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Mon, 26 Jan 2026 07:36:17 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id 0uVEAHEZd2kKZQAAD6G6ig (envelope-from ); Mon, 26 Jan 2026 07:36:17 +0000 Message-ID: <2b116198-b27a-4b20-90b2-951343f9fff1@suse.cz> Date: Mon, 26 Jan 2026 08:36:16 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/slab: avoid allocating slabobj_ext array from its own slab Content-Language: en-US To: Harry Yoo , akpm@linux-foundation.org Cc: linux-mm@kvack.org, cl@gentwo.org, rientjes@google.com, surenb@google.com, hao.li@linux.dev, kernel test robot , stable@vger.kernel.org References: <20260124104614.9739-1-harry.yoo@oracle.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: <20260124104614.9739-1-harry.yoo@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: ECBFC4000F X-Stat-Signature: gukxh9nfjbx8bqx6cefxxrn7gxy5fyiq X-Rspam-User: X-HE-Tag: 1769412978-641995 X-HE-Meta: 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 ajCWcjNM BJSZBZ+wO2JqYgR//dPAHx66oMZMmrKkoWcmDRrYmk85CPjwiGoE2jQNnBGRuhbx6ozzpdkn/ReAsk4ewKQosYPl08/VO2j/K/3ux1WqMs1sZAl07idSG5+DLQfKAlvOTg84aStP3r6kZpby0tyFII3qkYCnhBWc/XOE76FHHU61ghZalwll2+tSJ8AVsLA0GzxGNlE2bIwyn1N1byxsDlt54D+oOulgTXRt9+XKhKI7ASzMDz+SfM84C5czvoOmldma/tRZIvEXMWMHcb2DK11Yq+hqajOmmIDko+iJrKqLDKdWwbhE4E4C+O9wqjAR81gC0+XEi3vgS7n+2Wf6wdmHdUxUuRTKJ11ATCdm5WLZlS8AvUlmPTDaGmF/ZxF+kP6pJ6f0aVbIHjydPMS5DcjrSKA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 1/24/26 11:46, Harry Yoo wrote: > When allocating slabobj_ext array in alloc_slab_obj_exts(), the array > can be allocated from the same slab we're allocating the array for. > This led to obj_exts_in_slab() incorrectly returning true [1], > although the array is not allocated from wasted space of the slab. > > Vlastimil Babka observed that this problem should be fixed even when > ignoring its incompatibility with obj_exts_in_slab(), because it creates > slabs that are never freed as there is always at least one allocated > object. > > To avoid this, use the next kmalloc size or large kmalloc when > kmalloc_slab() returns the same cache we're allocating the array for. > > In case of random kmalloc caches, there are multiple kmalloc caches for > the same size and the cache is selected based on the caller address. > Because it is fragile to ensure the same caller address is passed to > kmalloc_slab(), kmalloc_noprof(), and kmalloc_node_noprof(), fall back > to (s->object_size + 1) when the sizes are equal. > > Note that this doesn't happen when memory allocation profiling is > disabled, as when the allocation of the array is triggered by memory > cgroup (KMALLOC_CGROUP), the array is allocated from KMALLOC_NORMAL. > > Reported-by: kernel test robot > Closes: https://lore.kernel.org/oe-lkp/202601231457.f7b31e09-lkp@intel.com [1] > Cc: stable@vger.kernel.org > Fixes: 4b8736964640 ("mm/slab: add allocation accounting into slab allocation and free paths") > Signed-off-by: Harry Yoo Thanks! Just wondering if we could simplify a bit. > --- > mm/slub.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++------- > 1 file changed, 55 insertions(+), 7 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index 3ff1c475b0f1..43ddb96c4081 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2104,6 +2104,52 @@ static inline void init_slab_obj_exts(struct slab *slab) > slab->obj_exts = 0; > } > > +/* > + * Calculate the allocation size for slabobj_ext array. > + * > + * When memory allocation profiling is enabled, the obj_exts array > + * could be allocated from the same slab cache it's being allocated for. > + * This would prevent the slab from ever being freed because it would > + * always contain at least one allocated object (its own obj_exts array). > + * > + * To avoid this, increase the allocation size when we detect the array > + * would come from the same cache, forcing it to use a different cache. > + */ > +static inline size_t obj_exts_alloc_size(struct kmem_cache *s, > + struct slab *slab, gfp_t gfp) > +{ > + size_t sz = sizeof(struct slabobj_ext) * slab->objects; > + struct kmem_cache *obj_exts_cache; > + > + /* > + * slabobj_ext array for KMALLOC_CGROUP allocations > + * are served from KMALLOC_NORMAL caches. > + */ > + if (!mem_alloc_profiling_enabled()) > + return sz; > + > + if (sz > KMALLOC_MAX_CACHE_SIZE) > + return sz; Could we bail out here immediately if !is_kmalloc_normal(s)? > + > + obj_exts_cache = kmalloc_slab(sz, NULL, gfp, 0); Then do this. > + if (s == obj_exts_cache) > + return obj_exts_cache->object_size + 1; But not this. > + /* > + * Random kmalloc caches have multiple caches per size, and the cache > + * is selected by the caller address. Since caller address may differ > + * between kmalloc_slab() and actual allocation, bump size when both > + * are normal kmalloc caches of same size. > + */ > + if (IS_ENABLED(CONFIG_RANDOM_KMALLOC_CACHES) && Instead just compare object_size unconditionally. > + is_kmalloc_normal(s) && This we already checked. > + is_kmalloc_normal(obj_exts_cache) && I think this is guaranteed thanks to "gfp &= ~OBJCGS_CLEAR_MASK;" below so we don't need it. > + (s->object_size == obj_exts_cache->object_size)) > + return obj_exts_cache->object_size + 1; > + > + return sz; > +} > + > int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > gfp_t gfp, bool new_slab) > { > @@ -2112,26 +2158,26 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > unsigned long new_exts; > unsigned long old_exts; > struct slabobj_ext *vec; > + size_t sz; > > gfp &= ~OBJCGS_CLEAR_MASK; > /* Prevent recursive extension vector allocation */ > gfp |= __GFP_NO_OBJ_EXT; > > + sz = obj_exts_alloc_size(s, slab, gfp); > + > /* > * Note that allow_spin may be false during early boot and its > * restricted GFP_BOOT_MASK. Due to kmalloc_nolock() only supporting > * architectures with cmpxchg16b, early obj_exts will be missing for > * very early allocations on those. > */ > - if (unlikely(!allow_spin)) { > - size_t sz = objects * sizeof(struct slabobj_ext); > - > + if (unlikely(!allow_spin)) > vec = kmalloc_nolock(sz, __GFP_ZERO | __GFP_NO_OBJ_EXT, > slab_nid(slab)); > - } else { > - vec = kcalloc_node(objects, sizeof(struct slabobj_ext), gfp, > - slab_nid(slab)); > - } > + else > + vec = kmalloc_node(sz, gfp | __GFP_ZERO, slab_nid(slab)); > + > if (!vec) { > /* > * Try to mark vectors which failed to allocate. > @@ -2145,6 +2191,8 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > return -ENOMEM; > } > > + VM_WARN_ON_ONCE(virt_to_slab(vec)->slab_cache == s); > + > new_exts = (unsigned long)vec; > if (unlikely(!allow_spin)) > new_exts |= OBJEXTS_NOSPIN_ALLOC;