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

Bart Van Assche
2024-05-20T23:43:14Z

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.