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From: Shakeel Butt To: Harry Yoo Cc: Venkat Rao Bagalkote , Vlastimil Babka , Carlos Maiolino , Johannes Weiner , Michal Hocko , Roman Gushchin , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, LKML , Madhavan Srinivasan , Ritesh Harjani , ojaswin@linux.ibm.com, Muchun Song , Cgroups , "linux-mm@kvack.org" , Hao Li Subject: Re: [next-20260216]NULL pointer dereference in drain_obj_stock() (RCU free path) Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 0D08D1A0006 X-Stat-Signature: mwgdtmecoj1pk4upfang66ua8dq9pwgd X-HE-Tag: 1771803414-981646 X-HE-Meta: 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 MB6JBeIX voxES5Xo7XLGa6zrNU8CnT0aoXpBtlv5H61qUXK7iMvozpHoVJhm7g+OnkHOw5BtUBR0g2FeeE15Y/BPrED0Rf4xqZkhl0rdu/XhbXTe3Phqnweathhel5A3TItfYVBNv5KgxGgOnVDsrUI06HeGPEDLH90Vi6Jmwq5YAdCf1eE7dfdSl0EEhSgpLzl44jPYIypJlt7zceRSztyJJ46TJvHwfrWJWuKnus9sFjqTWS2let7WtZd0XYPDBP6m2TNjgfHXK6Ig+GEmbkdaYzIanRbgFnTSl79Qek45g5PrmK6im/Wk= 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 Sun, Feb 22, 2026 at 08:47:03PM +0900, Harry Yoo wrote: [...] > > It seems it crashed while dereferencing objcg->ref->data->count. > I think that implies that obj_cgroup_release()->percpu_ref_exit() > is already called due to the refcount reaching zero and set > ref->data = NULL. > > Wait, was the stock->objcg ever a valid objcg? > I think it should be valid when refilling the obj stock, otherwise > it should have crashed in refill_obj_stock() -> obj_cgroup_get() path > in the first place, rather than crashing when draining. > > And that sounds like we're somehow calling obj_cgroup_put() more times > than obj_cgroup_get(). > > Anyway, this is my theory that it may be due to mis-refcounting of objcgs. > I have not looked deeper into recent slub changes (sheafs or obj_exts savings) but one thing looks weird to me: allocate_slab() // for cache with SLAB_OBJ_EXT_IN_OBJ -> alloc_slab_obj_exts_early() -> slab_set_stride(slab, s->size) -> account_slab() -> alloc_slab_obj_exts() -> slab_set_stride(slab, sizeof(struct slabobj_ext)); Unconditional overwrite of stride. Not sure if it is issue or even related to this crash but looks odd.