[dm-devel] [PATCH 6/6] dm: introduce dm_ask_for_duplicate_bios

[Mikulas Patocka]

On Fri, 25 Apr 2014, Mike Snitzer wrote:

> On Fri, Mar 14 2014 at  6:44pm -0400,
> Mikulas Patocka <mpatocka at 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 at 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

tio->num_bios is a pointer. You don't increase tio->num_bios, you increase 
the value it points to. It points to the local variable that is the limit 
of the for loop.

Mikulas