[PATCH 2/2] dm-thin: Allow specifying an offset

[Demi Marie Obenour <demi@invisiblethingslab.com>]

This allows exposing only part of a thin volume without having to layer
dm-linear.  One use-case is a hypervisor replacing a partition table.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
 drivers/md/dm-thin.c | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index d85fdbd782ae5426003c99a4b4bf53818cc85efa..87f14933375b050a950a5f58e98c13b4d28f6af0 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -357,6 +357,7 @@ struct thin_c {
 	 */
 	refcount_t refcount;
 	struct completion can_destroy;
+	u64 offset;
 };
 
 /*----------------------------------------------------------------*/
@@ -1180,9 +1181,9 @@ static void process_prepared_discard_passdown_pt1(struct dm_thin_new_mapping *m)
 	discard_parent = bio_alloc(NULL, 1, 0, GFP_NOIO);
 	discard_parent->bi_end_io = passdown_endio;
 	discard_parent->bi_private = m;
- 	if (m->maybe_shared)
- 		passdown_double_checking_shared_status(m, discard_parent);
- 	else {
+	if (m->maybe_shared)
+		passdown_double_checking_shared_status(m, discard_parent);
+	else {
 		struct discard_op op;
 
 		begin_discard(&op, tc, discard_parent);
@@ -4149,7 +4150,7 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
 
 	mutex_lock(&dm_thin_pool_table.mutex);
 
-	if (argc != 2 && argc != 3) {
+	if (argc < 2 || argc > 4) {
 		ti->error = "Invalid argument count";
 		r = -EINVAL;
 		goto out_unlock;
@@ -4168,7 +4169,8 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
 	bio_list_init(&tc->retry_on_resume_list);
 	tc->sort_bio_list = RB_ROOT;
 
-	if (argc == 3) {
+	/* Use "/" to indicate "no origin device" while providing an offset */
+	if (argc >= 3 && strcmp(argv[2], "/")) {
 		if (!strcmp(argv[0], argv[2])) {
 			ti->error = "Error setting origin device";
 			r = -EINVAL;
@@ -4196,6 +4198,23 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
 		goto bad_common;
 	}
 
+	tc->offset = 0;
+	if (argc > 3) {
+		sector_t sector_offset;
+
+		if (kstrtoull(argv[3], 10, &tc->offset)) {
+			ti->error = "Invalid offset";
+			r = -EINVAL;
+			goto bad_common;
+		}
+
+		if (check_add_overflow(tc->offset, ti->len, &sector_offset)) {
+			ti->error = "Offset + len overflows sector_t";
+			r = -EINVAL;
+			goto bad_common;
+		}
+	}
+
 	pool_md = dm_get_md(tc->pool_dev->bdev->bd_dev);
 	if (!pool_md) {
 		ti->error = "Couldn't get pool mapped device";
@@ -4285,8 +4304,9 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
 
 static int thin_map(struct dm_target *ti, struct bio *bio)
 {
-	bio->bi_iter.bi_sector = dm_target_offset(ti, bio->bi_iter.bi_sector);
+	struct thin_c *tc = ti->private;
 
+	bio->bi_iter.bi_sector = dm_target_offset(ti, bio->bi_iter.bi_sector) + tc->offset;
 	return thin_bio_map(ti, bio);
 }
 
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab