Re: [PATCH v20 02/12] Add infrastructure for copy offload in block and request layer.

[Nitesh Shetty]

On 21/05/24 09:01AM, Hannes Reinecke wrote:
> On 5/20/24 12:20, Nitesh Shetty wrote:
> > We add two new opcode REQ_OP_COPY_DST, REQ_OP_COPY_SRC.
> > Since copy is a composite operation involving src and dst sectors/lba,
> > each needs to be represented by a separate bio to make it compatible
> > with device mapper.
> > We expect caller to take a plug and send bio with destination information,
> > followed by bio with source information.
> > Once the dst bio arrives we form a request and wait for source
> > bio. Upon arrival of source bio we merge these two bio's and send
> > corresponding request down to device driver.
> > Merging non copy offload bio is avoided by checking for copy specific
> > opcodes in merge function.
> >
> > Signed-off-by: Nitesh Shetty <nj.shetty@xxxxxxxxxxx>
> > Signed-off-by: Anuj Gupta <anuj20.g@xxxxxxxxxxx>
> > ---
> >  block/blk-core.c          |  7 +++++++
> >  block/blk-merge.c         | 41 +++++++++++++++++++++++++++++++++++++++
> >  block/blk.h               | 16 +++++++++++++++
> >  block/elevator.h          |  1 +
> >  include/linux/bio.h       |  6 +-----
> >  include/linux/blk_types.h | 10 ++++++++++
> >  6 files changed, 76 insertions(+), 5 deletions(-)
> I am a bit unsure about leveraging 'merge' here. As Bart pointed out, 
> this is arguably as mis-use of the 'merge' functionality as we don't
> actually merge bios, but rather use the information from these bios to
> form the actual request.
> Wouldn't it be better to use bio_chain here, and send out the eventual
> request from the end_io function of the bio chain?
With above bio chain approach, I see a difficulty while dealing with
stacked/partitioned devices, as they might get remapped.
Or am I missing something here ?

Bart, Hannes,
Regarding merge, does it looks any better, if we use single request
operation such as REQ_OP_COPY and use op_flags(REQ_COPY_DST/REQ_COPY_SRC)
to identify dst and src bios ?


Regards,
Nitesh Shetty