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Date:   Thu, 21 May 2020 13:01:38 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org,
        Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH v3 04/19] mm: slub: implement SLUB version of
 obj_to_index()

On 5/20/20 11:00 PM, Roman Gushchin wrote:
> 
> From beeaecdac85c3a395dcfb99944dc8c858b541cbf Mon Sep 17 00:00:00 2001
> From: Roman Gushchin <guro@...com>
> Date: Mon, 29 Jul 2019 18:18:42 -0700
> Subject: [PATCH v3.2 04/19] mm: slub: implement SLUB version of obj_to_index()
> 
> 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.
> 
> Vlastimil Babka noticed, that SLUB does have already a similar
> function called slab_index(), which is defined only if SLUB_DEBUG
> is enabled. The function does a similar math, but with a division,
> and it also takes a page address instead of a page pointer.
> 
> Let's remove slab_index() and replace it with the new helper
> __obj_to_index(), which takes a page address. obj_to_index()
> will be a simple wrapper taking a page pointer and passing
> page_address(page) into __obj_to_index().
> 
> Signed-off-by: Roman Gushchin <guro@...com>

Looks good!

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  include/linux/slub_def.h | 16 ++++++++++++++++
>  mm/slub.c                | 15 +++++----------
>  2 files changed, 21 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
> index d2153789bd9f..30e91c83d401 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 +184,18 @@ static inline void *nearest_obj(struct kmem_cache *cache, struct page *page,
>  	return result;
>  }
>  
> +/* Determine object index from a given position */
> +static inline unsigned int __obj_to_index(const struct kmem_cache *cache,
> +					  void *addr, void *obj)
> +{
> +	return reciprocal_divide(kasan_reset_tag(obj) - addr,
> +				 cache->reciprocal_size);
> +}
> +
> +static inline unsigned int obj_to_index(const struct kmem_cache *cache,
> +					const struct page *page, void *obj)
> +{
> +	return __obj_to_index(cache, page_address(page), obj);
> +}
> +
>  #endif /* _LINUX_SLUB_DEF_H */
> diff --git a/mm/slub.c b/mm/slub.c
> index 2df4d4a420d1..d605d18b3c1b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -313,12 +313,6 @@ static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
>  		__p < (__addr) + (__objects) * (__s)->size; \
>  		__p += (__s)->size)
>  
> -/* Determine object index from a given position */
> -static inline unsigned int slab_index(void *p, struct kmem_cache *s, void *addr)
> -{
> -	return (kasan_reset_tag(p) - addr) / s->size;
> -}
> -
>  static inline unsigned int order_objects(unsigned int order, unsigned int size)
>  {
>  	return ((unsigned int)PAGE_SIZE << order) / size;
> @@ -461,7 +455,7 @@ static unsigned long *get_map(struct kmem_cache *s, struct page *page)
>  	bitmap_zero(object_map, page->objects);
>  
>  	for (p = page->freelist; p; p = get_freepointer(s, p))
> -		set_bit(slab_index(p, s, addr), object_map);
> +		set_bit(__obj_to_index(s, addr, p), object_map);
>  
>  	return object_map;
>  }
> @@ -3682,6 +3676,7 @@ 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
> @@ -3788,7 +3783,7 @@ static void list_slab_objects(struct kmem_cache *s, struct page *page,
>  	map = get_map(s, page);
>  	for_each_object(p, s, addr, page->objects) {
>  
> -		if (!test_bit(slab_index(p, s, addr), map)) {
> +		if (!test_bit(__obj_to_index(s, addr, p), map)) {
>  			pr_err("INFO: Object 0x%p @offset=%tu\n", p, p - addr);
>  			print_tracking(s, p);
>  		}
> @@ -4513,7 +4508,7 @@ static void validate_slab(struct kmem_cache *s, struct page *page)
>  	/* Now we know that a valid freelist exists */
>  	map = get_map(s, page);
>  	for_each_object(p, s, addr, page->objects) {
> -		u8 val = test_bit(slab_index(p, s, addr), map) ?
> +		u8 val = test_bit(__obj_to_index(s, addr, p), map) ?
>  			 SLUB_RED_INACTIVE : SLUB_RED_ACTIVE;
>  
>  		if (!check_object(s, page, p, val))
> @@ -4704,7 +4699,7 @@ static void process_slab(struct loc_track *t, struct kmem_cache *s,
>  
>  	map = get_map(s, page);
>  	for_each_object(p, s, addr, page->objects)
> -		if (!test_bit(slab_index(p, s, addr), map))
> +		if (!test_bit(__obj_to_index(s, addr, p), map))
>  			add_location(t, s, get_track(s, p, alloc));
>  	put_map(map);
>  }
> 

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