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

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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.




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