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

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Message-ID: <eda6c198-3a29-4da4-94db-305cfe28d3d6@acm.org>
Date: Fri, 24 May 2024 13:33:39 -0700
From: Bart Van Assche <bvanassche@....org>
To: Nitesh Shetty <nj.shetty@...sung.com>, Jens Axboe <axboe@...nel.dk>,
 Jonathan Corbet <corbet@....net>, Alasdair Kergon <agk@...hat.com>,
 Mike Snitzer <snitzer@...nel.org>, Mikulas Patocka <mpatocka@...hat.com>,
 Keith Busch <kbusch@...nel.org>, Christoph Hellwig <hch@....de>,
 Sagi Grimberg <sagi@...mberg.me>, Chaitanya Kulkarni <kch@...dia.com>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Cc: martin.petersen@...cle.com, david@...morbit.com, hare@...e.de,
 damien.lemoal@...nsource.wdc.com, anuj20.g@...sung.com, joshi.k@...sung.com,
 nitheshshetty@...il.com, gost.dev@...sung.com, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 dm-devel@...ts.linux.dev, linux-nvme@...ts.infradead.org,
 linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v20 02/12] Add infrastructure for copy offload in block
 and request layer.

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.