Re: [PATCH][v2] mm/mempool: Untangle CONFIG_SLUB_DEBUG_ON abuse and switch to static key

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 6/2/26 08:21, lirongqing wrote:
> From: Li RongQing <lirongqing@baidu.com>
> 
> The mempool subsystem historically wrapped its debugging logic inside an
> merely defines compile-time defaults for SLUB and caused two flaws:
> 
> 1. On production kernels where CONFIG_SLUB_DEBUG=y but
>    CONFIG_SLUB_DEBUG_ON=n, mempool debugging was completely compiled out
>    at compile time.
> 2. On kernels with CONFIG_SLUB_DEBUG_ON=y, mempool debugging stayed active
>    even if a user explicitly disabled slub debugging at boot time.
> 
> Clean up this mess by removing the #ifdef and switching to a runtime static
> key (mempool_debug_enabled), allowing mempool debugging to be toggled
> cleanly via its own boot parameter.
> 
> Suggested-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> Cc: Vlastimil Babka <vbabka@kernel.org>
> Cc: Harry Yoo <harry@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Hao Li <hao.li@linux.dev>
> Cc: Christoph Lameter <cl@gentwo.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Usama Arif <usama.arif@linux.dev>
> ---
> Diff with v1:
> 	Rewrite commit message, change early_param to __setup
> 
>  Documentation/admin-guide/kernel-parameters.txt |  5 ++++
>  mm/mempool.c                                    | 32 ++++++++++++++++++-------
>  2 files changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 642659b..89b5994 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3980,6 +3980,11 @@ Kernel parameters
>  			Note that even when enabled, there are a few cases where
>  			the feature is not effective.
>  
> +	mempool_debug	[MM]
> +			Enable mempool debugging. This enables element
> +			poison checking when freeing elements back to the
> +			pool. Useful for debugging mempool corruption.
> +
>  	memtest=	[KNL,X86,ARM,M68K,PPC,RISCV,EARLY] Enable memtest
>  			Format: <integer>
>  			default : 0 <disable>
> diff --git a/mm/mempool.c b/mm/mempool.c
> index db23e0e..71e4b54 100644
> --- a/mm/mempool.c
> +++ b/mm/mempool.c
> @@ -16,11 +16,28 @@
>  #include <linux/export.h>
>  #include <linux/mempool.h>
>  #include <linux/writeback.h>
> +#include <linux/static_key.h>
> +#include <linux/init.h>
>  #include "slab.h"
>  
>  static DECLARE_FAULT_ATTR(fail_mempool_alloc);
>  static DECLARE_FAULT_ATTR(fail_mempool_alloc_bulk);
>  
> +/*
> + * Debugging support for mempool using static key.
> + *
> + * This allows enabling mempool debug at boot time via:
> + *   mempool_debug
> + */
> +static DEFINE_STATIC_KEY_FALSE(mempool_debug_enabled);
> +
> +static int __init mempool_debug_setup(char *str)
> +{
> +	static_branch_enable(&mempool_debug_enabled);
> +	return 1;
> +}
> +__setup("mempool_debug", mempool_debug_setup);
> +
>  static int __init mempool_faul_inject_init(void)
>  {
>  	int error;
> @@ -37,7 +54,6 @@ static int __init mempool_faul_inject_init(void)
>  }
>  late_initcall(mempool_faul_inject_init);
>  
> -#ifdef CONFIG_SLUB_DEBUG_ON
>  static void poison_error(struct mempool *pool, void *element, size_t size,
>  			 size_t byte)
>  {
> @@ -73,6 +89,9 @@ static void __check_element(struct mempool *pool, void *element, size_t size)
>  
>  static void check_element(struct mempool *pool, void *element)
>  {
> +	if (!static_branch_unlikely(&mempool_debug_enabled))
> +		return;

With static keys it's better to have the check in the caller of
check_element so we skip over that call if it's disabled, and not do a call
that immediately returns. Since there's a single caller, there's no need for
an extra inline wrapper.

> +
>  	/* Skip checking: KASAN might save its metadata in the element. */
>  	if (kasan_enabled())
>  		return;
> @@ -112,6 +131,9 @@ static void __poison_element(void *element, size_t size)
>  
>  static void poison_element(struct mempool *pool, void *element)
>  {
> +	if (!static_branch_unlikely(&mempool_debug_enabled))
> +		return;

Ditto.

> +
>  	/* Skip poisoning: KASAN might save its metadata in the element. */
>  	if (kasan_enabled())
>  		return;
> @@ -140,14 +162,6 @@ static void poison_element(struct mempool *pool, void *element)
>  #endif
>  	}
>  }
> -#else /* CONFIG_SLUB_DEBUG_ON */
> -static inline void check_element(struct mempool *pool, void *element)
> -{
> -}
> -static inline void poison_element(struct mempool *pool, void *element)
> -{
> -}
> -#endif /* CONFIG_SLUB_DEBUG_ON */
>  
>  static __always_inline bool kasan_poison_element(struct mempool *pool,
>  		void *element)