[PATCH v6 0/4] mm/zsmalloc: reduce lock contention in zs_free()

[PATCH v6 0/4] mm/zsmalloc: reduce lock contention in zs_free()

[Wenchao Hao]

From: Wenchao Hao <haowenchao@xiaomi.com>

This series reduces lock contention in zs_free(), which dominates the
unmap path under memory pressure on Android (LMK kills) and on x86
servers running zswap-heavy workloads.

The current zs_free() takes pool->lock (rwlock, read side) just to
look up the size_class for a handle, then takes class->lock and holds
it across __free_zspage() which can call into the buddy allocator and
acquire zone->lock.  Two costs follow:

  * pool->lock reader-counter cacheline bouncing among concurrent
    zs_free() callers.
  * class->lock held across folio_put(), so any zone->lock wait
    fans out to every other zs_free() on the same class.

The series tackles both:

  Patch 1: encode size_class index into obj alongside PFN and obj_idx,
           so zs_free() can locate the class without pool->lock.
  Patch 2: drop pool->lock from zs_free() on 64-bit; 32-bit unchanged.
  Patch 3: move zspage page-freeing out of class->lock.
  Patch 4: document the three free_zspage helper variants that result
           from the split in patch 3.

Performance results:

Test: each process independently mmap 256MB, write data, madvise
MADV_PAGEOUT to swap out via zram (lzo-rle), then concurrent munmap.

Raspberry Pi 4B (4-core ARM64 Cortex-A72):

  mode        Base       Patched     Speedup
  single      59.0ms     56.0ms      1.05x
  multi 2p    94.6ms     66.7ms      1.42x
  multi 4p    202.9ms    110.6ms     1.83x

x86 (20-core Intel i7-12700, 16 concurrent processes):

  mode        Base       Patched     Speedup
  single      11.7ms     9.8ms       1.19x
  multi 2p    24.1ms     17.2ms      1.40x
  multi 4p    63.0ms     45.3ms      1.39x

[1] https://lore.kernel.org/linux-mm/20260508061910.3882831-1-haowenchao@xiaomi.com/
[2] https://lore.kernel.org/linux-mm/20260527115930.3138213-1-haowenchao@xiaomi.com/
[3] https://lore.kernel.org/linux-mm/20260605084242.1549811-1-haowenchao22@gmail.com/
[4] https://lore.kernel.org/linux-mm/20260609113520.3507783-1-haowenchao22@gmail.com/
[5] https://lore.kernel.org/all/20260616031309.3036234-1-haowenchao22@gmail.com/

Changes since v5:
- 2/4: rename obj_handle_class_lock() to obj_class_get_and_lock().
- Pick up Reviewed-by from Barry Song on 2/4, 3/4, 4/4.

Changes since v4:
- 1/4: rename macros to make their intent clearer, following the
  kernel's *_BITS convention (Nhat Pham, Barry Song).
- 3/4: drop the unused pool argument from __free_zspage_lockless().

Changes since v3:
- 1/4: gate ZS_OBJ_CLASS_BITS on a new spare-bit check so unsuitable
  configs (e.g. UML, no sparsemem) fall back to plain obj layout
  instead of breaking the build.  (sashiko AI review)
- 2/4: annotate handle_to_obj()/record_obj() with READ_ONCE/WRITE_ONCE
  to silence KCSAN; sync the #if predicate with patch 1.  (sashiko AI review)
- 4/4: clarify when async_free_zspage() runs; pick up Reviewed-by.  (Nhat Pham)
- Drop Reviewed-by from Nhat Pham on 1/4 and 2/4: substantial logic
  changes since v3, please re-review.

Changes since v2:
- 3/4: drop likely() hint and tidy up else-branch braces, per review
  nits from Nhat Pham and Joshua Hahn.
- 4/4 (new): document free_zspage helper variants in a single comment
  block, per Nhat Pham.
- Pick up Reviewed-by tags from Nhat Pham (1/4, 2/4, 3/4) and
  Joshua Hahn (3/4).
- Fix patch 1/4 and 2/4 From: to match Signed-off-by, per Barry Song.

Changes since v1:
- Rename obj-encoding macros for clarity.
- Make 32-bit / 64-bit handling transparent at call sites
  (no #ifdef in callers).
- Extract a helper to keep zs_free() unified.

Wenchao Hao (3):
  mm/zsmalloc: encode class index in obj value for lockless class lookup
  mm/zsmalloc: drop pool->lock from zs_free on 64-bit systems
  mm/zsmalloc: document free_zspage helper variants

Xueyuan Chen (1):
  mm/zsmalloc: drop class lock before freeing zspage

 mm/zsmalloc.c | 225 ++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 190 insertions(+), 35 deletions(-)

--
2.34.1

[PATCH v6 1/4] mm/zsmalloc: encode class index in obj value for lockless class lookup

[Wenchao Hao]

From: Wenchao Hao <haowenchao@xiaomi.com>

Encode the size_class index (class_idx) into the obj value so that
zs_free() can determine the correct size_class without dereferencing
the handle->obj->PFN->zpdesc->zspage->class chain under pool->lock.
class_idx is invariant across page migration (only PFN is rewritten),
so a lockless read of obj always yields a valid class_idx.

Where obj has more bits below the PFN field than obj_idx alone
needs, split that space into class_idx and obj_idx subfields:

 |<-- _PFN_BITS -->|<-- ZS_OBJ_CLASS_BITS -->|<-- ZS_OBJ_IDX_BITS -->|
 +-----------------+-------------------------+-----------------------+
 |       PFN       |        class_idx        |        obj_idx        |
 +-----------------+-------------------------+-----------------------+
MSB                ^                                                LSB
                   |
                   +-- ZS_OBJ_PFN_SHIFT

The macro layout changes as follows:

    Before            After               Meaning
    ----------------  ------------------  ----------------------------
    OBJ_INDEX_BITS    ZS_OBJ_IDX_BITS     width of obj_idx subfield
    OBJ_INDEX_MASK    ZS_OBJ_IDX_MASK     mask  of obj_idx subfield
    (n/a)             ZS_OBJ_CLASS_BITS   width of class_idx subfield
    (n/a)             ZS_OBJ_CLASS_MASK   mask  of class_idx subfield
    (n/a)             ZS_OBJ_PFN_SHIFT    bit offset of PFN in obj

ZS_OBJ_CLASS_BITS folds to 0 (and the layout collapses to
[PFN | obj_idx]) when obj has no spare bits, i.e. on 32-bit
or on 64-bit fallback paths where MAX_POSSIBLE_PHYSMEM_BITS ==
BITS_PER_LONG (e.g. UML); zs_free() then falls back to
pool->lock.

Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Wenchao Hao <haowenchao@xiaomi.com>
---
 mm/zsmalloc.c | 105 +++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 91 insertions(+), 14 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 63128ddb7959..c4fc06b259af 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -67,8 +67,8 @@
 #define MAX_POSSIBLE_PHYSMEM_BITS MAX_PHYSMEM_BITS
 #else
 /*
- * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
- * be PAGE_SHIFT
+ * If this definition of MAX_PHYSMEM_BITS is used, ZS_OBJ_PFN_SHIFT will
+ * just be PAGE_SHIFT
  */
 #define MAX_POSSIBLE_PHYSMEM_BITS BITS_PER_LONG
 #endif
@@ -88,8 +88,23 @@
 #define OBJ_TAG_BITS	1
 #define OBJ_TAG_MASK	OBJ_ALLOCATED_TAG
 
-#define OBJ_INDEX_BITS	(BITS_PER_LONG - _PFN_BITS)
-#define OBJ_INDEX_MASK	((_AC(1, UL) << OBJ_INDEX_BITS) - 1)
+/*
+ * obj is encoded as [PFN | class_idx | obj_idx] within an unsigned long:
+ *
+ *   |<-- _PFN_BITS -->|<-- ZS_OBJ_CLASS_BITS -->|<-- ZS_OBJ_IDX_BITS -->|
+ *   +-----------------+-------------------------+-----------------------+
+ *   |       PFN       |        class_idx        |        obj_idx        |
+ *   +-----------------+-------------------------+-----------------------+
+ *  MSB                ^                                                LSB
+ *                     |
+ *                     +-- ZS_OBJ_PFN_SHIFT
+ *
+ * Encoding class_idx into obj lets zs_free() locate the size_class
+ * without holding pool->lock; class_idx is invariant across page
+ * migration (only PFN changes), so a lockless read of the obj value
+ * always yields a valid class_idx.
+ */
+#define ZS_OBJ_PFN_SHIFT	(BITS_PER_LONG - _PFN_BITS)
 
 #define HUGE_BITS	1
 #define FULLNESS_BITS	4
@@ -98,9 +113,61 @@
 
 #define ZS_MAX_PAGES_PER_ZSPAGE	(_AC(CONFIG_ZSMALLOC_CHAIN_SIZE, UL))
 
+/*
+ * Bits to index a page within a zspage = ceil(log2(ZS_MAX_PAGES_PER_ZSPAGE)).
+ * Computed at preprocessor time, for use in #if below.  Kconfig
+ * restricts ZSMALLOC_CHAIN_SIZE to [4, 16].
+ */
+#if ZS_MAX_PAGES_PER_ZSPAGE <= 4
+#define ZS_PAGES_PER_ZSPAGE_BITS	2
+#elif ZS_MAX_PAGES_PER_ZSPAGE <= 8
+#define ZS_PAGES_PER_ZSPAGE_BITS	3
+#elif ZS_MAX_PAGES_PER_ZSPAGE <= 16
+#define ZS_PAGES_PER_ZSPAGE_BITS	4
+#else
+#error "ZSMALLOC_CHAIN_SIZE out of expected range [4,16]"
+#endif
+
+/*
+ * Bits to index an object within a single PAGE_SIZE at the smallest
+ * possible object size: log2(PAGE_SIZE / 32) = PAGE_SHIFT - 5.
+ * 32 is the hard floor of ZS_MIN_ALLOC_SIZE.
+ */
+#define ZS_OBJS_PER_PAGE_BITS	(PAGE_SHIFT - 5)
+
+/*
+ * Bits to index any object in the densest possible zspage.  Below this,
+ * ZS_MIN_ALLOC_SIZE is auto-raised by the MAX(32, ...) formula -- still
+ * correct, but objects are coarser.
+ */
+#define ZS_OBJS_PER_ZSPAGE_BITS \
+	(ZS_PAGES_PER_ZSPAGE_BITS + ZS_OBJS_PER_PAGE_BITS)
+
+/*
+ * Encode class_idx only when obj has spare bits; otherwise
+ * ZS_OBJ_CLASS_BITS folds to 0 (32-bit, or 64-bit UML/fallback).
+ */
+#if BITS_PER_LONG >= 64 && \
+	ZS_OBJ_PFN_SHIFT >= (CLASS_BITS + 1) + ZS_OBJS_PER_ZSPAGE_BITS
+#define ZS_OBJ_CLASS_BITS	(CLASS_BITS + 1)
+#else
+#define ZS_OBJ_CLASS_BITS	0
+#endif
+#define ZS_OBJ_CLASS_MASK	((_AC(1, UL) << ZS_OBJ_CLASS_BITS) - 1)
+
+#define ZS_OBJ_IDX_BITS		(ZS_OBJ_PFN_SHIFT - ZS_OBJ_CLASS_BITS)
+#define ZS_OBJ_IDX_MASK		((_AC(1, UL) << ZS_OBJ_IDX_BITS) - 1)
+
+/*
+ * Belt-and-suspenders: the #if above already guarantees this when
+ * class_idx is enabled.  Catches future tweaks that bypass it.
+ */
+static_assert(ZS_OBJ_IDX_BITS >= ZS_PAGES_PER_ZSPAGE_BITS,
+	      "zsmalloc: ZS_MIN_ALLOC_SIZE would exceed ZS_MAX_ALLOC_SIZE");
+
 /* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
 #define ZS_MIN_ALLOC_SIZE \
-	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
+	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> ZS_OBJ_IDX_BITS))
 /* each chunk includes extra space to keep handle */
 #define ZS_MAX_ALLOC_SIZE	PAGE_SIZE
 
@@ -721,26 +788,35 @@ static struct zpdesc *get_next_zpdesc(struct zpdesc *zpdesc)
 static void obj_to_location(unsigned long obj, struct zpdesc **zpdesc,
 				unsigned int *obj_idx)
 {
-	*zpdesc = pfn_zpdesc(obj >> OBJ_INDEX_BITS);
-	*obj_idx = (obj & OBJ_INDEX_MASK);
+	*zpdesc = pfn_zpdesc(obj >> ZS_OBJ_PFN_SHIFT);
+	*obj_idx = (obj & ZS_OBJ_IDX_MASK);
 }
 
 static void obj_to_zpdesc(unsigned long obj, struct zpdesc **zpdesc)
 {
-	*zpdesc = pfn_zpdesc(obj >> OBJ_INDEX_BITS);
+	*zpdesc = pfn_zpdesc(obj >> ZS_OBJ_PFN_SHIFT);
+}
+
+/* Folds to 0 when ZS_OBJ_CLASS_BITS == 0; no ifdef needed at callers. */
+static unsigned int obj_to_class_idx(unsigned long obj)
+{
+	return (obj >> ZS_OBJ_IDX_BITS) & ZS_OBJ_CLASS_MASK;
 }
 
 /**
- * location_to_obj - get obj value encoded from (<zpdesc>, <obj_idx>)
+ * location_to_obj - encode (<zpdesc>, <obj_idx>, <class_idx>) into obj value
  * @zpdesc: zpdesc object resides in zspage
  * @obj_idx: object index
+ * @class_idx: size class index; ignored when ZS_OBJ_CLASS_BITS == 0
  */
-static unsigned long location_to_obj(struct zpdesc *zpdesc, unsigned int obj_idx)
+static unsigned long location_to_obj(struct zpdesc *zpdesc, unsigned int obj_idx,
+				     unsigned int class_idx)
 {
 	unsigned long obj;
 
-	obj = zpdesc_pfn(zpdesc) << OBJ_INDEX_BITS;
-	obj |= obj_idx & OBJ_INDEX_MASK;
+	obj  = zpdesc_pfn(zpdesc) << ZS_OBJ_PFN_SHIFT;
+	obj |= (unsigned long)(class_idx & ZS_OBJ_CLASS_MASK) << ZS_OBJ_IDX_BITS;
+	obj |= obj_idx & ZS_OBJ_IDX_MASK;
 
 	return obj;
 }
@@ -1276,7 +1352,7 @@ static unsigned long obj_malloc(struct zs_pool *pool,
 	kunmap_local(vaddr);
 	mod_zspage_inuse(zspage, 1);
 
-	obj = location_to_obj(m_zpdesc, obj);
+	obj = location_to_obj(m_zpdesc, obj, zspage->class);
 	record_obj(handle, obj);
 
 	return obj;
@@ -1762,7 +1838,8 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 
 			old_obj = handle_to_obj(handle);
 			obj_to_location(old_obj, &dummy, &obj_idx);
-			new_obj = (unsigned long)location_to_obj(newzpdesc, obj_idx);
+			new_obj = location_to_obj(newzpdesc, obj_idx,
+						  obj_to_class_idx(old_obj));
 			record_obj(handle, new_obj);
 		}
 	}
-- 
2.34.1

[PATCH v6 2/4] mm/zsmalloc: drop pool->lock from zs_free on 64-bit systems

[Wenchao Hao]

From: Wenchao Hao <haowenchao@xiaomi.com>

With class_idx encoded in obj, zs_free() can locate the size_class
without holding pool->lock on 64-bit systems.  Page migration also
takes class->lock and only rewrites the PFN field of obj, so:

  1. read obj locklessly,
  2. lock the size_class derived from obj's class_idx,
  3. re-read obj under class->lock to get a stable PFN.

This eliminates the rwlock read-side cacheline bouncing between
zs_free() and migration/compaction on multi-core systems.

Annotate handle_to_obj()/record_obj() with READ_ONCE()/WRITE_ONCE() to
prevent load/store tearing on the lockless read path and silence KCSAN
data race reports.

When ZS_OBJ_CLASS_BITS == 0 (32-bit, or 64-bit with obj too narrow to
hold class_idx), zs_free() keeps pool->lock.

Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Signed-off-by: Wenchao Hao <haowenchao@xiaomi.com>
---
 mm/zsmalloc.c | 75 ++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 60 insertions(+), 15 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index c4fc06b259af..210a777081b7 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -21,6 +21,10 @@
  *	pool->lock
  *	class->lock
  *	zspage->lock
+ *
+ * When ZS_OBJ_CLASS_BITS > 0, zs_free() skips pool->lock; it picks
+ * the size_class from obj's encoded class_idx and serializes against
+ * page migration via class->lock.
  */
 
 #include <linux/module.h>
@@ -463,10 +467,13 @@ static void cache_free_zspage(struct zspage *zspage)
 	kmem_cache_free(zspage_cachep, zspage);
 }
 
-/* class->lock(which owns the handle) synchronizes races */
+/*
+ * Pairs with READ_ONCE() in handle_to_obj(): zs_free() may read the
+ * handle locklessly, so prevent store tearing here.
+ */
 static void record_obj(unsigned long handle, unsigned long obj)
 {
-	*(unsigned long *)handle = obj;
+	WRITE_ONCE(*(unsigned long *)handle, obj);
 }
 
 static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc)
@@ -823,7 +830,7 @@ static unsigned long location_to_obj(struct zpdesc *zpdesc, unsigned int obj_idx
 
 static unsigned long handle_to_obj(unsigned long handle)
 {
-	return *(unsigned long *)handle;
+	return READ_ONCE(*(unsigned long *)handle);
 }
 
 static inline bool obj_allocated(struct zpdesc *zpdesc, void *obj,
@@ -1457,10 +1464,58 @@ static void obj_free(int class_size, unsigned long obj)
 	mod_zspage_inuse(zspage, -1);
 }
 
+/*
+ * Resolve @handle to its zspage / size_class and acquire class->lock.
+ *
+ * When class_idx is encoded in obj (ZS_OBJ_CLASS_BITS > 0), it is
+ * invariant under page migration, so the handle can be read locklessly
+ * to pick the size_class.  Once class->lock is held migration is
+ * blocked and the handle is re-read to obtain a stable PFN.
+ *
+ * Otherwise (32-bit, or 64-bit fallback paths like UML where the
+ * encoding is disabled), fall back to pool->lock for the lookup.
+ */
+#if ZS_OBJ_CLASS_BITS > 0
+static inline void obj_class_get_and_lock(struct zs_pool *pool, unsigned long handle,
+					 unsigned long *objp, struct zspage **zspagep,
+					 struct size_class **classp)
+	__acquires(&(*classp)->lock)
+{
+	struct zpdesc *f_zpdesc;
+	unsigned long obj;
+
+	obj = handle_to_obj(handle);
+	*classp = pool->size_class[obj_to_class_idx(obj)];
+	spin_lock(&(*classp)->lock);
+	/* Re-read under class->lock: PFN is now stable vs migration. */
+	obj = handle_to_obj(handle);
+	obj_to_zpdesc(obj, &f_zpdesc);
+	*zspagep = get_zspage(f_zpdesc);
+	*objp = obj;
+}
+#else
+static inline void obj_class_get_and_lock(struct zs_pool *pool, unsigned long handle,
+					 unsigned long *objp, struct zspage **zspagep,
+					 struct size_class **classp)
+	__acquires(&(*classp)->lock)
+{
+	struct zpdesc *f_zpdesc;
+	unsigned long obj;
+
+	read_lock(&pool->lock);
+	obj = handle_to_obj(handle);
+	obj_to_zpdesc(obj, &f_zpdesc);
+	*zspagep = get_zspage(f_zpdesc);
+	*classp = zspage_class(pool, *zspagep);
+	spin_lock(&(*classp)->lock);
+	read_unlock(&pool->lock);
+	*objp = obj;
+}
+#endif
+
 void zs_free(struct zs_pool *pool, unsigned long handle)
 {
 	struct zspage *zspage;
-	struct zpdesc *f_zpdesc;
 	unsigned long obj;
 	struct size_class *class;
 	int fullness;
@@ -1468,17 +1523,7 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
 	if (IS_ERR_OR_NULL((void *)handle))
 		return;
 
-	/*
-	 * The pool->lock protects the race with zpage's migration
-	 * so it's safe to get the page from handle.
-	 */
-	read_lock(&pool->lock);
-	obj = handle_to_obj(handle);
-	obj_to_zpdesc(obj, &f_zpdesc);
-	zspage = get_zspage(f_zpdesc);
-	class = zspage_class(pool, zspage);
-	spin_lock(&class->lock);
-	read_unlock(&pool->lock);
+	obj_class_get_and_lock(pool, handle, &obj, &zspage, &class);
 
 	class_stat_sub(class, ZS_OBJS_INUSE, 1);
 	obj_free(class->size, obj);
-- 
2.34.1

Re: [PATCH v6 2/4] mm/zsmalloc: drop pool->lock from zs_free on 64-bit systems

[Andrew Morton]

On Fri, 26 Jun 2026 09:50:01 +0800 Wenchao Hao <haowenchao22@gmail.com> wrote:

> With class_idx encoded in obj, zs_free() can locate the size_class
> without holding pool->lock on 64-bit systems.  Page migration also
> takes class->lock and only rewrites the PFN field of obj, so:
> 
>   1. read obj locklessly,
>   2. lock the size_class derived from obj's class_idx,
>   3. re-read obj under class->lock to get a stable PFN.
> 
> This eliminates the rwlock read-side cacheline bouncing between
> zs_free() and migration/compaction on multi-core systems.
> 
> Annotate handle_to_obj()/record_obj() with READ_ONCE()/WRITE_ONCE() to
> prevent load/store tearing on the lockless read path and silence KCSAN
> data race reports.
> 
> When ZS_OBJ_CLASS_BITS == 0 (32-bit, or 64-bit with obj too narrow to
> hold class_idx), zs_free() keeps pool->lock.

This thing is still causing problems.  How about this?

From: Andrew Morton <akpm@linux-foundation.org>
Subject: mm-zsmalloc-drop-pool-lock-from-zs_free-on-64-bit-systems-fix
Date: Mon Jun 29 07:48:04 PM PDT 2026

build fix

mm/zsmalloc.c: In function 'obj_class_get_and_lock':
mm/zsmalloc.c:1481:36: error: implicit declaration of function 'obj_to_class_idx' [-Wimplicit-function-declaration]
 1481 |         *classp = pool->size_class[obj_to_class_idx(obj)];
      |                                    ^~~~~~~~~~~~~~~~

Cc: Barry Song <baohua@kernel.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Wenchao Hao <haowenchao@xiaomi.com>
Cc: Xueyuan Chen <xueyuan.chen21@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/zsmalloc.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/mm/zsmalloc.c~mm-zsmalloc-drop-pool-lock-from-zs_free-on-64-bit-systems-fix
+++ a/mm/zsmalloc.c
@@ -1457,6 +1457,12 @@ static void obj_free(int class_size, uns
 	mod_zspage_inuse(zspage, -1);
 }
 
+/* Folds to 0 when ZS_OBJ_CLASS_BITS == 0; no ifdef needed at callers. */
+static unsigned int obj_to_class_idx(unsigned long obj)
+{
+	return (obj >> ZS_OBJ_IDX_BITS) & ZS_OBJ_CLASS_MASK;
+}
+
 /*
  * Resolve @handle to its zspage / size_class and acquire class->lock.
  *
@@ -1759,12 +1765,6 @@ static void lock_zspage(struct zspage *z
 	zspage_read_unlock(zspage);
 }
 
-/* Folds to 0 when ZS_OBJ_CLASS_BITS == 0; no ifdef needed at callers. */
-static unsigned int obj_to_class_idx(unsigned long obj)
-{
-	return (obj >> ZS_OBJ_IDX_BITS) & ZS_OBJ_CLASS_MASK;
-}
-
 static void replace_sub_page(struct size_class *class, struct zspage *zspage,
 				struct zpdesc *newzpdesc, struct zpdesc *oldzpdesc)
 {
_

[PATCH v6 3/4] mm/zsmalloc: drop class lock before freeing zspage

[Wenchao Hao]

From: Xueyuan Chen <xueyuan.chen21@gmail.com>

Currently in zs_free(), the class->lock is held until the zspage is
completely freed and the counters are updated. However, freeing pages
back to the buddy allocator requires acquiring the zone lock.

Under heavy memory pressure, zone lock contention can be severe. When
this happens, the CPU holding the class->lock will stall waiting for
the zone lock, thereby blocking all other CPUs attempting to acquire
the same class->lock.

This patch shrinks the critical section of the class->lock to reduce
lock contention. By moving the actual page freeing process outside the
class->lock, we can improve the concurrency performance of zs_free().

Testing on the RADXA O6 platform shows that with 12 CPUs concurrently
performing zs_free() operations, the execution time is reduced by 20%.

Signed-off-by: Xueyuan Chen <xueyuan.chen21@gmail.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Signed-off-by: Wenchao Hao <haowenchao@xiaomi.com>
---
 mm/zsmalloc.c | 29 +++++++++++++++++++++++------
 1 file changed, 23 insertions(+), 6 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 210a777081b7..f3045345e3a8 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -884,13 +884,10 @@ static int trylock_zspage(struct zspage *zspage)
 	return 0;
 }
 
-static void __free_zspage(struct zs_pool *pool, struct size_class *class,
-				struct zspage *zspage)
+static inline void __free_zspage_lockless(struct zspage *zspage)
 {
 	struct zpdesc *zpdesc, *next;
 
-	assert_spin_locked(&class->lock);
-
 	VM_BUG_ON(get_zspage_inuse(zspage));
 	VM_BUG_ON(zspage->fullness != ZS_INUSE_RATIO_0);
 
@@ -906,7 +903,13 @@ static void __free_zspage(struct zs_pool *pool, struct size_class *class,
 	} while (zpdesc != NULL);
 
 	cache_free_zspage(zspage);
+}
 
+static void __free_zspage(struct zs_pool *pool, struct size_class *class,
+			  struct zspage *zspage)
+{
+	assert_spin_locked(&class->lock);
+	__free_zspage_lockless(zspage);
 	class_stat_sub(class, ZS_OBJS_ALLOCATED, class->objs_per_zspage);
 	atomic_long_sub(class->pages_per_zspage, &pool->pages_allocated);
 }
@@ -1519,6 +1522,7 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
 	unsigned long obj;
 	struct size_class *class;
 	int fullness;
+	struct zspage *zspage_to_free = NULL;
 
 	if (IS_ERR_OR_NULL((void *)handle))
 		return;
@@ -1529,10 +1533,23 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
 	obj_free(class->size, obj);
 
 	fullness = fix_fullness_group(class, zspage);
-	if (fullness == ZS_INUSE_RATIO_0)
-		free_zspage(pool, class, zspage);
+	if (fullness == ZS_INUSE_RATIO_0) {
+		if (trylock_zspage(zspage)) {
+			remove_zspage(class, zspage);
+			class_stat_sub(class, ZS_OBJS_ALLOCATED,
+				       class->objs_per_zspage);
+			zspage_to_free = zspage;
+		} else {
+			kick_deferred_free(pool);
+		}
+	}
 
 	spin_unlock(&class->lock);
+
+	if (zspage_to_free) {
+		__free_zspage_lockless(zspage_to_free);
+		atomic_long_sub(class->pages_per_zspage, &pool->pages_allocated);
+	}
 	cache_free_handle(handle);
 }
 EXPORT_SYMBOL_GPL(zs_free);
-- 
2.34.1

[PATCH v6 4/4] mm/zsmalloc: document free_zspage helper variants

[Wenchao Hao]

From: Wenchao Hao <haowenchao@xiaomi.com>

After splitting __free_zspage() into a lockless core and a wrapper that
does the class-stat bookkeeping, three similarly-named helpers coexist:
free_zspage / __free_zspage / __free_zspage_lockless.

Add a comment block above them describing what each does and where it
is used, so the names are not easy to confuse.

No functional change.

Suggested-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Signed-off-by: Wenchao Hao <haowenchao@xiaomi.com>
---
 mm/zsmalloc.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index f3045345e3a8..bf74e695d977 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -884,6 +884,22 @@ static int trylock_zspage(struct zspage *zspage)
 	return 0;
 }
 
+/*
+ * Three free helpers, kept apart here:
+ *
+ * __free_zspage_lockless(): bare core; walks zpdescs and returns pages
+ *   to the buddy allocator.  Caller owns all zpdesc locks and has
+ *   removed the zspage from its class list.  Used by zs_free() outside
+ *   class->lock so the buddy-side work does not stall the class.
+ *
+ * __free_zspage(): __free_zspage_lockless() + per-class accounting,
+ *   under class->lock.  Used by async_free_zspage(), the worker for
+ *   zspages whose trylock_zspage() failed.
+ *
+ * free_zspage(): full wrapper - trylock zpdescs, remove from class
+ *   list, call __free_zspage(); kicks deferred free on contention.
+ *   Used by compaction.
+ */
 static inline void __free_zspage_lockless(struct zspage *zspage)
 {
 	struct zpdesc *zpdesc, *next;
-- 
2.34.1

Re: [PATCH v6 0/4] mm/zsmalloc: reduce lock contention in zs_free()

[Andrew Morton]

On Fri, 26 Jun 2026 09:49:59 +0800 Wenchao Hao <haowenchao22@gmail.com> wrote:

> From: Wenchao Hao <haowenchao@xiaomi.com>
> 
> This series reduces lock contention in zs_free(), which dominates the
> unmap path under memory pressure on Android (LMK kills) and on x86
> servers running zswap-heavy workloads.
> 
> The current zs_free() takes pool->lock (rwlock, read side) just to
> look up the size_class for a handle, then takes class->lock and holds
> it across __free_zspage() which can call into the buddy allocator and
> acquire zone->lock.  Two costs follow:
> 
>   * pool->lock reader-counter cacheline bouncing among concurrent
>     zs_free() callers.
>   * class->lock held across folio_put(), so any zone->lock wait
>     fans out to every other zs_free() on the same class.
> 
> The series tackles both:
> 
>   Patch 1: encode size_class index into obj alongside PFN and obj_idx,
>            so zs_free() can locate the class without pool->lock.
>   Patch 2: drop pool->lock from zs_free() on 64-bit; 32-bit unchanged.
>   Patch 3: move zspage page-freeing out of class->lock.
>   Patch 4: document the three free_zspage helper variants that result
>            from the split in patch 3.
> 
> Performance results:
> 
> Test: each process independently mmap 256MB, write data, madvise
> MADV_PAGEOUT to swap out via zram (lzo-rle), then concurrent munmap.
> 
> Raspberry Pi 4B (4-core ARM64 Cortex-A72):
> 
>   mode        Base       Patched     Speedup
>   single      59.0ms     56.0ms      1.05x
>   multi 2p    94.6ms     66.7ms      1.42x
>   multi 4p    202.9ms    110.6ms     1.83x
> 
> x86 (20-core Intel i7-12700, 16 concurrent processes):
> 
>   mode        Base       Patched     Speedup
>   single      11.7ms     9.8ms       1.19x
>   multi 2p    24.1ms     17.2ms      1.40x
>   multi 4p    63.0ms     45.3ms      1.39x

Well that's a nice result.

Sashiko AI review said ....  nothing.  I don't recall seeing that
before ;)

I'll add this series to mm.git for the next step, thanks.

Re: [PATCH v6 0/4] mm/zsmalloc: reduce lock contention in zs_free()

[Sergey Senozhatsky]

On (26/06/26 09:49), Wenchao Hao wrote:
[..]
> Changes since v5:
> - 2/4: rename obj_handle_class_lock() to obj_class_get_and_lock().
> - Pick up Reviewed-by from Barry Song on 2/4, 3/4, 4/4.
> 
> Changes since v4:
> - 1/4: rename macros to make their intent clearer, following the
>   kernel's *_BITS convention (Nhat Pham, Barry Song).
> - 3/4: drop the unused pool argument from __free_zspage_lockless().
> 
> Changes since v3:
> - 1/4: gate ZS_OBJ_CLASS_BITS on a new spare-bit check so unsuitable
>   configs (e.g. UML, no sparsemem) fall back to plain obj layout
>   instead of breaking the build.  (sashiko AI review)
> - 2/4: annotate handle_to_obj()/record_obj() with READ_ONCE/WRITE_ONCE
>   to silence KCSAN; sync the #if predicate with patch 1.  (sashiko AI review)
> - 4/4: clarify when async_free_zspage() runs; pick up Reviewed-by.  (Nhat Pham)
> - Drop Reviewed-by from Nhat Pham on 1/4 and 2/4: substantial logic
>   changes since v3, please re-review.
> 
> Changes since v2:
> - 3/4: drop likely() hint and tidy up else-branch braces, per review
>   nits from Nhat Pham and Joshua Hahn.
> - 4/4 (new): document free_zspage helper variants in a single comment
>   block, per Nhat Pham.
> - Pick up Reviewed-by tags from Nhat Pham (1/4, 2/4, 3/4) and
>   Joshua Hahn (3/4).
> - Fix patch 1/4 and 2/4 From: to match Signed-off-by, per Barry Song.
> 
> Changes since v1:
> - Rename obj-encoding macros for clarity.
> - Make 32-bit / 64-bit handling transparent at call sites
>   (no #ifdef in callers).
> - Extract a helper to keep zs_free() unified.
> 
> Wenchao Hao (3):
>   mm/zsmalloc: encode class index in obj value for lockless class lookup
>   mm/zsmalloc: drop pool->lock from zs_free on 64-bit systems
>   mm/zsmalloc: document free_zspage helper variants
> 
> Xueyuan Chen (1):
>   mm/zsmalloc: drop class lock before freeing zspage

Folks, really sorry for not being around and not being helpful,
I'm catching up on emails and will look at this in the coming days.
Many, many thanks to everyone who worked on it, reviewed it,
contributed to this work in one way or another.  Thank you!