dm-zero.c

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/*
 * Copyright (C) 2003 Christophe Saout <[email protected]>
 *
 * This file is released under the GPL.
 */

#include <linux/device-mapper.h>

#include <linux/module.h>
#include <linux/init.h>
#include <linux/bio.h>

#define DM_MSG_PREFIX "zero"

/*
 * Construct a dummy mapping that only returns zeros
 */
static int zero_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
    if (argc != 0) {
        ti->error = "No arguments required";
        return -EINVAL;
    }

    /*
     * Silently drop discards, avoiding -EOPNOTSUPP.
     */
    ti->num_discard_requests = 1;

    return 0;
}

/*
 * Return zeros only on reads
 */
static int zero_map(struct dm_target *ti, struct bio *bio,
              union map_info *map_context)
{
    switch(bio_rw(bio)) {
    case READ:
        zero_fill_bio(bio);
        break;
    case READA:
        /* readahead of null bytes only wastes buffer cache */
        return -EIO;
    case WRITE:
        /* writes get silently dropped */
        break;
    }

    bio_endio(bio, 0);

    /* accepted bio, don't make new request */
    return DM_MAPIO_SUBMITTED;
}

static struct target_type zero_target = {
    .name   = "zero",
    .version = {1, 0, 0},
    .module = THIS_MODULE,
    .ctr    = zero_ctr,
    .map    = zero_map,
};

static int __init dm_zero_init(void)
{
    int r = dm_register_target(&zero_target);

    if (r < 0)
        DMERR("register failed %d", r);

    return r;
}

static void __exit dm_zero_exit(void)
{
    dm_unregister_target(&zero_target);
}

module_init(dm_zero_init)
module_exit(dm_zero_exit)

MODULE_AUTHOR("Christophe Saout <[email protected]>");
MODULE_DESCRIPTION(DM_NAME " dummy target returning zeros");
MODULE_LICENSE("GPL");