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Subject: Re: [PATCH v3 04/19] mm: slub: implement SLUB version of
 obj_to_index()
To:     Roman Gushchin <guro@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org,
        Christoph Lameter <cl@linux.com>
References: <20200422204708.2176080-1-guro@fb.com>
 <20200422204708.2176080-5-guro@fb.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <0e0616f2-6c5a-8911-7d37-6f2027c2930b@suse.cz>
Date:   Wed, 20 May 2020 15:51:45 +0200
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On 4/22/20 10:46 PM, Roman Gushchin wrote:
> This commit implements SLUB version of the obj_to_index() function,
> which will be required to calculate the offset of obj_cgroup in the
> obj_cgroups vector to store/obtain the objcg ownership data.
> 
> To make it faster, let's repeat the SLAB's trick introduced by
> commit 6a2d7a955d8d ("[PATCH] SLAB: use a multiply instead of a
> divide in obj_to_index()") and avoid an expensive division.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Acked-by: Christoph Lameter <cl@linux.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

There's already a slab_index() doing the same without the trick, with only
SLUB_DEBUG callers. Maybe just improve it and perhaps rename? (obj_to_index()
seems more descriptive). The difference is that it takes the result of
page_addr() instead of doing that, as it's being called in a loop on objects
from a single page, so you'd have to perhaps split to obj_to_index(page) and
__obj_to_index(addr) or something.

> ---
>  include/linux/slub_def.h | 9 +++++++++
>  mm/slub.c                | 1 +
>  2 files changed, 10 insertions(+)
> 
> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
> index d2153789bd9f..200ea292f250 100644
> --- a/include/linux/slub_def.h
> +++ b/include/linux/slub_def.h
> @@ -8,6 +8,7 @@
>   * (C) 2007 SGI, Christoph Lameter
>   */
>  #include <linux/kobject.h>
> +#include <linux/reciprocal_div.h>
>  
>  enum stat_item {
>  	ALLOC_FASTPATH,		/* Allocation from cpu slab */
> @@ -86,6 +87,7 @@ struct kmem_cache {
>  	unsigned long min_partial;
>  	unsigned int size;	/* The size of an object including metadata */
>  	unsigned int object_size;/* The size of an object without metadata */
> +	struct reciprocal_value reciprocal_size;
>  	unsigned int offset;	/* Free pointer offset */
>  #ifdef CONFIG_SLUB_CPU_PARTIAL
>  	/* Number of per cpu partial objects to keep around */
> @@ -182,4 +184,11 @@ static inline void *nearest_obj(struct kmem_cache *cache, struct page *page,
>  	return result;
>  }
>  
> +static inline unsigned int obj_to_index(const struct kmem_cache *cache,
> +					const struct page *page, void *obj)
> +{
> +	return reciprocal_divide(kasan_reset_tag(obj) - page_address(page),
> +				 cache->reciprocal_size);
> +}
> +
>  #endif /* _LINUX_SLUB_DEF_H */
> diff --git a/mm/slub.c b/mm/slub.c
> index 03071ae5ff07..8d16babe1829 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3660,6 +3660,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>  	 */
>  	size = ALIGN(size, s->align);
>  	s->size = size;
> +	s->reciprocal_size = reciprocal_value(size);
>  	if (forced_order >= 0)
>  		order = forced_order;
>  	else
>