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? 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 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) Thanks, Bart.
