Re: [PATCH 6/6] dm: introduce dm_ask_for_duplicate_bios

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Mar 14 2014 at  6:44pm -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> This function can be used if the target needs to receive another duplicate
> of the current bio.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  drivers/md/dm.c               |   24 +++++++++++++++++++-----
>  include/linux/device-mapper.h |    2 ++
>  2 files changed, 21 insertions(+), 5 deletions(-)
> 
> Index: linux-3.14-rc6/drivers/md/dm.c
> ===================================================================
> --- linux-3.14-rc6.orig/drivers/md/dm.c	2014-03-14 23:25:27.000000000 +0100
> +++ linux-3.14-rc6/drivers/md/dm.c	2014-03-14 23:33:09.000000000 +0100
> @@ -1129,9 +1129,9 @@ EXPORT_SYMBOL_GPL(dm_set_target_max_io_l
>   *	 to make it empty)
>   * The target requires that region 3 is to be sent in the next bio.
>   *
> - * If the target wants to receive multiple copies of the bio with num_write_bios
> - * or num_write_same_bios, the partially processed part (the sum of regions 1+2)
> - * must be the same for all copies of the bio.
> + * If the target wants to receive multiple copies of the bio with num_*_bios or
> + * dm_ask_for_duplicate_bio, the partially processed part (the sum of regions
> + * 1+2) must be the same for all copies of the bio.
>   */
>  void dm_accept_partial_bio(struct bio *bio, unsigned n_sectors)
>  {
> @@ -1144,6 +1144,17 @@ void dm_accept_partial_bio(struct bio *b
>  }
>  EXPORT_SYMBOL_GPL(dm_accept_partial_bio);
>  
> +/*
> + * The target driver can call this function only from the map routine. The
> + * target driver requests that the dm sends more duplicates of the current bio.
> + */
> +void dm_ask_for_duplicate_bios(struct bio *bio, unsigned n_duplicates)
> +{
> +	struct dm_target_io *tio = container_of(bio, struct dm_target_io, clone);
> +	(*tio->num_bios) += n_duplicates;
> +}
> +EXPORT_SYMBOL_GPL(dm_ask_for_duplicate_bios);
> +
>  static void __map_bio(struct dm_target_io *tio)
>  {
>  	int r;

Hey Mikulas,

I'm not immediately seeing how increasing tio->num_bios from within a
target's map method, using dm_ask_for_duplicate_bios, will cause DM core
to send duplicates.

Nothing in DM core actually consumes tio->num_bios -- please advise.

Thanks,
Mike