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Wed, 21 Jan 2026 06:57:20 +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 wiJhCdB4cGl6JQAAD6G6ig (envelope-from ); Wed, 21 Jan 2026 06:57:20 +0000 From: Vlastimil Babka Date: Wed, 21 Jan 2026 07:57:10 +0100 Subject: [PATCH v2] slab: replace cache_from_obj() with inline checks Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260121-b4-remove_cache_from_obj-v2-1-7213d36b89d5@suse.cz> X-B4-Tracking: v=1; b=H4sIAMV4cGkC/42NQQ7CIBBFr9LMWgyDTbWuvIdpCB0HwaTFgBK14 e5iT+DyveS/v0Di6DnBsVkgcvbJh7mC2jRAzsxXFv5SGZRUnUQlxdiKyFPIrMmQY21jmHQYbwJ 7aclY7PaHHur8Htn615o+D5WdT48Q3+tTxp/9I5pRoDDEO0ktMpM9pWfiLX1gKKV8AXZlhH++A AAA X-Change-ID: 20260120-b4-remove_cache_from_obj-190fcaf16789 To: Harry Yoo Cc: Eric Dumazet , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, Vlastimil Babka X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; 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MX_GOOD(-0.01)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCVD_TLS_ALL(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; FUZZY_RATELIMITED(0.00)[rspamd.com]; TO_DN_SOME(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; MID_RHS_MATCH_FROM(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_SEVEN(0.00)[10]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.cz:dkim,suse.cz:mid,suse.cz:email,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,oracle.com:email,linux.dev:email]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.cz:+] X-Spam-Level: X-Rspamd-Action: no action X-Rspamd-Queue-Id: 3960E5BCCA X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Spam-Flag: NO Eric Dumazet has noticed cache_from_obj() is not inlined with clang and suggested splitting it into two functions, where the smaller inlined one assumes the fastpath is !CONFIG_SLAB_FREELIST_HARDENED. However most distros enable it these days and so this would likely add a function call to the object free fastpaths. Instead take a step back and consider that cache_from_obj() is a relict from when memcgs created their separate kmem_cache copies, as the outdated comment in build_detached_freelist() reminds us. Meanwhile hardening/debugging had reused cache_from_obj() to validate that the freed object really belongs to a slab from the cache we think we are freeing from. In build_detached_freelist() simply remove this, because it did not handle the NULL result from cache_from_obj() failure properly, nor validate objects (for the NULL slab->slab_cache pointer) when called via kfree_bulk(). If anyone is motivated to implement it properly, it should be possible in a similar way to kmem_cache_free(). In kmem_cache_free(), do the hardening/debugging checks directly so they are inlined by definition and virt_to_slab(obj) is performed just once. In case they failed, call a newly introduced warn_free_bad_obj() that performs the warnings outside of the fastpath, and leak the object. As an intentional change, leak the object when slab->slab_cache differs from the cache given to kmem_cache_free(). Previously we would only leak when the object is not in a valid slab page or the slab->slab_cache pointer is NULL, and otherwise trust the slab->slab_cache over the kmem_cache_free() argument. But if those differ, it means something went wrong enough that it's best not to continue freeing. As a result the fastpath should be inlined in all configs and the warnings are moved away. Reported-by: Eric Dumazet Closes: https://lore.kernel.org/all/20260115130642.3419324-1-edumazet@googl= e.com/ Reviewed-by: Harry Yoo Reviewed-by: Hao Li Signed-off-by: Vlastimil Babka Acked-by: Eric Dumazet Tested-by: David Rientjes --- I'll put this to slab/for-next. Thanks for the reviews! Changes in v2: - Add a comment and mention in the commit log that we are leaking the freed object also in cases where s !=3D slab->slab_cache (Hao Li) - Link to v1: https://patch.msgid.link/20260120-b4-remove_cache_from_obj-v1= -1-ace30c41eecf@suse.cz --- mm/slub.c | 56 +++++++++++++++++++++++++++++++++----------------------- 1 file changed, 33 insertions(+), 23 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index 861592ac5425..fd915ea95121 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -6738,30 +6738,26 @@ void ___cache_free(struct kmem_cache *cache, void *= x, unsigned long addr) } #endif =20 -static inline struct kmem_cache *virt_to_cache(const void *obj) +static noinline void warn_free_bad_obj(struct kmem_cache *s, void *obj) { + struct kmem_cache *cachep; struct slab *slab; =20 slab =3D virt_to_slab(obj); - if (WARN_ONCE(!slab, "%s: Object is not a Slab page!\n", __func__)) - return NULL; - return slab->slab_cache; -} - -static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void= *x) -{ - struct kmem_cache *cachep; + if (WARN_ONCE(!slab, + "kmem_cache_free(%s, %p): object is not in a slab page\n", + s->name, obj)) + return; =20 - if (!IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) && - !kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) - return s; + cachep =3D slab->slab_cache; =20 - cachep =3D virt_to_cache(x); - if (WARN(cachep && cachep !=3D s, - "%s: Wrong slab cache. %s but object is from %s\n", - __func__, s->name, cachep->name)) - print_tracking(cachep, x); - return cachep; + if (WARN_ONCE(cachep !=3D s, + "kmem_cache_free(%s, %p): object belongs to different cache %s\n", + s->name, obj, cachep ? cachep->name : "(NULL)")) { + if (cachep) + print_tracking(cachep, obj); + return; + } } =20 /** @@ -6774,11 +6770,25 @@ static inline struct kmem_cache *cache_from_obj(str= uct kmem_cache *s, void *x) */ void kmem_cache_free(struct kmem_cache *s, void *x) { - s =3D cache_from_obj(s, x); - if (!s) - return; + struct slab *slab; + + slab =3D virt_to_slab(x); + + if (IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) || + kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) { + + /* + * Intentionally leak the object in these cases, because it + * would be too dangerous to continue. + */ + if (unlikely(!slab || (slab->slab_cache !=3D s))) { + warn_free_bad_obj(s, x); + return; + } + } + trace_kmem_cache_free(_RET_IP_, x, s); - slab_free(s, virt_to_slab(x), x, _RET_IP_); + slab_free(s, slab, x, _RET_IP_); } EXPORT_SYMBOL(kmem_cache_free); =20 @@ -7305,7 +7315,7 @@ int build_detached_freelist(struct kmem_cache *s, siz= e_t size, df->s =3D slab->slab_cache; } else { df->slab =3D slab; - df->s =3D cache_from_obj(s, object); /* Support for memcg */ + df->s =3D s; } =20 /* Start new detached freelist */ --- base-commit: 0f61b1860cc3f52aef9036d7235ed1f017632193 change-id: 20260120-b4-remove_cache_from_obj-190fcaf16789 Best regards, --=20 Vlastimil Babka