drivers/md/dm-zero.c

   1 /*
   2  * Copyright (C) 2003 Christophe Saout <christophe@saout.de>
   3  *
   4  * This file is released under the GPL.
   5  */
   6
   7 #include <linux/device-mapper.h>
   8
   9 #include <linux/module.h>
  10 #include <linux/init.h>
  11 #include <linux/bio.h>
  12
  13 #define DM_MSG_PREFIX "zero"
  14
  15 /*
  16  * Construct a dummy mapping that only returns zeros
  17  */
  18 static int zero_ctr(struct dm_target *ti, unsigned int argc, char **argv)
  19 {
  20         if (argc != 0) {
  21                 ti->error = "No arguments required";
  22                 return -EINVAL;
  23         }
  24
  25         /*
  26          * Silently drop discards, avoiding -EOPNOTSUPP.
  27          */
  28         ti->num_discard_requests = 1;
  29
  30         return 0;
  31 }
  32
  33 /*
  34  * Return zeros only on reads
  35  */
  36 static int zero_map(struct dm_target *ti, struct bio *bio,
  37                       union map_info *map_context)
  38 {
  39         switch(bio_rw(bio)) {
  40         case READ:
  41                 zero_fill_bio(bio);
  42                 break;
  43         case READA:
  44                 /* readahead of null bytes only wastes buffer cache */
  45                 return -EIO;
  46         case WRITE:
  47                 /* writes get silently dropped */
  48                 break;
  49         }
  50
  51         bio_endio(bio, 0);
  52
  53         /* accepted bio, don't make new request */
  54         return DM_MAPIO_SUBMITTED;
  55 }
  56
  57 static struct target_type zero_target = {
  58         .name   = "zero",
  59         .version = {1, 0, 0},
  60         .module = THIS_MODULE,
  61         .ctr    = zero_ctr,
  62         .map    = zero_map,
  63 };
  64
  65 static int __init dm_zero_init(void)
  66 {
  67         int r = dm_register_target(&zero_target);
  68
  69         if (r < 0)
  70                 DMERR("register failed %d", r);
  71
  72         return r;
  73 }
  74
  75 static void __exit dm_zero_exit(void)
  76 {
  77         dm_unregister_target(&zero_target);
  78 }
  79
  80 module_init(dm_zero_init)
  81 module_exit(dm_zero_exit)
  82
  83 MODULE_AUTHOR("Christophe Saout <christophe@saout.de>");
  84 MODULE_DESCRIPTION(DM_NAME " dummy target returning zeros");
  85 MODULE_LICENSE("GPL");