This is the dm-loop target - a replacement for the regular loop driver
with better performance. The dm-loop target builds a map of the file in
the constructor and it just remaps bios according to this map.
Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
---
drivers/md/Kconfig | 9 +
drivers/md/Makefile | 1
drivers/md/dm-loop.c | 404 +++++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 414 insertions(+)
Index: linux-2.6/drivers/md/Kconfig
===================================================================
--- linux-2.6.orig/drivers/md/Kconfig 2025-03-02 21:09:46.000000000 +0100
+++ linux-2.6/drivers/md/Kconfig 2025-03-02 21:09:46.000000000 +0100
@@ -646,6 +646,15 @@ config DM_ZONED
If unsure, say N.
+config DM_LOOP
+ tristate "Loop target"
+ depends on BLK_DEV_DM
+ help
+ This device-mapper target allows you to treat a regular file as
+ a block device.
+
+ If unsure, say N.
+
config DM_AUDIT
bool "DM audit events"
depends on BLK_DEV_DM
Index: linux-2.6/drivers/md/Makefile
===================================================================
--- linux-2.6.orig/drivers/md/Makefile 2025-03-02 21:09:46.000000000 +0100
+++ linux-2.6/drivers/md/Makefile 2025-03-02 21:09:46.000000000 +0100
@@ -79,6 +79,7 @@ obj-$(CONFIG_DM_CLONE) += dm-clone.o
obj-$(CONFIG_DM_LOG_WRITES) += dm-log-writes.o
obj-$(CONFIG_DM_INTEGRITY) += dm-integrity.o
obj-$(CONFIG_DM_ZONED) += dm-zoned.o
+obj-$(CONFIG_DM_LOOP) += dm-loop.o
obj-$(CONFIG_DM_WRITECACHE) += dm-writecache.o
obj-$(CONFIG_SECURITY_LOADPIN_VERITY) += dm-verity-loadpin.o
Index: linux-2.6/drivers/md/dm-loop.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6/drivers/md/dm-loop.c 2025-03-02 21:41:36.000000000 +0100
@@ -0,0 +1,404 @@
+#include <linux/device-mapper.h>
+
+#include <linux/module.h>
+#include <linux/pagemap.h>
+
+#define DM_MSG_PREFIX "loop"
+
+struct loop_c {
+ struct file *filp;
+ char *path;
+ loff_t offset;
+ struct block_device *bdev;
+ struct inode *inode;
+ unsigned blkbits;
+ bool read_only;
+ sector_t mapped_sectors;
+
+ sector_t nr_extents;
+ struct dm_loop_extent *map;
+};
+
+struct dm_loop_extent {
+ sector_t start; /* start sector in mapped device */
+ sector_t to; /* start sector on target device */
+ sector_t len; /* length in sectors */
+};
+
+static sector_t blk2sect(struct loop_c *lc, blkcnt_t block)
+{
+ return block << (lc->blkbits - SECTOR_SHIFT);
+}
+
+static blkcnt_t sec2blk(struct loop_c *lc, sector_t sector)
+{
+ return sector >> (lc->blkbits - SECTOR_SHIFT);
+}
+
+static blkcnt_t sec2blk_roundup(struct loop_c *lc, sector_t sector)
+{
+ return (sector + (1 << (lc->blkbits - SECTOR_SHIFT)) - 1) >> (lc->blkbits - SECTOR_SHIFT);
+}
+
+static struct dm_loop_extent *extent_binary_lookup(struct loop_c *lc, sector_t sector)
+{
+ ssize_t first = 0;
+ ssize_t last = lc->nr_extents - 1;
+
+ while (first <= last) {
+ ssize_t middle = (first + last) >> 1;
+ struct dm_loop_extent *ex = &lc->map[middle];
+ if (sector < ex->start) {
+ last = middle - 1;
+ continue;
+ }
+ if (likely(sector >= ex->start + ex->len)) {
+ first = middle + 1;
+ continue;
+ }
+ return ex;
+ }
+
+ return NULL;
+}
+
+static int loop_map(struct dm_target *ti, struct bio *bio)
+{
+ struct loop_c *lc = ti->private;
+ sector_t sector, len;
+ struct dm_loop_extent *ex;
+
+ sector = dm_target_offset(ti, bio->bi_iter.bi_sector);
+ ex = extent_binary_lookup(lc, sector);
+ if (!ex)
+ return DM_MAPIO_KILL;
+
+ bio_set_dev(bio, lc->bdev);
+ bio->bi_iter.bi_sector = ex->to + (sector - ex->start);
+ len = ex->len - (sector - ex->start);
+ if (len < bio_sectors(bio))
+ dm_accept_partial_bio(bio, len);
+
+ if (unlikely(!ex->to)) {
+ if (unlikely(!lc->read_only))
+ return DM_MAPIO_KILL;
+ zero_fill_bio(bio);
+ bio_endio(bio);
+ return DM_MAPIO_SUBMITTED;
+ }
+
+ return DM_MAPIO_REMAPPED;
+}
+
+static void loop_status(struct dm_target *ti, status_type_t type,
+ unsigned status_flags, char *result, unsigned maxlen)
+{
+ struct loop_c *lc = ti->private;
+ size_t sz = 0;
+
+ switch (type) {
+ case STATUSTYPE_INFO:
+ result[0] = '\0';
+ break;
+ case STATUSTYPE_TABLE:
+ DMEMIT("%s %llu", lc->path, lc->offset);
+ break;
+ case STATUSTYPE_IMA:
+ DMEMIT_TARGET_NAME_VERSION(ti->type);
+ DMEMIT(",file_name=%s,offset=%llu;", lc->path, lc->offset);
+ break;
+ }
+}
+
+static int loop_iterate_devices(struct dm_target *ti,
+ iterate_devices_callout_fn fn, void *data)
+{
+ return 0;
+}
+
+static int extent_range(struct loop_c *lc,
+ sector_t logical_blk, sector_t last_blk,
+ sector_t *begin_blk, sector_t *nr_blks,
+ char **error)
+{
+ sector_t dist = 0, phys_blk, probe_blk = logical_blk;
+ int r;
+
+ /* Find beginning physical block of extent starting at logical_blk. */
+ *begin_blk = probe_blk;
+ *nr_blks = 0;
+ r = bmap(lc->inode, begin_blk);
+ if (r) {
+ *error = "bmap failed";
+ return r;
+ }
+ if (!*begin_blk) {
+ if (!lc->read_only) {
+ *error = "File is sparse";
+ return -ENXIO;
+ }
+ }
+
+ for (phys_blk = *begin_blk; phys_blk == *begin_blk + dist; dist += !!*begin_blk) {
+ cond_resched();
+
+ (*nr_blks)++;
+ if (++probe_blk > last_blk)
+ break;
+
+ phys_blk = probe_blk;
+ r = bmap(lc->inode, &phys_blk);
+ if (r) {
+ *error = "bmap failed";
+ return r;
+ }
+ if (unlikely(!phys_blk)) {
+ if (!lc->read_only) {
+ *error = "File is sparse";
+ return -ENXIO;
+ }
+ }
+ }
+
+ return 0;
+}
+
+static int loop_extents(struct loop_c *lc, sector_t *nr_extents,
+ struct dm_loop_extent *map, char **error)
+{
+ int r;
+ sector_t start = 0;
+ sector_t nr_blks, begin_blk;
+ sector_t after_last_blk = sec2blk_roundup(lc,
+ (lc->mapped_sectors + (lc->offset >> 9)));
+ sector_t logical_blk = sec2blk(lc, lc->offset >> 9);
+
+ *nr_extents = 0;
+
+ /* for each block in the mapped region */
+ while (logical_blk < after_last_blk) {
+ r = extent_range(lc, logical_blk, after_last_blk - 1,
+ &begin_blk, &nr_blks, error);
+
+ if (unlikely(r))
+ return r;
+
+ if (map) {
+ if (*nr_extents >= lc->nr_extents) {
+ *error = "The file changed while mapping it";
+ return -EBUSY;
+ }
+ map[*nr_extents].start = start;
+ map[*nr_extents].to = blk2sect(lc, begin_blk);
+ map[*nr_extents].len = blk2sect(lc, nr_blks);
+ }
+
+ (*nr_extents)++;
+ start += blk2sect(lc, nr_blks);
+ logical_blk += nr_blks;
+ }
+
+ if (*nr_extents != lc->nr_extents) {
+ *error = "The file changed while mapping it";
+ return -EBUSY;
+ }
+
+ return 0;
+}
+
+static int setup_block_map(struct loop_c *lc, struct dm_target *ti)
+{
+ int r;
+ sector_t n_file_sectors, offset_sector, nr_extents_tmp;
+
+ if (!S_ISREG(lc->inode->i_mode) || !lc->inode->i_sb || !lc->inode->i_sb->s_bdev) {
+ ti->error = "The file is not a regular file";
+ return -ENXIO;
+ }
+
+ lc->bdev = lc->inode->i_sb->s_bdev;
+ lc->blkbits = lc->inode->i_blkbits;
+ n_file_sectors = i_size_read(lc->inode) >> lc->blkbits << (lc->blkbits - 9);
+
+ if (lc->offset & ((1 << lc->blkbits) - 1)) {
+ ti->error = "Unaligned offset";
+ return -EINVAL;
+ }
+ offset_sector = lc->offset >> 9;
+ if (offset_sector >= n_file_sectors) {
+ ti->error = "Offset is greater than file size";
+ return -EINVAL;
+ }
+ if (ti->len > (n_file_sectors - offset_sector)) {
+ ti->error = "Target maps area after file end";
+ return -EINVAL;
+ }
+ lc->mapped_sectors = ti->len >> (lc->blkbits - 9) << (lc->blkbits - 9);
+
+ r = loop_extents(lc, &lc->nr_extents, NULL, &ti->error);
+ if (r)
+ return r;
+
+ if (lc->nr_extents != (size_t)lc->nr_extents) {
+ ti->error = "Too many extents";
+ return -EOVERFLOW;
+ }
+
+ lc->map = kvcalloc(lc->nr_extents, sizeof(struct dm_loop_extent), GFP_KERNEL);
+ if (!lc->map) {
+ ti->error = "Failed to allocate extent map";
+ return -ENOMEM;
+ }
+
+ r = loop_extents(lc, &nr_extents_tmp, lc->map, &ti->error);
+ if (r)
+ return r;
+
+ return 0;
+}
+
+static int loop_lock_inode(struct inode *inode)
+{
+ int r;
+ inode_lock(inode);
+ if (IS_SWAPFILE(inode)) {
+ inode_unlock(inode);
+ return -EBUSY;
+ }
+ inode->i_flags |= S_SWAPFILE;
+ r = inode_drain_writes(inode);
+ if (r) {
+ inode->i_flags &= ~S_SWAPFILE;
+ inode_unlock(inode);
+ return r;
+ }
+ inode_unlock(inode);
+ return 0;
+}
+
+static void loop_unlock_inode(struct inode *inode)
+{
+ inode_lock(inode);
+ inode->i_flags &= ~S_SWAPFILE;
+ inode_unlock(inode);
+}
+
+static void loop_free(struct loop_c *lc)
+{
+ if (!lc)
+ return;
+ if (!IS_ERR_OR_NULL(lc->filp)) {
+ loop_unlock_inode(lc->inode);
+ filp_close(lc->filp, NULL);
+ }
+ kvfree(lc->map);
+ kfree(lc->path);
+ kfree(lc);
+}
+
+static int loop_ctr(struct dm_target *ti, unsigned argc, char **argv)
+{
+ struct loop_c *lc = NULL;
+ int r;
+ char dummy;
+
+ if (argc != 2) {
+ r = -EINVAL;
+ ti->error = "Invalid number of arguments";
+ goto err;
+ }
+
+ lc = kzalloc(sizeof(*lc), GFP_KERNEL);
+ if (!lc) {
+ r = -ENOMEM;
+ ti->error = "Cannot allocate loop context";
+ goto err;
+ }
+ ti->private = lc;
+
+ lc->path = kstrdup(argv[0], GFP_KERNEL);
+ if (!lc->path) {
+ r = -ENOMEM;
+ ti->error = "Cannot allocate loop path";
+ goto err;
+ }
+
+ if (sscanf(argv[1], "%lld%c", &lc->offset, &dummy) != 1) {
+ r = -EINVAL;
+ ti->error = "Invalid file offset";
+ goto err;
+ }
+
+ lc->read_only = !(dm_table_get_mode(ti->table) & FMODE_WRITE);
+
+ lc->filp = filp_open(lc->path, lc->read_only ? O_RDONLY : O_RDWR, 0);
+ if (IS_ERR(lc->filp)) {
+ r = PTR_ERR(lc->filp);
+ ti->error = "Could not open backing file";
+ goto err;
+ }
+
+ lc->inode = lc->filp->f_mapping->host;
+
+ r = loop_lock_inode(lc->inode);
+ if (r) {
+ ti->error = "Could not lock inode";
+ goto err;
+ }
+
+ r = setup_block_map(lc, ti);
+ if (r) {
+ goto err;
+ }
+
+ return 0;
+
+err:
+ loop_free(lc);
+ return r;
+}
+
+static void loop_dtr(struct dm_target *ti)
+{
+ struct loop_c *lc = ti->private;
+ loop_free(lc);
+}
+
+static struct target_type loop_target = {
+ .name = "loop",
+ .version = {1, 0, 0},
+ .module = THIS_MODULE,
+ .ctr = loop_ctr,
+ .dtr = loop_dtr,
+ .map = loop_map,
+ .status = loop_status,
+ .iterate_devices = loop_iterate_devices,
+};
+
+static int __init dm_loop_init(void)
+{
+ int r;
+
+ r = dm_register_target(&loop_target);
+ if (r < 0) {
+ DMERR("register failed %d", r);
+ goto err_target;
+ }
+
+ return 0;
+
+err_target:
+ return r;
+}
+
+static void __exit dm_loop_exit(void)
+{
+ dm_unregister_target(&loop_target);
+}
+
+module_init(dm_loop_init);
+module_exit(dm_loop_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Mikulas Patocka <mpatocka@xxxxxxxxxx>");
+MODULE_DESCRIPTION("device-mapper loop target");
