lib/flex_array.c

   1 /*
   2  * Flexible array managed in PAGE_SIZE parts
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License as published by
   6  * the Free Software Foundation; either version 2 of the License, or
   7  * (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU General Public License
  15  * along with this program; if not, write to the Free Software
  16  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  17  *
  18  * Copyright IBM Corporation, 2009
  19  *
  20  * Author: Dave Hansen <dave@linux.vnet.ibm.com>
  21  */
  22
  23 #include <linux/flex_array.h>
  24 #include <linux/slab.h>
  25 #include <linux/stddef.h>
  26
  27 struct flex_array_part {
  28         char elements[FLEX_ARRAY_PART_SIZE];
  29 };
  30
  31 static inline int __elements_per_part(int element_size)
  32 {
  33         return FLEX_ARRAY_PART_SIZE / element_size;
  34 }
  35
  36 static inline int bytes_left_in_base(void)
  37 {
  38         int element_offset = offsetof(struct flex_array, parts);
  39         int bytes_left = FLEX_ARRAY_BASE_SIZE - element_offset;
  40         return bytes_left;
  41 }
  42
  43 static inline int nr_base_part_ptrs(void)
  44 {
  45         return bytes_left_in_base() / sizeof(struct flex_array_part *);
  46 }
  47
  48 /*
  49  * If a user requests an allocation which is small
  50  * enough, we may simply use the space in the
  51  * flex_array->parts[] array to store the user
  52  * data.
  53  */
  54 static inline int elements_fit_in_base(struct flex_array *fa)
  55 {
  56         int data_size = fa->element_size * fa->total_nr_elements;
  57         if (data_size <= bytes_left_in_base())
  58                 return 1;
  59         return 0;
  60 }
  61
  62 /**
  63  * flex_array_alloc - allocate a new flexible array
  64  * @element_size:       the size of individual elements in the array
  65  * @total:              total number of elements that this should hold
  66  *
  67  * Note: all locking must be provided by the caller.
  68  *
  69  * @total is used to size internal structures.  If the user ever
  70  * accesses any array indexes >=@total, it will produce errors.
  71  *
  72  * The maximum number of elements is defined as: the number of
  73  * elements that can be stored in a page times the number of
  74  * page pointers that we can fit in the base structure or (using
  75  * integer math):
  76  *
  77  *      (PAGE_SIZE/element_size) * (PAGE_SIZE-8)/sizeof(void *)
  78  *
  79  * Here's a table showing example capacities.  Note that the maximum
  80  * index that the get/put() functions is just nr_objects-1.   This
  81  * basically means that you get 4MB of storage on 32-bit and 2MB on
  82  * 64-bit.
  83  *
  84  *
  85  * Element size | Objects | Objects |
  86  * PAGE_SIZE=4k |  32-bit |  64-bit |
  87  * ---------------------------------|
  88  *      1 bytes | 4186112 | 2093056 |
  89  *      2 bytes | 2093056 | 1046528 |
  90  *      3 bytes | 1395030 |  697515 |
  91  *      4 bytes | 1046528 |  523264 |
  92  *     32 bytes |  130816 |   65408 |
  93  *     33 bytes |  126728 |   63364 |
  94  *   2048 bytes |    2044 |    1022 |
  95  *   2049 bytes |    1022 |     511 |
  96  *       void * | 1046528 |  261632 |
  97  *
  98  * Since 64-bit pointers are twice the size, we lose half the
  99  * capacity in the base structure.  Also note that no effort is made
 100  * to efficiently pack objects across page boundaries.
 101  */
 102 struct flex_array *flex_array_alloc(int element_size, int total, gfp_t flags)
 103 {
 104         struct flex_array *ret;
 105         int max_size = nr_base_part_ptrs() * __elements_per_part(element_size);
 106
 107         /* max_size will end up 0 if element_size > PAGE_SIZE */
 108         if (total > max_size)
 109                 return NULL;
 110         ret = kzalloc(sizeof(struct flex_array), flags);
 111         if (!ret)
 112                 return NULL;
 113         ret->element_size = element_size;
 114         ret->total_nr_elements = total;
 115         return ret;
 116 }
 117
 118 static int fa_element_to_part_nr(struct flex_array *fa, int element_nr)
 119 {
 120         return element_nr / __elements_per_part(fa->element_size);
 121 }
 122
 123 /**
 124  * flex_array_free_parts - just free the second-level pages
 125  * @src:        address of data to copy into the array
 126  * @element_nr: index of the position in which to insert
 127  *              the new element.
 128  *
 129  * This is to be used in cases where the base 'struct flex_array'
 130  * has been statically allocated and should not be free.
 131  */
 132 void flex_array_free_parts(struct flex_array *fa)
 133 {
 134         int part_nr;
 135         int max_part = nr_base_part_ptrs();
 136
 137         if (elements_fit_in_base(fa))
 138                 return;
 139         for (part_nr = 0; part_nr < max_part; part_nr++)
 140                 kfree(fa->parts[part_nr]);
 141 }
 142
 143 void flex_array_free(struct flex_array *fa)
 144 {
 145         flex_array_free_parts(fa);
 146         kfree(fa);
 147 }
 148
 149 static int fa_index_inside_part(struct flex_array *fa, int element_nr)
 150 {
 151         return element_nr % __elements_per_part(fa->element_size);
 152 }
 153
 154 static int index_inside_part(struct flex_array *fa, int element_nr)
 155 {
 156         int part_offset = fa_index_inside_part(fa, element_nr);
 157         return part_offset * fa->element_size;
 158 }
 159
 160 static struct flex_array_part *
 161 __fa_get_part(struct flex_array *fa, int part_nr, gfp_t flags)
 162 {
 163         struct flex_array_part *part = fa->parts[part_nr];
 164         if (!part) {
 165                 /*
 166                  * This leaves the part pages uninitialized
 167                  * and with potentially random data, just
 168                  * as if the user had kmalloc()'d the whole.
 169                  * __GFP_ZERO can be used to zero it.
 170                  */
 171                 part = kmalloc(FLEX_ARRAY_PART_SIZE, flags);
 172                 if (!part)
 173                         return NULL;
 174                 fa->parts[part_nr] = part;
 175         }
 176         return part;
 177 }
 178
 179 /**
 180  * flex_array_put - copy data into the array at @element_nr
 181  * @src:        address of data to copy into the array
 182  * @element_nr: index of the position in which to insert
 183  *              the new element.
 184  *
 185  * Note that this *copies* the contents of @src into
 186  * the array.  If you are trying to store an array of
 187  * pointers, make sure to pass in &ptr instead of ptr.
 188  *
 189  * Locking must be provided by the caller.
 190  */
 191 int flex_array_put(struct flex_array *fa, int element_nr, void *src, gfp_t flags)
 192 {
 193         int part_nr = fa_element_to_part_nr(fa, element_nr);
 194         struct flex_array_part *part;
 195         void *dst;
 196
 197         if (element_nr >= fa->total_nr_elements)
 198                 return -ENOSPC;
 199         if (elements_fit_in_base(fa))
 200                 part = (struct flex_array_part *)&fa->parts[0];
 201         else
 202                 part = __fa_get_part(fa, part_nr, flags);
 203         if (!part)
 204                 return -ENOMEM;
 205         dst = &part->elements[index_inside_part(fa, element_nr)];
 206         memcpy(dst, src, fa->element_size);
 207         return 0;
 208 }
 209
 210 /**
 211  * flex_array_prealloc - guarantee that array space exists
 212  * @start:      index of first array element for which space is allocated
 213  * @end:        index of last (inclusive) element for which space is allocated
 214  *
 215  * This will guarantee that no future calls to flex_array_put()
 216  * will allocate memory.  It can be used if you are expecting to
 217  * be holding a lock or in some atomic context while writing
 218  * data into the array.
 219  *
 220  * Locking must be provided by the caller.
 221  */
 222 int flex_array_prealloc(struct flex_array *fa, int start, int end, gfp_t flags)
 223 {
 224         int start_part;
 225         int end_part;
 226         int part_nr;
 227         struct flex_array_part *part;
 228
 229         if (start >= fa->total_nr_elements || end >= fa->total_nr_elements)
 230                 return -ENOSPC;
 231         if (elements_fit_in_base(fa))
 232                 return 0;
 233         start_part = fa_element_to_part_nr(fa, start);
 234         end_part = fa_element_to_part_nr(fa, end);
 235         for (part_nr = start_part; part_nr <= end_part; part_nr++) {
 236                 part = __fa_get_part(fa, part_nr, flags);
 237                 if (!part)
 238                         return -ENOMEM;
 239         }
 240         return 0;
 241 }
 242
 243 /**
 244  * flex_array_get - pull data back out of the array
 245  * @element_nr: index of the element to fetch from the array
 246  *
 247  * Returns a pointer to the data at index @element_nr.  Note
 248  * that this is a copy of the data that was passed in.  If you
 249  * are using this to store pointers, you'll get back &ptr.
 250  *
 251  * Locking must be provided by the caller.
 252  */
 253 void *flex_array_get(struct flex_array *fa, int element_nr)
 254 {
 255         int part_nr = fa_element_to_part_nr(fa, element_nr);
 256         struct flex_array_part *part;
 257         int index;
 258
 259         if (element_nr >= fa->total_nr_elements)
 260                 return NULL;
 261         if (!fa->parts[part_nr])
 262                 return NULL;
 263         if (elements_fit_in_base(fa))
 264                 part = (struct flex_array_part *)&fa->parts[0];
 265         else
 266                 part = fa->parts[part_nr];
 267         index = index_inside_part(fa, element_nr);
 268         return &part->elements[index_inside_part(fa, element_nr)];
 269 }