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Thu, 5 Mar 2026 13:38:02 +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 7MYHKDqHqWloPgAAD6G6ig (envelope-from ); Thu, 05 Mar 2026 13:38:02 +0000 Date: Thu, 5 Mar 2026 13:38:00 +0000 From: Pedro Falcato To: Harry Yoo Cc: vbabka@suse.cz, akpm@linux-foundation.org, cgroups@vger.kernel.org, cl@gentwo.org, hannes@cmpxchg.org, hao.li@linux.dev, linux-mm@kvack.org, mhocko@kernel.org, muchun.song@linux.dev, rientjes@google.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, surenb@google.com, venkat88@linux.ibm.com Subject: Re: [PATCH] mm/slab: change stride type from unsigned short to unsigned int Message-ID: References: <20260303135722.2680521-1-harry.yoo@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260303135722.2680521-1-harry.yoo@oracle.com> X-Rspamd-Action: no action X-Rspamd-Queue-Id: C331B40010 X-Rspamd-Server: rspam07 X-Stat-Signature: xk9xrsrqan7hy1cfzzhbzmnkiaz89mmm X-Rspam-User: X-HE-Tag: 1772717886-790787 X-HE-Meta: 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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 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 > --- > > 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! -- Pedro