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"); 82