drivers/md/dm-zero.c

   1 /*
   2  * Copyright (C) 2003 Jana Saout <jana@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_op(bio)) {
  39         case REQ_OP_READ:
  40                 if (bio->bi_opf & REQ_RAHEAD) {
  41                         /* readahead of null bytes only wastes buffer cache */
  42                         return DM_MAPIO_KILL;
  43                 }
  44                 zero_fill_bio(bio);
  45                 break;
  46         case REQ_OP_WRITE:
  47                 /* writes get silently dropped */
  48                 break;
  49         default:
  50                 return DM_MAPIO_KILL;
  51         }
  52
  53         bio_endio(bio);
  54
  55         /* accepted bio, don't make new request */
  56         return DM_MAPIO_SUBMITTED;
  57 }
  58
  59 static struct target_type zero_target = {
  60         .name   = "zero",
  61         .version = {1, 1, 0},
  62         .features = DM_TARGET_NOWAIT,
  63         .module = THIS_MODULE,
  64         .ctr    = zero_ctr,
  65         .map    = zero_map,
  66 };
  67
  68 static int __init dm_zero_init(void)
  69 {
  70         int r = dm_register_target(&zero_target);
  71
  72         if (r < 0)
  73                 DMERR("register failed %d", r);
  74
  75         return r;
  76 }
  77
  78 static void __exit dm_zero_exit(void)
  79 {
  80         dm_unregister_target(&zero_target);
  81 }
  82
  83 module_init(dm_zero_init)
  84 module_exit(dm_zero_exit)
  85
  86 MODULE_AUTHOR("Jana Saout <jana@saout.de>");
  87 MODULE_DESCRIPTION(DM_NAME " dummy target returning zeros");
  88 MODULE_LICENSE("GPL");