drivers/md/dm-zero.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Copyright (C) 2003 Jana Saout <jana@saout.de>
   4  *
   5  * This file is released under the GPL.
   6  */
   7
   8 #include <linux/device-mapper.h>
   9
  10 #include <linux/module.h>
  11 #include <linux/init.h>
  12 #include <linux/bio.h>
  13
  14 #define DM_MSG_PREFIX "zero"
  15
  16 /*
  17  * Construct a dummy mapping that only returns zeros
  18  */
  19 static int zero_ctr(struct dm_target *ti, unsigned int argc, char **argv)
  20 {
  21         if (argc != 0) {
  22                 ti->error = "No arguments required";
  23                 return -EINVAL;
  24         }
  25
  26         /*
  27          * Silently drop discards, avoiding -EOPNOTSUPP.
  28          */
  29         ti->num_discard_bios = 1;
  30         ti->discards_supported = true;
  31
  32         return 0;
  33 }
  34
  35 /*
  36  * Return zeros only on reads
  37  */
  38 static int zero_map(struct dm_target *ti, struct bio *bio)
  39 {
  40         switch (bio_op(bio)) {
  41         case REQ_OP_READ:
  42                 if (bio->bi_opf & REQ_RAHEAD) {
  43                         /* readahead of null bytes only wastes buffer cache */
  44                         return DM_MAPIO_KILL;
  45                 }
  46                 zero_fill_bio(bio);
  47                 break;
  48         case REQ_OP_WRITE:
  49         case REQ_OP_DISCARD:
  50                 /* writes get silently dropped */
  51                 break;
  52         default:
  53                 return DM_MAPIO_KILL;
  54         }
  55
  56         bio_endio(bio);
  57
  58         /* accepted bio, don't make new request */
  59         return DM_MAPIO_SUBMITTED;
  60 }
  61
  62 static void zero_io_hints(struct dm_target *ti, struct queue_limits *limits)
  63 {
  64         limits->max_hw_discard_sectors = UINT_MAX;
  65         limits->discard_granularity = 512;
  66 }
  67
  68 static struct target_type zero_target = {
  69         .name   = "zero",
  70         .version = {1, 2, 0},
  71         .features = DM_TARGET_NOWAIT,
  72         .module = THIS_MODULE,
  73         .ctr    = zero_ctr,
  74         .map    = zero_map,
  75         .io_hints = zero_io_hints,
  76 };
  77 module_dm(zero);
  78
  79 MODULE_AUTHOR("Jana Saout <jana@saout.de>");
  80 MODULE_DESCRIPTION(DM_NAME " dummy target returning zeros");
  81 MODULE_LICENSE("GPL");