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

[Joe Thornber <thornber@xxxxxxxxxx>]

Nack.  I'm not building a linear target into every other target.  Layering targets is simple.

On Tue, Feb 7, 2023 at 7:56 AM Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:

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@xxxxxxxxxxxxxxxxxxxxxx>
---
 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

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