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

Nitesh Shetty
2024-05-29T06:34:12Z

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On 24/05/24 01:33PM, Bart Van Assche wrote:
>On 5/20/24 03:20, 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.
>
>In this patch I don't see any changes for blk_attempt_bio_merge(). Does
>this mean that combining REQ_OP_COPY_DST and REQ_OP_COPY_SRC will never
>happen if the QUEUE_FLAG_NOMERGES request queue flag has been set?
>
Yes, in this case copy won't work, as both src and dst bio reach driver
as part of separate requests.
We will add this as part of documentation.

>Can it happen that the REQ_NOMERGE flag is set by __bio_split_to_limits()
>for REQ_OP_COPY_DST or REQ_OP_COPY_SRC bios? Will this happen if the
>following condition is met?
>
>dst_bio->nr_phys_segs + src_bio->nr_phys_segs > max_segments
>
No, this should not happen. We don't use bio_split_rw for copy.
We have added a separate function to check for split incase of
copy(bio_split_copy), which doesn't allow copy bio splits,
hence REQ_NOMERGE flag won't be set.

>Is it allowed to set REQ_PREFLUSH or REQ_FUA for REQ_OP_COPY_DST or
>REQ_OP_COPY_SRC bios? I'm asking this because these flags disable merging.
>
>From include/linux/blk_types.h:
>
>#define REQ_NOMERGE_FLAGS (REQ_NOMERGE | REQ_PREFLUSH | REQ_FUA)
>
No, setting these flags won't allow copy bio's to merge and hence copy
won't work.
We suggest to use helper API blkdev_copy_offload to achieve the
copy which won't be setting these flags.

Thank You,
Nitesh Shetty

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