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"); 85