From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 61A1EC2EA for ; Wed, 29 Jul 2026 04:14:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785298490; cv=none; b=c50prJYH/+clus2hehU14yXguzFwZGKUwnGSitF7RKb/cjDhz+AJ5gmQKppIWGquqrjdoOqxUBUSDwgC0fNU7Bdm1f7erluORmFuX421ID7IwwWMhyx20WI9j5xsqJx9S2VADsGwZDQceEGf9d5NHwKucxpbeurG1ekiSVOh5lo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785298490; c=relaxed/simple; bh=9kGQtk7s7sNa+Bhy7zapbpt0gs3rAsQCaD8B6pOpe5c=; h=Date:To:From:Subject:Message-Id; b=idju48clKcYdA18wnbXBPozqGOBtmZ34hkXsq7X3UFrcIA0gbCDkj1E8wg5j1lcIzITjoX1WcIeixICRu/Tb1wME6qAK/YAm9o5vsOy44FLMf9pZ1pW5Xz6ecdi523CaYL8Vbyx08FrDSmUky5FPE6t3b1sMlcFEaQQuYpA/2JI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=b6qDNzlY; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="b6qDNzlY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 368081F000E9; Wed, 29 Jul 2026 04:14:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1785298489; bh=JyG7g2l7adLALcGDKYcUyGbj6BRn/1oYRJ0OQHlJ8C8=; h=Date:To:From:Subject; b=b6qDNzlY/ide9MKyMqM0W1p5AUrSPtsqb0n1pUsdXowWtbw7o0N0VP5BQa0OuhpI5 L7uqbhlbhc3/ndnWRCw969l6iQfwy7Q825pCKOZCieXhvI6d3BJ14Bwqr8Vk3kADNu STZ6XYqDmwZZqgV+6deNKTdgdN1JqV67hrPONUis= Date: Tue, 28 Jul 2026 21:14:48 -0700 To: mm-commits@vger.kernel.org,xueyuan.chen21@gmail.com,senozhatsky@chromium.org,nphamcs@gmail.com,minchan@kernel.org,joshua.hahnjy@gmail.com,baohua@kernel.org,haowenchao@xiaomi.com,akpm@linux-foundation.org From: Andrew Morton Subject: [merged mm-stable] mm-zsmalloc-encode-class-index-in-obj-value-for-lockless-class-lookup.patch removed from -mm tree Message-Id: <20260729041449.368081F000E9@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The quilt patch titled Subject: mm/zsmalloc: encode class index in obj value for lockless class lookup has been removed from the -mm tree. Its filename was mm-zsmalloc-encode-class-index-in-obj-value-for-lockless-class-lookup.patch This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm ------------------------------------------------------ From: Wenchao Hao Subject: mm/zsmalloc: encode class index in obj value for lockless class lookup Date: Fri, 26 Jun 2026 09:50:00 +0800 Patch series "mm/zsmalloc: reduce lock contention in zs_free()", v6. 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 This patch (of 4): 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. [akpm@linux-foundation.org: fix obj_to_class_idx() defined but not used, remove duplicated #ifdef] Link: https://lore.kernel.org/20260626015003.2965881-1-haowenchao22@gmail.com Link: https://lore.kernel.org/20260626015003.2965881-2-haowenchao22@gmail.com Signed-off-by: Wenchao Hao Reviewed-by: Nhat Pham Cc: Barry Song Cc: Joshua Hahn Cc: Minchan Kim Cc: Sergey Senozhatsky Cc: Xueyuan Chen Signed-off-by: Andrew Morton --- mm/zsmalloc.c | 106 ++++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 90 insertions(+), 16 deletions(-) --- a/mm/zsmalloc.c~mm-zsmalloc-encode-class-index-in-obj-value-for-lockless-class-lookup +++ a/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 @@ -720,26 +787,29 @@ static struct zpdesc *get_next_zpdesc(st 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); } /** - * location_to_obj - get obj value encoded from (, ) + * location_to_obj - encode (, , ) 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; } @@ -1275,7 +1345,7 @@ static unsigned long obj_malloc(struct z 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; @@ -1643,9 +1713,12 @@ static void lock_zspage(struct zspage *z } zspage_read_unlock(zspage); } -#endif /* CONFIG_COMPACTION */ -#ifdef CONFIG_COMPACTION +/* 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) @@ -1761,7 +1834,8 @@ static int zs_page_migrate(struct 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); } } _ Patches currently in -mm which might be from haowenchao@xiaomi.com are