linux-kernel - Re: [PATCH v2 2/2] slab: __GFP_ZERO is incompatible with a constructor

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Date:   Wed, 11 Apr 2018 08:44:23 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     linux-mm@...ck.org, Matthew Wilcox <mawilcox@...rosoft.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH v2 2/2] slab: __GFP_ZERO is incompatible with a
 constructor

On Tue, 10 Apr 2018, Matthew Wilcox wrote:

> diff --git a/mm/slab.h b/mm/slab.h
> index 3cd4677953c6..896818c7b30a 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -515,6 +515,13 @@ static inline void dump_unreclaimable_slab(void)
>
>  void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr);
>
> +static inline bool slab_no_ctor(struct kmem_cache *s)
> +{
> +	if (IS_ENABLED(CONFIG_DEBUG_VM))
> +		return !WARN_ON_ONCE(s->ctor);
> +	return true;
> +}
> +
>  #ifdef CONFIG_SLAB_FREELIST_RANDOM
>  int cache_random_seq_create(struct kmem_cache *cachep, unsigned int count,
>  			gfp_t gfp);

Move that to mm/slab.c? Debugging is runtime enabled with SLUB not compile
time as with SLAB.

> +++ b/mm/slub.c
> @@ -2725,7 +2726,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
>  		stat(s, ALLOC_FASTPATH);
>  	}
>
> -	if (unlikely(gfpflags & __GFP_ZERO) && object)
> +	if (unlikely(gfpflags & __GFP_ZERO) && object && slab_no_ctor(s))
>  		memset(object, 0, s->object_size);
>
>  	slab_post_alloc_hook(s, gfpflags, 1, &object);

Please put this in a code path that is enabled by specifying

slub_debug

on the kernel command line.