[Devel] Re: [PATCH v3 09/16] sl[au]b: always get the cache from its page in kfree

[Christoph Lameter]

On Tue, 18 Sep 2012, Glauber Costa wrote:

> index f2d760c..18de3f6 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3938,9 +3938,12 @@ EXPORT_SYMBOL(__kmalloc);
>   * Free an object which was previously allocated from this
>   * cache.
>   */
> -void kmem_cache_free(struct kmem_cache *cachep, void *objp)
> +void kmem_cache_free(struct kmem_cache *s, void *objp)
>  {
>  	unsigned long flags;
> +	struct kmem_cache *cachep = virt_to_cache(objp);
> +
> +	VM_BUG_ON(!slab_equal_or_parent(cachep, s));
>

This is an extremely hot path of the kernel and you are adding significant
processing. Check how the benchmarks are influenced by this change.
virt_to_cache can be a bit expensive.

> diff --git a/mm/slub.c b/mm/slub.c
> index 4778548..a045dfc 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2604,7 +2604,9 @@ void kmem_cache_free(struct kmem_cache *s, void *x)
>
>  	page = virt_to_head_page(x);
>
> -	slab_free(s, page, x, _RET_IP_);
> +	VM_BUG_ON(!slab_equal_or_parent(page->slab, s));
> +
> +	slab_free(page->slab, page, x, _RET_IP_);
>

Less of a problem here but you are eroding one advantage that slab has had
in the past over slub in terms of freeing objects.