Re: [PATCH v20 12/12] null_blk: add support for copy offload

[Bart Van Assche <bvanassche@acm.org>]

On 5/20/24 03:20, Nitesh Shetty wrote:
> +	if (blk_rq_nr_phys_segments(req) != BLK_COPY_MAX_SEGMENTS)
> +		return status;

Why is this check necessary?

> +	/*
> +	 * First bio contains information about destination and last bio
> +	 * contains information about source.
> +	 */

Please check this at runtime (WARN_ON_ONCE()?).

> +	__rq_for_each_bio(bio, req) {
> +		if (seg == blk_rq_nr_phys_segments(req)) {
> +			sector_in = bio->bi_iter.bi_sector;
> +			if (rem != bio->bi_iter.bi_size)
> +				return status;
> +		} else {
> +			sector_out = bio->bi_iter.bi_sector;
> +			rem = bio->bi_iter.bi_size;
> +		}
> +		seg++;
> +	}

_rq_for_each_bio() iterates over the bios in a request. Does a copy
offload request always have two bios - one copy destination bio and
one copy source bio? If so, is 'seg' a bio counter? Why is that bio
counter compared with the number of physical segments in the request?

> +	trace_nullb_copy_op(req, sector_out << SECTOR_SHIFT,
> +			    sector_in << SECTOR_SHIFT, rem);
> +
> +	spin_lock_irq(&nullb->lock);
> +	while (rem > 0) {
> +		chunk = min_t(size_t, nullb->dev->blocksize, rem);
> +		offset_in = (sector_in & SECTOR_MASK) << SECTOR_SHIFT;
> +		offset_out = (sector_out & SECTOR_MASK) << SECTOR_SHIFT;
> +
> +		if (null_cache_active(nullb) && !is_fua)
> +			null_make_cache_space(nullb, PAGE_SIZE);
> +
> +		t_page_in = null_lookup_page(nullb, sector_in, false,
> +					     !null_cache_active(nullb));
> +		if (!t_page_in)
> +			goto err;
> +		t_page_out = null_insert_page(nullb, sector_out,
> +					      !null_cache_active(nullb) ||
> +					      is_fua);
> +		if (!t_page_out)
> +			goto err;
> +
> +		in = kmap_local_page(t_page_in->page);
> +		out = kmap_local_page(t_page_out->page);
> +
> +		memcpy(out + offset_out, in + offset_in, chunk);
> +		kunmap_local(out);
> +		kunmap_local(in);
> +		__set_bit(sector_out & SECTOR_MASK, t_page_out->bitmap);
> +
> +		if (is_fua)
> +			null_free_sector(nullb, sector_out, true);
> +
> +		rem -= chunk;
> +		sector_in += chunk >> SECTOR_SHIFT;
> +		sector_out += chunk >> SECTOR_SHIFT;
> +	}
> +
> +	status = 0;
> +err:
> +	spin_unlock_irq(&nullb->lock);

In the worst case, how long does this loop disable interrupts?

> +TRACE_EVENT(nullb_copy_op,
> +		TP_PROTO(struct request *req,
> +			 sector_t dst, sector_t src, size_t len),
> +		TP_ARGS(req, dst, src, len),
> +		TP_STRUCT__entry(
> +				 __array(char, disk, DISK_NAME_LEN)
> +				 __field(enum req_op, op)
> +				 __field(sector_t, dst)
> +				 __field(sector_t, src)
> +				 __field(size_t, len)
> +		),

Isn't __string() preferred over __array() since the former occupies less space
in the trace buffer?

Thanks,

Bart.