Re: [PATCH] mm/slab: change stride type from unsigned short to unsigned int

[Hao Li <hao.li@linux.dev>]

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:

Wow, such a complex issue boiled down to something so straightforward.
This kind of corner case can be really hard to debug. Thanks!

Reviewed-by: Hao Li <hao.li@linux.dev>

> 
>   ------------[ 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 <SF,EE,ME,IR,DR,RI,LE>  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 <venkat88@linux.ibm.com>
> 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 <harry.yoo@oracle.com>
> ---
> 
> 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...
> and I discovered this while decoding ppc assembly suspecting memory
> corruption or a compiler bug, which came down to:
>   
>     "Hmm... why is the size truncated to 2 bytes?... OH WAIT!"
> 
>  mm/slab.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index f6ef862b60ef..e9ab292acd22 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -59,7 +59,7 @@ struct freelist_counters {
>  					 * to save memory. In case ->stride field is not available,
>  					 * such optimizations are disabled.
>  					 */
> -					unsigned short stride;
> +					unsigned int stride;
>  #endif
>  				};
>  			};
> @@ -559,20 +559,20 @@ static inline void put_slab_obj_exts(unsigned long obj_exts)
>  }
>  
>  #ifdef CONFIG_64BIT
> -static inline void slab_set_stride(struct slab *slab, unsigned short stride)
> +static inline void slab_set_stride(struct slab *slab, unsigned int stride)
>  {
>  	slab->stride = stride;
>  }
> -static inline unsigned short slab_get_stride(struct slab *slab)
> +static inline unsigned int slab_get_stride(struct slab *slab)
>  {
>  	return slab->stride;
>  }
>  #else
> -static inline void slab_set_stride(struct slab *slab, unsigned short stride)
> +static inline void slab_set_stride(struct slab *slab, unsigned int stride)
>  {
>  	VM_WARN_ON_ONCE(stride != sizeof(struct slabobj_ext));
>  }
> -static inline unsigned short slab_get_stride(struct slab *slab)
> +static inline unsigned int slab_get_stride(struct slab *slab)
>  {
>  	return sizeof(struct slabobj_ext);
>  }
> -- 
> 2.43.0
>