mm/slab: put the freelist at the end of slab page

Currently, the freelist is at the front of slab page.  This requires
extra space to meet object alignment requirement.  If we put the
freelist at the end of a slab page, objects could start at page boundary
and will be at correct alignment.  This is possible because freelist has
no alignment constraint itself.

This gives us two benefits: It removes extra memory space for the
freelist alignment and remove complex calculation at cache
initialization step.  I can't think notable drawback here.

I mentioned that this would reduce extra memory space, but, this benefit
is rather theoretical because it can be applied to very few cases.
Following is the example cache type that can get benefit from this
change.

  size align num before after
    32    8  124  4100  4092
    64    8   63  4103  4095
    88    8   46  4102  4094
   272    8   15  4103  4095
   408    8   10  4098  4090
    32   16  124  4108  4092
    64   16   63  4111  4095
    32   32  124  4124  4092
    64   32   63  4127  4095
    96   32   42  4106  4074

before means whole size for objects and aligned freelist before applying
patch and after shows the result of this patch.

Since before is more than 4096, number of object should decrease and
memory waste happens.

Anyway, this patch removes complex calculation so looks beneficial to
me.

[akpm@linux-foundation.org: fix kerneldoc]
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab.c

@@ -456,55 +456,12 @@ static inline struct array_cache *cpu_cache_get(struct kmem_cache *cachep)
 	return this_cpu_ptr(cachep->cpu_cache);
 }
 
-static size_t calculate_freelist_size(int nr_objs, size_t align)
-{
-	size_t freelist_size;
-
-	freelist_size = nr_objs * sizeof(freelist_idx_t);
-	if (align)
-		freelist_size = ALIGN(freelist_size, align);
-
-	return freelist_size;
-}
-
-static int calculate_nr_objs(size_t slab_size, size_t buffer_size,
-				size_t idx_size, size_t align)
-{
-	int nr_objs;
-	size_t remained_size;
-	size_t freelist_size;
-
-	/*
-	 * Ignore padding for the initial guess. The padding
-	 * is at most @align-1 bytes, and @buffer_size is at
-	 * least @align. In the worst case, this result will
-	 * be one greater than the number of objects that fit
-	 * into the memory allocation when taking the padding
-	 * into account.
-	 */
-	nr_objs = slab_size / (buffer_size + idx_size);
-
-	/*
-	 * This calculated number will be either the right
-	 * amount, or one greater than what we want.
-	 */
-	remained_size = slab_size - nr_objs * buffer_size;
-	freelist_size = calculate_freelist_size(nr_objs, align);
-	if (remained_size < freelist_size)
-		nr_objs--;
-
-	return nr_objs;
-}
-
 /*
  * Calculate the number of objects and left-over bytes for a given buffer size.
  */
 static void cache_estimate(unsigned long gfporder, size_t buffer_size,
-			   size_t align, int flags, size_t *left_over,
-			   unsigned int *num)
+		unsigned long flags, size_t *left_over, unsigned int *num)
 {
-	int nr_objs;
-	size_t mgmt_size;
 	size_t slab_size = PAGE_SIZE << gfporder;
 
 	/*
@@ -512,9 +469,12 @@ static void cache_estimate(unsigned long gfporder, size_t buffer_size,
 	 * on it. For the latter case, the memory allocated for a
 	 * slab is used for:
 	 *
-	 * - One freelist_idx_t for each object
-	 * - Padding to respect alignment of @align
 	 * - @buffer_size bytes for each object
+	 * - One freelist_idx_t for each object
+	 *
+	 * We don't need to consider alignment of freelist because
+	 * freelist will be at the end of slab page. The objects will be
+	 * at the correct alignment.
 	 *
 	 * If the slab management structure is off the slab, then the
 	 * alignment will already be calculated into the size. Because
@@ -522,16 +482,13 @@ static void cache_estimate(unsigned long gfporder, size_t buffer_size,
 	 * correct alignment when allocated.
 	 */
 	if (flags & CFLGS_OFF_SLAB) {
-		mgmt_size = 0;
-		nr_objs = slab_size / buffer_size;
-
+		*num = slab_size / buffer_size;
+		*left_over = slab_size % buffer_size;
 	} else {
-		nr_objs = calculate_nr_objs(slab_size, buffer_size,
-					sizeof(freelist_idx_t), align);
-		mgmt_size = calculate_freelist_size(nr_objs, align);
+		*num = slab_size / (buffer_size + sizeof(freelist_idx_t));
+		*left_over = slab_size %
+			(buffer_size + sizeof(freelist_idx_t));
 	}
-	*num = nr_objs;
-	*left_over = slab_size - nr_objs*buffer_size - mgmt_size;
 }
 
 #if DEBUG
@@ -1911,7 +1868,6 @@ static void slabs_destroy(struct kmem_cache *cachep, struct list_head *list)
  * calculate_slab_order - calculate size (page order) of slabs
  * @cachep: pointer to the cache that is being created
  * @size: size of objects to be created in this cache.
- * @align: required alignment for the objects.
  * @flags: slab allocation flags
  *
  * Also calculates the number of objects per slab.
@@ -1921,7 +1877,7 @@ static void slabs_destroy(struct kmem_cache *cachep, struct list_head *list)
  * towards high-order requests, this should be changed.
  */
 static size_t calculate_slab_order(struct kmem_cache *cachep,
-			size_t size, size_t align, unsigned long flags)
+				size_t size, unsigned long flags)
 {
 	unsigned long offslab_limit;
 	size_t left_over = 0;
@@ -1931,7 +1887,7 @@ static size_t calculate_slab_order(struct kmem_cache *cachep,
 		unsigned int num;
 		size_t remainder;
 
-		cache_estimate(gfporder, size, align, flags, &remainder, &num);
+		cache_estimate(gfporder, size, flags, &remainder, &num);
 		if (!num)
 			continue;
 
@@ -2207,12 +2163,12 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
 	if (FREELIST_BYTE_INDEX && size < SLAB_OBJ_MIN_SIZE)
 		size = ALIGN(SLAB_OBJ_MIN_SIZE, cachep->align);
 
-	left_over = calculate_slab_order(cachep, size, cachep->align, flags);
+	left_over = calculate_slab_order(cachep, size, flags);
 
 	if (!cachep->num)
 		return -E2BIG;
 
-	freelist_size = calculate_freelist_size(cachep->num, cachep->align);
+	freelist_size = cachep->num * sizeof(freelist_idx_t);
 
 	/*
 	 * If the slab has been placed off-slab, and we have enough space then
@@ -2223,11 +2179,6 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
 		left_over -= freelist_size;
 	}
 
-	if (flags & CFLGS_OFF_SLAB) {
-		/* really off slab. No need for manual alignment */
-		freelist_size = calculate_freelist_size(cachep->num, 0);
-	}
-
 	cachep->colour_off = cache_line_size();
 	/* Offset must be a multiple of the alignment. */
 	if (cachep->colour_off < cachep->align)
@@ -2443,6 +2394,9 @@ static void *alloc_slabmgmt(struct kmem_cache *cachep,
 	void *freelist;
 	void *addr = page_address(page);
 
+	page->s_mem = addr + colour_off;
+	page->active = 0;
+
 	if (OFF_SLAB(cachep)) {
 		/* Slab management obj is off-slab. */
 		freelist = kmem_cache_alloc_node(cachep->freelist_cache,
@@ -2450,11 +2404,11 @@ static void *alloc_slabmgmt(struct kmem_cache *cachep,
 		if (!freelist)
 			return NULL;
 	} else {
-		freelist = addr + colour_off;
-		colour_off += cachep->freelist_size;
+		/* We will use last bytes at the slab for freelist */
+		freelist = addr + (PAGE_SIZE << cachep->gfporder) -
+				cachep->freelist_size;
 	}
-	page->active = 0;
-	page->s_mem = addr + colour_off;
+
 	return freelist;
 }