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

[Joe Thornber]

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 at invisiblethingslab.com> 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 at 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
>
> --
> dm-devel mailing list
> dm-devel at redhat.com
> https://listman.redhat.com/mailman/listinfo/dm-devel
>
>
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