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_bios = 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 {
  38         switch(bio_rw(bio)) {
  39         case READ:
  40                 zero_fill_bio(bio);
  41                 break;
  42         case READA:
  43                 /* readahead of null bytes only wastes buffer cache */
  44                 return -EIO;
  45         case WRITE:
  46                 /* writes get silently dropped */
  47                 break;
  48         }
  49
  50         bio_endio(bio, 0);
  51
  52         /* accepted bio, don't make new request */
  53         return DM_MAPIO_SUBMITTED;
  54 }
  55
  56 static struct target_type zero_target = {
  57         .name   = "zero",
  58         .version = {1, 1, 0},
  59         .module = THIS_MODULE,
  60         .ctr    = zero_ctr,
  61         .map    = zero_map,
  62 };
  63
  64 static int __init dm_zero_init(void)
  65 {
  66         int r = dm_register_target(&zero_target);
  67
  68         if (r < 0)
  69                 DMERR("register failed %d", r);
  70
  71         return r;
  72 }
  73
  74 static void __exit dm_zero_exit(void)
  75 {
  76         dm_unregister_target(&zero_target);
  77 }
  78
  79 module_init(dm_zero_init)
  80 module_exit(dm_zero_exit)
  81
  82 MODULE_AUTHOR("Christophe Saout <christophe@saout.de>");
  83 MODULE_DESCRIPTION(DM_NAME " dummy target returning zeros");
  84 MODULE_LICENSE("GPL");