Re: [PATCH] mm/slub: clamp slab_min_order to MAX_PAGE_ORDER

[Harry Yoo <harry@kernel.org>]

On Thu, Oct 08, 2026 at 01:34:02AM +0900, Sang-Heon Jeon wrote:
> setup_slub_max_order() clamps slab_max_order to MAX_PAGE_ORDER, but
> setup_slub_min_order() has no such clamp.
> 
> The buddy allocator cannot allocate more than 2^MAX_PAGE_ORDER pages
> at once, so slab_min_order above it makes no sense.
> 
> If slab_min_order is set above MAX_PAGE_ORDER, it can even panic at
> boot or invoke undefined behavior in order_objects().
> 
> So clamp slab_min_order to MAX_PAGE_ORDER as well.
> 
> Fixes: 818cf5909701 ("slub: enforce MAX_ORDER")

I think Fixes: should be the commit that introduced slub_min_order.

> Cc: stable@vger.kernel.org

And no need for Cc: stable? It's been there for decades and people
should realize the kernel parameter is broken when it doesn't boot.

Otherwise looks good to me.

> Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> ---
> QEMU-based test results
> 
> - Booted with "slab_min_order=20"
> 
> 1) AS-IS (before fix)
> [    0.094926] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
> [    0.097776] Kernel panic - not syncing: Creation of kmalloc slab kmem_cache_node size=64 failed. Reason -22
> [    0.099542] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.2.0 #1 PREEMPT(undef) 
> [    0.100903] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [    0.102607] Call Trace:
> [    0.103040]  <TASK>
> [    0.103417]  vpanic+0x32a/0x4b0
> [    0.103976]  panic+0x5e/0x60
> [    0.104491]  create_boot_cache+0xe3/0xf0
> [    0.105194]  kmem_cache_init+0x12d/0x410
> [    0.105890]  ? mem_init+0x1fc/0x210
> [    0.106521]  mm_core_init+0xba/0x140
> [    0.107153]  start_kernel+0x441/0x8b0
> [    0.107814]  x86_64_start_reservations+0x18/0x30
> [    0.108638]  x86_64_start_kernel+0x10c/0x120
> [    0.109401]  common_startup_64+0x13e/0x158
> [    0.110145]  </TASK>
> [    0.110584] ---[ end Kernel panic - not syncing: Creation of kmalloc slab kmem_cache_node size=64 failed. Reason -22 ]---
> 
> 2) TO-BE (after fix)
> [    0.008976] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
> [    0.011366] SLUB: HWalign=64, Order=10-10, MinObjects=0, CPUs=4, Nodes=1
> 
> Also, I'm not sure about the Fixes commit. Please change it if you know
> a better one.
> 
> ---
>  mm/slub.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 1559cbace469..8400df7c222e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -8354,6 +8354,8 @@ static int __init setup_slub_min_order(const char *str, const struct kernel_para
>  	if (ret)
>  		return ret;
>  
> +	slub_min_order = min_t(unsigned int, slub_min_order, MAX_PAGE_ORDER);
> +
>  	if (slub_min_order > slub_max_order)
>  		slub_max_order = slub_min_order;
>  
> 
> base-commit: d755441b7672c74f29d1241ffdd2b067ecab1ca0
> -- 
> 2.43.0
> 

-- 
Cheers,
Harry / Hyeonggon