linux-kernel - Re: [PATCH v2] mm/slub: disable user tracing for kmemleak caches by default

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Date:   Thu, 14 Jan 2021 12:07:54 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Johannes Berg <johannes@...solutions.net>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Johannes Berg <johannes.berg@...el.com>
Subject: Re: [PATCH v2] mm/slub: disable user tracing for kmemleak caches by
 default

On 1/13/21 9:51 PM, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
> 
> If kmemleak is enabled, it uses a kmem cache for its own objects.
> These objects are used to hold information kmemleak uses, including
> a stack trace. If slub_debug is also turned on, each of them has
> *another* stack trace, so the overhead adds up, and on my tests (on
> ARCH=um, admittedly) 2/3rds of the allocations end up being doing
> the stack tracing.
> 
> Turn off SLAB_STORE_USER if SLAB_NOLEAKTRACE was given, to avoid
> storing the essentially same data twice.
> 
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
> Perhaps instead it should go the other way around, and kmemleak
> could even use/access the stack trace that's already in there ...
> But I don't really care too much, I can just turn off slub debug
> for the kmemleak caches via the command line anyway :-)
> 
> v2:
>  - strip SLAB_STORE_USER only coming from slub_debug so that the
>    command line args always take effect
> 
> ---
>  mm/slub.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 34dcc09e2ec9..a66c9948c529 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1412,6 +1412,15 @@ slab_flags_t kmem_cache_flags(unsigned int object_size,
>  	size_t len;
>  	char *next_block;
>  	slab_flags_t block_flags;
> +	slab_flags_t slub_debug_local = slub_debug;
> +
> +	/*
> +	 * If the slab cache is for debugging (e.g. kmemleak) then
> +	 * don't store user (stack trace) information by default,
> +	 * but let the user enable it via the command line below.
> +	 */
> +	if (flags & SLAB_NOLEAKTRACE)
> +		slub_debug_local &= ~SLAB_STORE_USER;
>  
>  	len = strlen(name);
>  	next_block = slub_debug_string;
> @@ -1446,7 +1455,7 @@ slab_flags_t kmem_cache_flags(unsigned int object_size,
>  		}
>  	}
>  
> -	return flags | slub_debug;
> +	return flags | slub_debug_local;
>  }
>  #else /* !CONFIG_SLUB_DEBUG */
>  static inline void setup_object_debug(struct kmem_cache *s,
>