Root/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 */
18static 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 */
36static 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
56static 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
64static 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
74static void __exit dm_zero_exit(void)
75{
76    dm_unregister_target(&zero_target);
77}
78
79module_init(dm_zero_init)
80module_exit(dm_zero_exit)
81
82MODULE_AUTHOR("Christophe Saout <christophe@saout.de>");
83MODULE_DESCRIPTION(DM_NAME " dummy target returning zeros");
84MODULE_LICENSE("GPL");
85

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