Re: [PATCH v2 5/6] mm/slab: Support disabling KMALLOC_PARTITION_CACHES at boot

[Kees Cook <kees@kernel.org>]

On Thu, Oct 29, 2026 at 02:09:09PM +0800, Chris Down wrote:
> KMALLOC_PARTITION_CACHES creates several copies of each normal kmalloc
> cache and spreads allocations across them. That costs a fair bit of
> memory: Harry measured about 18 MiB more slab memory after boot on a 64
> CPU, 64 GiB VM, mostly from under-used slabs [1]. Like SLAB_BUCKETS, it
> can only be chosen at build time.
> 
> Add a kmalloc_partition= early parameter, enabled by default on kernels
> built with CONFIG_KMALLOC_PARTITION_CACHES=y. When it's off, the copies
> alias the normal caches rather than being created.
> 
> Let's leave kmalloc_type() unchanged since each index simply resolves to
> the same cache when partitioning is off.
> 
> Link: https://lore.kernel.org/r/179312451613.474906.1475329637924619088@kernel.org [1]
> Signed-off-by: Chris Down <chris@chrisdown.name>
> ---
>  .../admin-guide/kernel-parameters.txt         |  7 +++++++
>  mm/Kconfig                                    |  5 +++++
>  mm/slab_common.c                              | 19 ++++++++++++++++++-
>  3 files changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index de2fd6963a16..6633a553a79c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3054,6 +3054,13 @@ Kernel parameters
>  			Configure the RouterBoard 532 series on-chip
>  			Ethernet adapter MAC address.
>  
> +	kmalloc_partition= [MM,EARLY]
> +			Format: <bool>
> +			Enable or disable kmalloc cache partitioning on
> +			kernels built with CONFIG_KMALLOC_PARTITION_CACHES=y.
> +			Ignored without that option.
> +			Default: on.
> +
>  	kmemleak=	[KNL,EARLY] Boot-time kmemleak enable/disable
>  			Valid arguments: on, off
>  			Default: on
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 48078c1fd26f..b90b3915fcd5 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -266,6 +266,11 @@ config KMALLOC_PARTITION_CACHES
>  	  limited degree of memory and CPU overhead that relates to hardware
>  	  and system workload.
>  
> +	  The partitioning can be disabled at boot with the
> +	  "kmalloc_partition=0" kernel parameter, in which case the copies
> +	  are not created and all normal kmalloc allocations use the
> +	  regular kmalloc caches.
> +
>  choice
>  	prompt "Partitioned slab cache mode"
>  	depends on KMALLOC_PARTITION_CACHES
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 9fe48da12010..5391149a2eed 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -781,6 +781,16 @@ unsigned long random_kmalloc_seed __ro_after_init;
>  EXPORT_SYMBOL(random_kmalloc_seed);
>  #endif
>  
> +#ifdef CONFIG_KMALLOC_PARTITION_CACHES
> +static bool kmalloc_partition_enabled __ro_after_init = true;
> +
> +static int __init setup_kmalloc_partition(char *str)
> +{
> +	return kstrtobool(str, &kmalloc_partition_enabled);
> +}
> +early_param("kmalloc_partition", setup_kmalloc_partition);
> +#endif
> +
>  /*
>   * Conversion table for small slabs sizes / 8 to the index in the
>   * kmalloc array. This is necessary for slabs < 192 since we have non power
> @@ -1013,8 +1023,15 @@ new_kmalloc_cache(int idx, enum kmalloc_cache_type type)
>  	}
>  
>  #ifdef CONFIG_KMALLOC_PARTITION_CACHES

Swap the ifdef for a CONFIG-controlled static branch that defaults off
without CONFIG_KMALLOC_PARTITION_CACHES.

> -	if (type >= KMALLOC_PARTITION_START && type <= KMALLOC_PARTITION_END)
> +	if (!kmalloc_partition_enabled) {
> +		/* Use the normal caches, which have already been created. */
> +		if (type > KMALLOC_PARTITION_START && type <= KMALLOC_PARTITION_END) {
> +			kmalloc_caches[type][idx] = kmalloc_caches[KMALLOC_NORMAL][idx];
> +			return;
> +		}
> +	} else if (type >= KMALLOC_PARTITION_START && type <= KMALLOC_PARTITION_END) {
>  		flags |= SLAB_NO_MERGE;
> +	}
>  #endif
>  
>  	/*
> -- 
> 2.54.0
> 

-- 
Kees Cook