Re: [PATCH v15 02/12] Add infrastructure for copy offload in block and request layer.

Nitesh Shetty
2023-09-07T17:36:37Z

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On 07/09/23 07:39AM, Hannes Reinecke wrote:
>On 9/6/23 18:38, Nitesh Shetty wrote:
>>We add two new opcode REQ_OP_COPY_SRC, REQ_OP_COPY_DST.
>>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 source information,
>>followed by bio with destination information.
>>Once the src bio arrives we form a request and wait for destination
>>bio. Upon arrival of destination we merge these two bio's and send
>>corresponding request down to device driver.
>>
>>Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
>>Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
>>---
>> 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(-)
>>
>Having two separate bios is okay, and what one would expect.
>What is slightly strange is the merging functionality;
>That could do with some more explanation why this approach was taken.

Combining the two bios is necessary to form a single copy command.
And that's what we do by putting two bios in the single request, and send
this down to the driver.
This helps to avoid putting payload (token) in the request.
This change came as a feedback, as sending payload that is not data to the
device is considered a bad idea [1].
Current approach is similar to bio merging in discard.

>And also some checks in the merging code to avoid merging non-copy
>offload bios.
blk_copy_offload_mergable takes care of this, as this checks REQ_OP_COPY_SRC
and REQ_OP_COPY_DST

[1] https://lore.kernel.org/lkml/20230605121732.28468-1-nj.shetty@samsung.com/T/#mfa7104c5f5f8579cd20f668a9d5e83b4ac8bc58a

Thank You,
Nitesh Shetty

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