linux-kernel - Re: [PATCH v20 09/12] dm: Add support for copy offload

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Date: Tue, 21 May 2024 09:11:21 +0200
From: Hannes Reinecke <hare@...e.de>
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, bvanassche@....org, david@...morbit.com,
 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 09/12] dm: Add support for copy offload

On 5/20/24 12:20, Nitesh Shetty wrote:
> Before enabling copy for dm target, check if underlying devices and
> dm target support copy. Avoid split happening inside dm target.
> Fail early if the request needs split, currently splitting copy
> request is not supported.
> 
> Signed-off-by: Nitesh Shetty <nj.shetty@...sung.com>
> ---
>   drivers/md/dm-table.c         | 37 +++++++++++++++++++++++++++++++++++
>   drivers/md/dm.c               |  7 +++++++
>   include/linux/device-mapper.h |  3 +++
>   3 files changed, 47 insertions(+)
> 
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index cc66a27c363a..d58c67ecd794 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -1899,6 +1899,38 @@ static bool dm_table_supports_nowait(struct dm_table *t)
>   	return true;
>   }
>   
> +static int device_not_copy_capable(struct dm_target *ti, struct dm_dev *dev,
> +				   sector_t start, sector_t len, void *data)
> +{
> +	struct request_queue *q = bdev_get_queue(dev->bdev);
> +
> +	return !q->limits.max_copy_sectors;
> +}
> +
> +static bool dm_table_supports_copy(struct dm_table *t)
> +{
> +	struct dm_target *ti;
> +	unsigned int i;
> +
> +	for (i = 0; i < t->num_targets; i++) {
> +		ti = dm_table_get_target(t, i);
> +
> +		if (!ti->copy_offload_supported)
> +			return false;
> +
> +		/*
> +		 * target provides copy support (as implied by setting
> +		 * 'copy_offload_supported')
> +		 * and it relies on _all_ data devices having copy support.
> +		 */
> +		if (!ti->type->iterate_devices ||
> +		    ti->type->iterate_devices(ti, device_not_copy_capable, NULL))
> +			return false;
> +	}
> +
> +	return true;
> +}
> +
>   static int device_not_discard_capable(struct dm_target *ti, struct dm_dev *dev,
>   				      sector_t start, sector_t len, void *data)
>   {
> @@ -1975,6 +2007,11 @@ int dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
>   		limits->discard_misaligned = 0;
>   	}
>   
> +	if (!dm_table_supports_copy(t)) {
> +		limits->max_copy_sectors = 0;
> +		limits->max_copy_hw_sectors = 0;
> +	}
> +
>   	if (!dm_table_supports_write_zeroes(t))
>   		limits->max_write_zeroes_sectors = 0;
>   
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 597dd7a25823..070b41b83a97 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1717,6 +1717,13 @@ static blk_status_t __split_and_process_bio(struct clone_info *ci)
>   	if (unlikely(ci->is_abnormal_io))
>   		return __process_abnormal_io(ci, ti);
>   
> +	if ((unlikely(op_is_copy(ci->bio->bi_opf)) &&
> +	    max_io_len(ti, ci->sector) < ci->sector_count)) {
> +		DMERR("Error, IO size(%u) > max target size(%llu)\n",
> +		      ci->sector_count, max_io_len(ti, ci->sector));
> +		return BLK_STS_IOERR;
> +	}
> +
>   	/*
>   	 * Only support bio polling for normal IO, and the target io is
>   	 * exactly inside the dm_io instance (verified in dm_poll_dm_io)
> diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
> index 82b2195efaca..6868941bc7d9 100644
> --- a/include/linux/device-mapper.h
> +++ b/include/linux/device-mapper.h
> @@ -397,6 +397,9 @@ struct dm_target {
>   	 * bio_set_dev(). NOTE: ideally a target should _not_ need this.
>   	 */
>   	bool needs_bio_set_dev:1;
> +
> +	/* copy offload is supported */
> +	bool copy_offload_supported:1;
>   };
>   
>   void *dm_per_bio_data(struct bio *bio, size_t data_size);

Errm. Not sure this will work. DM tables might be arbitrarily, requiring 
us to _split_ the copy offload request according to the underlying 
component devices. But we explicitly disallowed a split in one of the 
earlier patches.
Or am I wrong?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich