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a=xqWC_Br6kY4A:10 a=kj9zAlcOel0A:10 a=Yq5XynenixoA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=jiCTI4zE5U7BLdzWsZGv:22 a=BqU2WV_vvsyTyxaotp0D:22 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=yPCof4ZbAAAA:8 a=PH5Tzu57Fri5wPsWqzYA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-GUID: mMRmL5rukDAJtuInxYv5AkntE43NctVi X-Proofpoint-ORIG-GUID: mMRmL5rukDAJtuInxYv5AkntE43NctVi X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMzA2MDA1OSBTYWx0ZWRfX1F1TuTzQgxCa YskAuYGnLHI1ooBZ5JssNoFkmDjRfoUvmqxIVbu76L/jvIdAwAZG2512LN5pikB5hhsaa9V3nvV sOdJUNY5OepU3IhkpKZlnGoCds7VJrPO8Of2y2EChVl+ujInGOLCeEVyezMU1RWWHzGuQ73YT6P 2dA9y8u66fdfHU4/yIedM4qDaZzue3VVr9cQZ75DTG5A62l0NzuDCueZanpJ7S6DBcd2resjMZ9 xrHzQySGnaqG4/VBHLUNuuG9b0v5WTrRGt7IK54LLM4Ik6u73LBbqZ8Q3YLv1O7EywkBqjI77o0 8y1k5MH4BXSrM/EvXbA+lmVbUOi1amrQpgyVPbRAnynpb7f+bHFunyzNZRXOO/1VVrPaFossOB2 JzIDCJXfJ1f2niD76E+wo4KOfncW55DDdYFS8UNCQPEhiCIVrGJeUJkNK8/BlrlAuTTiNKwnaJV Yhuqr5VylYQnE8/ZgOg== On Thu, Mar 05, 2026 at 01:38:00PM +0000, Pedro Falcato wrote: > On Tue, Mar 03, 2026 at 10:57:22PM +0900, Harry Yoo wrote: > > Commit 7a8e71bc619d ("mm/slab: use stride to access slabobj_ext") > > defined the type of slab->stride as unsigned short, because the author > > initially planned to store stride within the lower 16 bits of the > > page_type field, but later stored it in unused bits in the counters > > field instead. > > > > However, the idea of having only 2-byte stride turned out to be a > > serious mistake. On systems with 64k pages, order-1 pages are 128k, > > which is larger than USHRT_MAX. It triggers a debug warning because > > s->size is 128k while stride, truncated to 2 bytes, becomes zero: > > > > ------------[ cut here ]------------ > > Warning! stride (0) != s->size (131072) > > WARNING: mm/slub.c:2231 at alloc_slab_obj_exts_early.constprop.0+0x524/0x534, CPU#6: systemd-sysctl/307 > > Modules linked in: > > CPU: 6 UID: 0 PID: 307 Comm: systemd-sysctl Not tainted 7.0.0-rc1+ #6 PREEMPTLAZY > > Hardware name: IBM,9009-22A POWER9 (architected) 0x4e0202 0xf000005 of:IBM,FW950.E0 (VL950_179) hv:phyp pSeries > > NIP: c0000000008a9ac0 LR: c0000000008a9abc CTR: 0000000000000000 > > REGS: c0000000141f7390 TRAP: 0700 Not tainted (7.0.0-rc1+) > > MSR: 8000000000029033 CR: 28004400 XER: 00000005 > > CFAR: c000000000279318 IRQMASK: 0 > > GPR00: c0000000008a9abc c0000000141f7630 c00000000252a300 c00000001427b200 > > GPR04: 0000000000000004 0000000000000000 c000000000278fd0 0000000000000000 > > GPR08: fffffffffffe0000 0000000000000000 0000000000000000 0000000022004400 > > GPR12: c000000000f644b0 c000000017ff8f00 0000000000000000 0000000000000000 > > GPR16: 0000000000000000 c0000000141f7aa0 0000000000000000 c0000000141f7a88 > > GPR20: 0000000000000000 0000000000400cc0 ffffffffffffffff c00000001427b180 > > GPR24: 0000000000000004 00000000000c0cc0 c000000004e89a20 c00000005de90011 > > GPR28: 0000000000010010 c00000005df00000 c000000006017f80 c00c000000177a00 > > NIP [c0000000008a9ac0] alloc_slab_obj_exts_early.constprop.0+0x524/0x534 > > LR [c0000000008a9abc] alloc_slab_obj_exts_early.constprop.0+0x520/0x534 > > Call Trace: > > [c0000000141f7630] [c0000000008a9abc] alloc_slab_obj_exts_early.constprop.0+0x520/0x534 (unreliable) > > [c0000000141f76c0] [c0000000008aafbc] allocate_slab+0x154/0x94c > > [c0000000141f7760] [c0000000008b41c0] refill_objects+0x124/0x16c > > [c0000000141f77c0] [c0000000008b4be0] __pcs_replace_empty_main+0x2b0/0x444 > > [c0000000141f7810] [c0000000008b9600] __kvmalloc_node_noprof+0x840/0x914 > > [c0000000141f7900] [c000000000a3dd40] seq_read_iter+0x60c/0xb00 > > [c0000000141f7a10] [c000000000b36b24] proc_reg_read_iter+0x154/0x1fc > > [c0000000141f7a50] [c0000000009cee7c] vfs_read+0x39c/0x4e4 > > [c0000000141f7b30] [c0000000009d0214] ksys_read+0x9c/0x180 > > [c0000000141f7b90] [c00000000003a8d0] system_call_exception+0x1e0/0x4b0 > > [c0000000141f7e50] [c00000000000d05c] system_call_vectored_common+0x15c/0x2ec > > > > This leads to slab_obj_ext() returning the first slabobj_ext or all > > objects and confuses the reference counting of object cgroups [1] and > > memory (un)charging for memory cgroups [2]. > > > > Fortunately, the counters field has 32 unused bits instead of 16 > > on 64-bit CPUs, which is wide enough to hold any value of s->size. > > Change the type to unsigned int. > > > > Reported-by: Venkat Rao Bagalkote > > Closes: https://lore.kernel.org/lkml/ca241daa-e7e7-4604-a48d-de91ec9184a5@linux.ibm.com [1] > > Closes: https://lore.kernel.org/all/ddff7c7d-c0c3-4780-808f-9a83268bbf0c@linux.ibm.com [2] > > Fixes: 7a8e71bc619d ("mm/slab: use stride to access slabobj_ext") > > Signed-off-by: Harry Yoo > > Acked-by: Pedro Falcato Thanks for the ack, Pedro! > > --- > > Hi Venkat, could you please test this on top of 7.0-rc2 (instead of > > 7.0-rc1) and see if the bugs [1] [2] are reproduced on your machine? > > > > I reproduced a debug warning on a ppc machine and fixed it. > > The bugs are expected to be resolved by this fix. > > > > p.s. After more debugging, I saw stride appeared as 0 even on the CPU > > that wrote it, which likely rules out a memory ordering issue... > > More fun than debugging memory ordering issues, is debugging memory ordering > issues that actually don't exist! Hehe, yeah. Just because memory barriers hid the problem doesn't mean it's always a memory ordering issue... a lesson learned the hard way ;) -- Cheers, Harry / Hyeonggon