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 B5M5V5+d 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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