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

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Message-ID: <1db7c986-25c4-884e-4fbf-9af348bdff6f@suse.cz>
Date:   Wed, 13 Jan 2021 17:59:43 +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>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH] mm/slub: disable user tracing for kmemleak caches

On 1/13/21 5:09 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>

How about stripping away SLAB_STORE_USER only if it's added from the global
slub_debug variable? In case somebody lists one of the kmemleak caches
explicitly in "slub_debug=..." instead of just booting with "slub_debug", we
should honor that.

> ---
> 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 :-)
> 
> ---
>  mm/slub.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 34dcc09e2ec9..625a32a6645b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1446,7 +1446,16 @@ slab_flags_t kmem_cache_flags(unsigned int object_size,
>  		}
>  	}
>  
> -	return flags | slub_debug;
> +	flags |= slub_debug;
> +
> +	/*
> +	 * If the slab cache is for debugging (e.g. kmemleak) then
> +	 * don't store user (stack trace) information.
> +	 */
> +	if (flags & SLAB_NOLEAKTRACE)
> +		flags &= ~SLAB_STORE_USER;
> +
> +	return flags;
>  }
>  #else /* !CONFIG_SLUB_DEBUG */
>  static inline void setup_object_debug(struct kmem_cache *s,
>