
+
 #ifdef CONFIG_ZSMALLOC_STAT
 	struct dentry *stat_dentry;
 #endif
@@ -260,6 +265,12 @@ struct zspage {
 	unsigned int freeobj;
 	struct page *first_page;
 	struct list_head list; /* fullness list */
+
+#ifdef CONFIG_ZPOOL
+	/* links the zspage to the lru list in the pool */
+	struct list_head lru;
+#endif
+
 	struct zs_pool *pool;
 #ifdef CONFIG_COMPACTION
 	rwlock_t lock;
@@ -953,6 +964,9 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
 	}

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

@@ -998,6 +1012,10 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
 		off %= PAGE_SIZE;
 	}

+#ifdef CONFIG_ZPOOL
+	INIT_LIST_HEAD(&zspage->lru);
+#endif
+
 	set_freeobj(zspage, 0);
 }

@@ -1270,6 +1288,31 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
 	obj_to_location(obj, &page, &obj_idx);
 	zspage = get_zspage(page);

+#ifdef CONFIG_ZPOOL
+	/*
+	 * Move the zspage to front of pool's LRU.
+	 *
+	 * Note that this is swap-specific, so by definition there are no ongoing
+	 * accesses to the memory while the page is swapped out that would make
+	 * it "hot". A new entry is hot, then ages to the tail until it gets either
+	 * written back or swaps back in.
+	 *
+	 * Furthermore, map is also called during writeback. We must not put an
+	 * isolated page on the LRU mid-reclaim.
+	 *
+	 * As a result, only update the LRU when the page is mapped for write
+	 * when it's first instantiated.
+	 *
+	 * This is a deviation from the other backends, which perform this update
+	 * in the allocation function (zbud_alloc, z3fold_alloc).
+	 */
+	if (mm == ZS_MM_WO) {
+		if (!list_empty(&zspage->lru))
+			list_del(&zspage->lru);
+		list_add(&zspage->lru, &pool->lru);
+	}
+#endif
+
 	/*
 	 * migration cannot move any zpages in this zspage. Here, pool->lock
 	 * is too heavy since callers would take some time until they calls
@@ -1988,6 +2031,9 @@ 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);
+#ifdef CONFIG_ZPOOL
+		list_del(&zspage->lru);
+#endif
 		__free_zspage(pool, class, zspage);
 		spin_unlock(&pool->lock);
 	}
@@ -2299,6 +2345,10 @@ struct zs_pool *zs_create_pool(const char *name)
 	 */
 	zs_register_shrinker(pool);

+#ifdef CONFIG_ZPOOL
+	INIT_LIST_HEAD(&pool->lru);
+#endif
+
 	return pool;

 err:
--
2.30.2

[PATCH v7 5/6] zsmalloc: Add zpool_ops field to zs_pool to store evict handlers

[Nhat Pham]

This adds a new field to zs_pool to store evict handlers for writeback,
analogous to the zbud allocator.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
---
 mm/zsmalloc.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index b1bc231d94a3..d06f9150b9da 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -242,6 +242,8 @@ struct zs_pool {
 #ifdef CONFIG_ZPOOL
 	/* List tracking the zspages in LRU order by most recently added object */
 	struct list_head lru;
+	struct zpool *zpool;
+	const struct zpool_ops *zpool_ops;
 #endif

 #ifdef CONFIG_ZSMALLOC_STAT
@@ -382,7 +384,14 @@ 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;
+	}
+
+	return pool;
 }

 static void zs_zpool_destroy(void *pool)
--
2.30.2

[PATCH v7 6/6] 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 | 194 +++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 183 insertions(+), 11 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index d06f9150b9da..9445bee6b014 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -271,12 +271,13 @@ struct zspage {
 #ifdef CONFIG_ZPOOL
 	/* 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;
 #endif

 	struct zs_pool *pool;
-#ifdef CONFIG_COMPACTION
 	rwlock_t lock;
-#endif
 };

 struct mapping_area {
@@ -297,10 +298,11 @@ 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);
@@ -308,9 +310,6 @@ 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) {}
@@ -413,6 +412,27 @@ 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)
 {
@@ -451,6 +471,7 @@ 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,
@@ -924,6 +945,25 @@ static int trylock_zspage(struct zspage *zspage)
 	return 0;
 }

+#ifdef CONFIG_ZPOOL
+/*
+ * 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;
+	}
+}
+#else
+static inline void free_handles(struct zs_pool *pool, struct zspage *zspage) {}
+#endif
+
 static void __free_zspage(struct zs_pool *pool, struct size_class *class,
 				struct zspage *zspage)
 {
@@ -938,6 +978,9 @@ 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);
@@ -1023,6 +1066,8 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)

 #ifdef CONFIG_ZPOOL
 	INIT_LIST_HEAD(&zspage->lru);
+	zspage->under_reclaim = false;
+	zspage->deferred_handles = NULL;
 #endif

 	set_freeobj(zspage, 0);
@@ -1572,12 +1617,26 @@ void zs_free(struct zs_pool *pool, unsigned long handle)

 	obj_free(class->size, obj);
 	class_stat_dec(class, OBJ_USED, 1);
+
+#ifdef CONFIG_ZPOOL
+	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;
+	}
+#endif
 	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);
 }
@@ -1777,7 +1836,7 @@ 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.
@@ -1819,6 +1878,24 @@ 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)
 {
@@ -1835,6 +1912,7 @@ 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);
@@ -2399,6 +2477,100 @@ 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;
+
+	/* 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);
+		cond_resched();
+
+		/* Lock backing pages into place */
+		lock_zspage(zspage);
+
+		obj_idx = 0;
+		page = get_first_page(zspage);
+		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->zpool_ops->evict(pool->zpool, 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 v7 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Johannes Weiner]

On Mon, Nov 28, 2022 at 11:16:13AM -0800, Nhat Pham wrote:
> This helps determines the coldest zspages as candidates for writeback.
> 
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>

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

Re: [PATCH v7 5/6] zsmalloc: Add zpool_ops field to zs_pool to store evict handlers

[Johannes Weiner]

On Mon, Nov 28, 2022 at 11:16:14AM -0800, Nhat Pham wrote:
> This adds a new field to zs_pool to store evict handlers for writeback,
> analogous to the zbud allocator.
> 
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> Acked-by: Minchan Kim <minchan@kernel.org>

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

Sergey was happy with this too:
https://lore.kernel.org/linux-mm/Y3wh0i5r+9Gk2Okf@google.com/

Re: [PATCH v7 6/6] zsmalloc: Implement writeback mechanism for zsmalloc

[Johannes Weiner]

On Mon, Nov 28, 2022 at 11:16:15AM -0800, Nhat Pham wrote:
> 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>

Ok, this now also has the get_first_page() and the cond_resched() that
Sergey had asked for. Looks good to me.

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

Re: [PATCH v7 1/6] zswap: fix writeback lock ordering for zsmalloc

[Sergey Senozhatsky]

On (22/11/28 11:16), Nhat Pham wrote:
> 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>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Re: [PATCH v7 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Sergey Senozhatsky]

On (22/11/28 11:16), Nhat Pham wrote:
> This helps determines the coldest zspages as candidates for writeback.
> 
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Re: [PATCH v7 5/6] zsmalloc: Add zpool_ops field to zs_pool to store evict handlers

[Sergey Senozhatsky]

On (22/11/28 11:16), Nhat Pham wrote:
> This adds a new field to zs_pool to store evict handlers for writeback,
> analogous to the zbud allocator.
> 
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> Acked-by: Minchan Kim <minchan@kernel.org>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Re: [PATCH v7 6/6] zsmalloc: Implement writeback mechanism for zsmalloc

[Sergey Senozhatsky]

On (22/11/28 11:16), Nhat Pham wrote:
> 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>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Re: [PATCH v7 3/6] zsmalloc: Consolidate zs_pool's migrate_lock and size_class's locks

[Sergey Senozhatsky]

On (22/11/28 11:16), Nhat Pham wrote:
> 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>
> Acked-by: Minchan Kim <minchan@kernel.org>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Re: [PATCH v7 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Vitaly Wool]

On Mon, Nov 28, 2022 at 8:16 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> This helps determines the coldest zspages as candidates for writeback.
>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> ---
>  mm/zsmalloc.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 50 insertions(+)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 5427a00a0518..b1bc231d94a3 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -239,6 +239,11 @@ struct zs_pool {
>         /* Compact classes */
>         struct shrinker shrinker;
>
> +#ifdef CONFIG_ZPOOL
> +       /* List tracking the zspages in LRU order by most recently added object */
> +       struct list_head lru;
> +#endif
> +
>  #ifdef CONFIG_ZSMALLOC_STAT
>         struct dentry *stat_dentry;
>  #endif
> @@ -260,6 +265,12 @@ struct zspage {
>         unsigned int freeobj;
>         struct page *first_page;
>         struct list_head list; /* fullness list */
> +
> +#ifdef CONFIG_ZPOOL
> +       /* links the zspage to the lru list in the pool */
> +       struct list_head lru;
> +#endif
> +
>         struct zs_pool *pool;
>  #ifdef CONFIG_COMPACTION
>         rwlock_t lock;
> @@ -953,6 +964,9 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
>         }
>
>         remove_zspage(class, zspage, ZS_EMPTY);
> +#ifdef CONFIG_ZPOOL
> +       list_del(&zspage->lru);
> +#endif
>         __free_zspage(pool, class, zspage);
>  }
>
> @@ -998,6 +1012,10 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
>                 off %= PAGE_SIZE;
>         }
>
> +#ifdef CONFIG_ZPOOL
> +       INIT_LIST_HEAD(&zspage->lru);
> +#endif
> +
>         set_freeobj(zspage, 0);
>  }
>
> @@ -1270,6 +1288,31 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
>         obj_to_location(obj, &page, &obj_idx);
>         zspage = get_zspage(page);
>
> +#ifdef CONFIG_ZPOOL
> +       /*
> +        * Move the zspage to front of pool's LRU.
> +        *
> +        * Note that this is swap-specific, so by definition there are no ongoing
> +        * accesses to the memory while the page is swapped out that would make
> +        * it "hot". A new entry is hot, then ages to the tail until it gets either
> +        * written back or swaps back in.
> +        *
> +        * Furthermore, map is also called during writeback. We must not put an
> +        * isolated page on the LRU mid-reclaim.
> +        *
> +        * As a result, only update the LRU when the page is mapped for write
> +        * when it's first instantiated.
> +        *
> +        * This is a deviation from the other backends, which perform this update
> +        * in the allocation function (zbud_alloc, z3fold_alloc).
> +        */
> +       if (mm == ZS_MM_WO) {
> +               if (!list_empty(&zspage->lru))
> +                       list_del(&zspage->lru);
> +               list_add(&zspage->lru, &pool->lru);
> +       }
> +#endif
> +
>         /*
>          * migration cannot move any zpages in this zspage. Here, pool->lock
>          * is too heavy since callers would take some time until they calls
> @@ -1988,6 +2031,9 @@ 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);
> +#ifdef CONFIG_ZPOOL
> +               list_del(&zspage->lru);
> +#endif
>                 __free_zspage(pool, class, zspage);
>                 spin_unlock(&pool->lock);
>         }
> @@ -2299,6 +2345,10 @@ struct zs_pool *zs_create_pool(const char *name)
>          */
>         zs_register_shrinker(pool);
>
> +#ifdef CONFIG_ZPOOL
> +       INIT_LIST_HEAD(&pool->lru);
> +#endif

I think the amount of #ifdefs here becomes absolutely overwhelming.
Not that zsmalloc code was very readable before, but now it is
starting to look like a plain disaster.

Thanks,
Vitaly

> +
>         return pool;
>
>  err:
> --
> 2.30.2

Re: [PATCH v7 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Sergey Senozhatsky]

On (22/11/29 12:53), Vitaly Wool wrote:
> I think the amount of #ifdefs here becomes absolutely overwhelming.
> Not that zsmalloc code was very readable before, but now it is
> starting to look like a plain disaster.

Presumably most of them will go away once LRU moved from
allocator to upper level.

Re: [PATCH v7 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Johannes Weiner]

On Tue, Nov 29, 2022 at 11:03:45PM +0900, Sergey Senozhatsky wrote:
> On (22/11/29 12:53), Vitaly Wool wrote:
> > I think the amount of #ifdefs here becomes absolutely overwhelming.
> > Not that zsmalloc code was very readable before, but now it is
> > starting to look like a plain disaster.
> 
> Presumably most of them will go away once LRU moved from
> allocator to upper level.

Yes consider it the "cut here" lines for refactoring the LRU into
zswap, which we want to do next. They're not here to stay, and that
work will remove a lot of duplication and complexity from all
backends. So I hope it's acceptable as a transitionary state.

Re: [PATCH v7 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Vitaly Wool]

On Tue, Nov 29, 2022 at 4:54 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Tue, Nov 29, 2022 at 11:03:45PM +0900, Sergey Senozhatsky wrote:
> > On (22/11/29 12:53), Vitaly Wool wrote:
> > > I think the amount of #ifdefs here becomes absolutely overwhelming.
> > > Not that zsmalloc code was very readable before, but now it is
> > > starting to look like a plain disaster.
> >
> > Presumably most of them will go away once LRU moved from
> > allocator to upper level.
>
> Yes consider it the "cut here" lines for refactoring the LRU into
> zswap, which we want to do next. They're not here to stay, and that
> work will remove a lot of duplication and complexity from all
> backends. So I hope it's acceptable as a transitionary state.

I see, thanks for the explanation.

~Vitaly

Re: [PATCH v7 0/6] Implement writeback for zsmalloc

[Thomas Weißschuh]

Hi,

On Mon, Nov 28, 2022 at 11:16:09AM -0800, Nhat Pham wrote:
> 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.

Then this part of Documentation/admin-guide/mm/zswap.rst should probably
also be updated at some point:

    However, zsmalloc does not implement compressed page eviction, so
    once zswap fills it cannot evict the oldest page, it can only reject
    new pages.

Thomas

Re: [PATCH v7 0/6] Implement writeback for zsmalloc

[Nhat Pham]

On Mon, Jan 2, 2023 at 8:57 PM Thomas Weißschuh <thomas@t-8ch.de> wrote:
>
> Hi,
>
> On Mon, Nov 28, 2022 at 11:16:09AM -0800, Nhat Pham wrote:
> > 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.
>
> Then this part of Documentation/admin-guide/mm/zswap.rst should probably
> also be updated at some point:
>
>     However, zsmalloc does not implement compressed page eviction, so
>     once zswap fills it cannot evict the oldest page, it can only reject
>     new pages.
>
> Thomas

Thanks for pointing this out, Thomas! I'll send a patch to update this.