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Thu, 25 Jun 2026 18:54:54 -0700 (PDT) Received: from ubuntu22.mioffice.cn ([43.224.245.232]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2c7f63b2bd7sm29047005ad.48.2026.06.25.18.54.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 25 Jun 2026 18:54:53 -0700 (PDT) From: Wenchao Hao To: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Minchan Kim , Sergey Senozhatsky Cc: Nhat Pham , Joshua Hahn , Barry Song , Wenchao Hao Subject: [PATCH v6 2/4] mm/zsmalloc: drop pool->lock from zs_free on 64-bit systems Date: Fri, 26 Jun 2026 09:50:01 +0800 Message-Id: <20260626015003.2965881-3-haowenchao22@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260626015003.2965881-1-haowenchao22@gmail.com> References: <20260626015003.2965881-1-haowenchao22@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 482C31C000B X-Stat-Signature: hgnyoroe18ncbotwtqks18y953jmcez4 X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1782438895-342278 X-HE-Meta: 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 Ox4PZDKp 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Wenchao Hao 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 Reviewed-by: Barry Song Signed-off-by: Wenchao Hao --- 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 @@ -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