linux-kernel - Re: [PATCH v3 1/3] mm/slub: enable debugging memory wasting of kmalloc

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Date:   Wed, 27 Jul 2022 12:20:47 +0200 (CEST)
From:   Christoph Lameter <cl@...two.de>
To:     Feng Tang <feng.tang@...el.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        John Garry <john.garry@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH v3 1/3] mm/slub: enable debugging memory wasting of
 kmalloc

On Wed, 27 Jul 2022, Feng Tang wrote:

> @@ -2905,7 +2950,7 @@ static inline void *get_freelist(struct kmem_cache *s, struct slab *slab)
>   * already disabled (which is the case for bulk allocation).
>   */
>  static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> -			  unsigned long addr, struct kmem_cache_cpu *c)
> +			  unsigned long addr, struct kmem_cache_cpu *c, unsigned int orig_size)
>  {
>  	void *freelist;
>  	struct slab *slab;
> @@ -3102,7 +3147,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>   * pointer.
>   */
>  static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> -			  unsigned long addr, struct kmem_cache_cpu *c)
> +			  unsigned long addr, struct kmem_cache_cpu *c, unsigned int orig_size)
>  {
>  	void *p;
>
> @@ -3115,7 +3160,7 @@ static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>  	c = slub_get_cpu_ptr(s->cpu_slab);
>  #endif
>
> -	p = ___slab_alloc(s, gfpflags, node, addr, c);
> +	p = ___slab_alloc(s, gfpflags, node, addr, c, orig_size);
>  #ifdef CONFIG_PREEMPT_COUNT
>  	slub_put_cpu_ptr(s->cpu_slab);

This is modifying and making execution of standard slab functions more
expensive. Could you restrict modifications to the kmalloc subsystem?

kmem_cache_alloc() and friends are not doing any rounding up to power of
two  sizes.

What is happening here is that you pass kmalloc object size info through
the kmem_cache_alloc functions so that the regular allocation functions
debug functionality can then save the kmalloc specific object request
size. This is active even when no debugging options are enabled.

Can you avoid that? Have kmalloc do the object allocation without passing
through the kmalloc request size and then add the original size info
to the debug field later after execution continues in the kmalloc functions?