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Tue, 24 Mar 2026 03:54:35 -0700 (PDT) Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20260322164943.37460-1-devnexen@gmail.com> <202603241635.qNXDPwjs-lkp@intel.com> In-Reply-To: <202603241635.qNXDPwjs-lkp@intel.com> From: David CARLIER Date: Tue, 24 Mar 2026 10:54:23 +0000 X-Gm-Features: AaiRm50vrYNk_mlAnRdMfp0nxFOpfo962oTnSfj2Jot4SGUmcie4lybU4o5qGTY Message-ID: Subject: Re: [PATCH] mm/memcontrol: fix obj_cgroup leak in mem_cgroup_css_online() error path To: Dan Carpenter Cc: oe-kbuild@lists.linux.dev, Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Qi Zheng , lkp@intel.com, oe-kbuild-all@lists.linux.dev, Linux Memory Management List , stable@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Hi Dan, On Tue, 24 Mar 2026 at 09:10, Dan Carpenter wrote: > > Hi David, > > kernel test robot noticed the following build warnings: > > url: https://github.com/intel-lab-lkp/linux/commits/David-Carlier/mm-memcontrol-fix-obj_cgroup-leak-in-mem_cgroup_css_online-error-path/20260324-010357 > base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything > patch link: https://lore.kernel.org/r/20260322164943.37460-1-devnexen%40gmail.com > patch subject: [PATCH] mm/memcontrol: fix obj_cgroup leak in mem_cgroup_css_online() error path > config: arm64-randconfig-r072-20260324 (https://download.01.org/0day-ci/archive/20260324/202603241635.qNXDPwjs-lkp@intel.com/config) > compiler: aarch64-linux-gcc (GCC) 14.3.0 > smatch: v0.5.0-9004-gb810ac53 > > If you fix the issue in a separate patch/commit (i.e. not just a new version of > the same patch/commit), kindly add following tags > | Reported-by: kernel test robot > | Reported-by: Dan Carpenter > | Closes: https://lore.kernel.org/r/202603241635.qNXDPwjs-lkp@intel.com/ > > smatch warnings: > mm/memcontrol.c:4180 mem_cgroup_css_online() warn: variable dereferenced before check 'pn' (see line 4176) > > vim +/pn +4180 mm/memcontrol.c > > 6f0df8e16eb543 Johannes Weiner 2023-08-23 4168 * regular ID destruction during offlining. > 6f0df8e16eb543 Johannes Weiner 2023-08-23 4169 */ > e77786b4682e69 Shakeel Butt 2025-12-25 4170 xa_store(&mem_cgroup_private_ids, memcg->id.id, memcg, GFP_KERNEL); > 6f0df8e16eb543 Johannes Weiner 2023-08-23 4171 > 2f7dd7a4100ad4 Johannes Weiner 2014-10-02 4172 return 0; > 098fad3e1621cb Qi Zheng 2026-03-05 4173 free_objcg: > 098fad3e1621cb Qi Zheng 2026-03-05 4174 for_each_node(nid) { > 098fad3e1621cb Qi Zheng 2026-03-05 4175 struct mem_cgroup_per_node *pn = memcg->nodeinfo[nid]; > 59f75a1877fbf7 David Carlier 2026-03-22 @4176 objcg = rcu_replace_pointer(pn->objcg, NULL, true); > ^^^^^^^^^ > Dereference > > 59f75a1877fbf7 David Carlier 2026-03-22 4177 if (objcg) > 59f75a1877fbf7 David Carlier 2026-03-22 4178 percpu_ref_kill(&objcg->refcnt); > 098fad3e1621cb Qi Zheng 2026-03-05 4179 > 4a2f95f5c79e02 Qi Zheng 2026-03-09 @4180 if (pn && pn->orig_objcg) { > ^^ > Checked too late. > > 098fad3e1621cb Qi Zheng 2026-03-05 4181 obj_cgroup_put(pn->orig_objcg); > 4a2f95f5c79e02 Qi Zheng 2026-03-09 4182 /* > 02b5fc7885d9f8 Andrew Morton 2026-03-09 4183 * Reset pn->orig_objcg to NULL to prevent > 02b5fc7885d9f8 Andrew Morton 2026-03-09 4184 * obj_cgroup_put() from being called again in > 02b5fc7885d9f8 Andrew Morton 2026-03-09 4185 * __mem_cgroup_free(). > 4a2f95f5c79e02 Qi Zheng 2026-03-09 4186 */ > 4a2f95f5c79e02 Qi Zheng 2026-03-09 4187 pn->orig_objcg = NULL; > 4a2f95f5c79e02 Qi Zheng 2026-03-09 4188 } > 098fad3e1621cb Qi Zheng 2026-03-05 4189 } > a0dd8b1942f5bf Muchun Song 2026-03-05 4190 free_shrinker_info(memcg); > da0efe30944476 Muchun Song 2022-03-22 4191 offline_kmem: > da0efe30944476 Muchun Song 2022-03-22 4192 memcg_offline_kmem(memcg); > e77786b4682e69 Shakeel Butt 2025-12-25 4193 mem_cgroup_private_id_remove(memcg); > da0efe30944476 Muchun Song 2022-03-22 4194 return -ENOMEM; > 8cdea7c0545426 Balbir Singh 2008-02-07 4195 } > > -- > 0-DAY CI Kernel Test Service > https://github.com/intel/lkp-tests/wiki > Seems Smatch is flagging the inconsistency, but pn cannot be NULL at the free_objcg label because all nodeinfo[] entries were fully allocated in mem_cgroup_alloc() before css_online() runs. The old pn && check was unnecessary defensive code. Kind regards.