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Received: from viro by zeniv.linux.org.uk with local (Exim 4.99.2 #2 (Red Hat Linux)) id 1wYGcy-00000003aT0-1Lre; Sat, 13 Jun 2026 05:09:52 +0000 From: Al Viro To: linux-mm@kvack.org Cc: Vlastimil Babka , Harry Yoo , linux-fsdevel@vger.kernel.org, Linus Torvalds , Christian Brauner , Jan Kara , Mateusz Guzik , linux-kernel@vger.kernel.org Subject: [RFC PATCH v3 03/10] allow preallocated kmem_cache instances in modules Date: Sat, 13 Jun 2026 06:09:44 +0100 Message-ID: <20260613050951.855141-4-viro@zeniv.linux.org.uk> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260613050951.855141-1-viro@zeniv.linux.org.uk> References: <20260611171425.1671254-1-viro@zeniv.linux.org.uk> <20260613050951.855141-1-viro@zeniv.linux.org.uk> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 0DAD11C0007 X-Rspam-User: X-Stat-Signature: fp43utdgfj83itswbumr6yhhys9yhoet X-Rspamd-Server: rspam09 X-HE-Tag: 1781327396-602120 X-HE-Meta: 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 JZh5Q/t+ tMylooYJDUb0427QF5MjQkffGRgXxm3cg2gKZFJUrEtENdGWWkTBNsXC7unF2BRNbXMBQMXB/ga96cnhliXh9ezKxcfw8JbJ99eFtQI7MQ5MKWQ1v8emLV/MndznPye8OX4KzOEJx8uPlENhZoACLuY1KINmAS9ddNqCXjVzbjIm1Ee8wyhE/DsJtiwm8QjuDrGIBGVUB1HQgxly0U8wDciK0JxE4wmOMmI4/lmRBWWdTTKzt2Tu7IkorhtyroS+BllFd+aIydO20rCLK1dUyMVrCHdsCwbHUQqvIKRbjWsKjIVhoS0QMQt+KiCOZBUg7J+Sk8wH0UCboN7OlLZ+9FxobWa/HAt6q6jg91jWBwnx1B1AZz7ZUBz12yf5STOsWQ2/ljrV/mZkkciVKswCWV/nS0cu20IZOkN0UOim32aWxAwKbzJik5oeWEi++c+Hz4ZevoA9jBwsgj4hqzQrUKvxU8v3RIniWazWSK7eDA2PpDffIe4Qzba9JuQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: We need to make sure that instance in a module will get to slab_kmem_cache_release() before the module data gets freed. That's only a problem on sysfs setups - otherwise it'll definitely be finished before kmem_cache_destroy() returns. Note that modules themselves have sysfs-exposed attributes, so a similar problem already exists there. That's dealt with by having mod_sysfs_teardown() wait for refcount of module->mkobj.kobj reaching zero. Let's make use of that - have static-duration-in-module kmem_cache instances grab a reference to that kobject upon setup and drop it in the end of slab_kmem_cache_release(). Let setup helpers store the kobject to be pinned in kmem_cache_args->owner (for preallocated; if somebody manually sets it for non-preallocated case, it'll be ignored). That would be &THIS_MODULE->mkobj.kobj for a module and NULL in built-in. If sysfs is enabled and we are dealing with preallocated instance, let create_cache() grab and stash that reference in kmem_cache->owner and let slab_kmem_cache_release() drop it instead of freeing kmem_cache instance. Preallocated kmem_cache instances in modules are forcibly made non-mergable - any subsequent alias could outlive the module (and module's .data) by arbitrary long time. If you really want your (modular) cache to be mergable, just have it dynamically allocated. Signed-off-by: Al Viro --- include/linux/slab-static.h | 11 +++++++---- include/linux/slab.h | 1 + mm/slab.h | 1 + mm/slab_common.c | 19 +++++++++++++++++-- 4 files changed, 26 insertions(+), 6 deletions(-) diff --git a/include/linux/slab-static.h b/include/linux/slab-static.h index 007fc0bd4e8c..16d1564b4a4b 100644 --- a/include/linux/slab-static.h +++ b/include/linux/slab-static.h @@ -5,18 +5,21 @@ #include #include -#ifdef MODULE -#error "can't use that in modules" -#endif - /* same size and alignment as struct kmem_cache: */ struct kmem_cache_opaque { unsigned char opaque[KMEM_CACHE_SIZE]; } __aligned(KMEM_CACHE_ALIGN); +#ifdef MODULE +#define THIS_MODULE_KOBJ &THIS_MODULE->mkobj.kobj +#else +#define THIS_MODULE_KOBJ NULL +#endif + #define __KMEM_CACHE_SETUP(cache, name, size, flags, ...) \ __kmem_cache_create_args((name), (size), \ &(struct kmem_cache_args) { \ + .owner = THIS_MODULE_KOBJ, \ .preallocated = (cache), \ __VA_ARGS__}, (flags)) diff --git a/include/linux/slab.h b/include/linux/slab.h index ec68aabf98df..c95ad771ece2 100644 --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -382,6 +382,7 @@ struct kmem_cache_args { */ unsigned int sheaf_capacity; struct kmem_cache *preallocated; + struct kobject *owner; }; struct kmem_cache *__kmem_cache_create_args(const char *name, diff --git a/mm/slab.h b/mm/slab.h index bf2f87acf5e3..8b1db0d03226 100644 --- a/mm/slab.h +++ b/mm/slab.h @@ -219,6 +219,7 @@ struct kmem_cache { struct list_head list; /* List of slab caches */ #ifdef CONFIG_SYSFS struct kobject kobj; /* For sysfs */ + struct kobject *owner; /* keep that pinned while alive */ #endif #ifdef CONFIG_SLAB_FREELIST_HARDENED unsigned long random; diff --git a/mm/slab_common.c b/mm/slab_common.c index 5b6aaa96d68d..1717c79d92be 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -255,6 +255,12 @@ static struct kmem_cache *create_cache(const char *name, kmem_cache_free(kmem_cache, s); return ERR_PTR(err); } +#ifdef CONFIG_SYSFS + if (flags & SLAB_PREALLOCATED) { + s->owner = args->owner; + kobject_get(s->owner); + } +#endif s->refcount = 1; list_add(&s->list, &slab_caches); return s; @@ -366,8 +372,11 @@ struct kmem_cache *__kmem_cache_create_args(const char *name, object_size - args->usersize < args->useroffset)) args->usersize = args->useroffset = 0; - if (args->preallocated) + if (args->preallocated) { flags |= SLAB_PREALLOCATED; + if (args->owner) + flags |= SLAB_NO_MERGE; + } s = __kmem_cache_alias(name, object_size, flags, args); if (s) @@ -524,7 +533,13 @@ void slab_kmem_cache_release(struct kmem_cache *s) { __kmem_cache_release(s); kfree_const(s->name); - kmem_cache_free(kmem_cache, s); + if (!(s->flags & SLAB_PREALLOCATED)) { + kmem_cache_free(kmem_cache, s); + return; + } +#ifdef CONFIG_SYSFS + kobject_put(s->owner); +#endif } void kmem_cache_destroy(struct kmem_cache *s) -- 2.47.3