Re: [PATCH] KFENCE: Clarify that sample allocations are not following NUMA or memory policies

[Vlastimil Babka]

On 1/23/25 23:44, Christoph Lameter via B4 Relay wrote:
> From: Christoph Lameter <cl@xxxxxxxxx>
> 
> KFENCE manages its own pools and redirects regular memory allocations
> to those pools in a sporadic way. The usual memory allocator features
> like NUMA, memory policies and pfmemalloc are not supported.

Can it also violate __GFP_THISNODE constraint? That could be a problem, I
recall a problem in the past where it could have been not honoured by the
page allocator, leading to corruption of slab lists.

> This means that one gets surprising object placement with KFENCE that
> may impact performance on some NUMA systems.
> 
> Update the description and make KFENCE depend on VM debugging
> having been enabled.
> 
> Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>
> ---
>  Documentation/dev-tools/kfence.rst |  4 +++-
>  lib/Kconfig.kfence                 | 10 ++++++----
>  2 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/dev-tools/kfence.rst b/Documentation/dev-tools/kfence.rst
> index 541899353865..27150780d6f5 100644
> --- a/Documentation/dev-tools/kfence.rst
> +++ b/Documentation/dev-tools/kfence.rst
> @@ -8,7 +8,9 @@ Kernel Electric-Fence (KFENCE) is a low-overhead sampling-based memory safety
>  error detector. KFENCE detects heap out-of-bounds access, use-after-free, and
>  invalid-free errors.
>  
> -KFENCE is designed to be enabled in production kernels, and has near zero
> +KFENCE is designed to be low overhead but does not implememnt the typical
> +memory allocation features for its samples like memory policies, NUMA and
> +management of emergency memory pools. It has near zero
>  performance overhead. Compared to KASAN, KFENCE trades performance for
>  precision. The main motivation behind KFENCE's design, is that with enough
>  total uptime KFENCE will detect bugs in code paths not typically exercised by
> diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
> index 6fbbebec683a..48d2a6a1be08 100644
> --- a/lib/Kconfig.kfence
> +++ b/lib/Kconfig.kfence
> @@ -5,14 +5,14 @@ config HAVE_ARCH_KFENCE
>  
>  menuconfig KFENCE
>  	bool "KFENCE: low-overhead sampling-based memory safety error detector"
> -	depends on HAVE_ARCH_KFENCE
> +	depends on HAVE_ARCH_KFENCE && DEBUG_VM
>  	select STACKTRACE
>  	select IRQ_WORK
>  	help
>  	  KFENCE is a low-overhead sampling-based detector of heap out-of-bounds
>  	  access, use-after-free, and invalid-free errors. KFENCE is designed
> -	  to have negligible cost to permit enabling it in production
> -	  environments.
> +	  to have negligible cost. KFENCE does not support NUMA features
> +	  and other memory allocator features for it sample allocations.
>  
>  	  See <file:Documentation/dev-tools/kfence.rst> for more details.
>  
> @@ -21,7 +21,9 @@ menuconfig KFENCE
>  	  detect, albeit at very different performance profiles. If you can
>  	  afford to use KASAN, continue using KASAN, for example in test
>  	  environments. If your kernel targets production use, and cannot
> -	  enable KASAN due to its cost, consider using KFENCE.
> +	  enable KASAN due to its cost and you are not using NUMA and have
> +	  no use of the memory reserve logic of the memory allocators,
> +	  consider using KFENCE.
>  
>  if KFENCE
>  
> 
> ---
> base-commit: d0d106a2bd21499901299160744e5fe9f4c83ddb
> change-id: 20250123-kfence_doc_update-93b4576c25bb
> 
> Best regards,