Re: [PATCH V4 1/8] null_blk: allow REQ_OP_WRITE_ZEROES

Shinichiro Kawasaki
2022-11-30T07:59:41Z

On Nov 29, 2022 / 15:28, Chaitanya Kulkarni wrote:
> Add a helper function to enable the REQ_OP_WRITE_ZEROES operations
> when null_blk.
>
> Since write-zeroes is a non-trivial I/O operation we need this to
> add a blktest so we can test the non-trivial I/O path from the
> application to the block layer.
>
> Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>

This motivation sounds good. I tried this patch. With a quick test it looks
working good for me. Please find minor comments in line.

[...]

> +static void null_zero_sector(struct nullb_device *d, sector_t sect,
> + sector_t nr_sects, bool cache)
> +{
> + struct radix_tree_root *root = cache ? &d->cache : &d->data;
> + struct nullb_page *t_page;
> + unsigned int offset;
> + void *dest;
> +
> + t_page = radix_tree_lookup(root, sect >> PAGE_SECTORS_SHIFT);
> + if (!t_page)
> + return;
> +
> + offset = (sect & SECTOR_MASK) << SECTOR_SHIFT;
> + dest = kmap_atomic(t_page->page);
> + memset(dest + offset, 0, SECTOR_SIZE * nr_sects);
> + kunmap_atomic(dest);
> +}

Did you consider to call null_lookup_page() for __null_lookup_page() from
null_zero_sector()? It may simplify this function a bit.

> +
> static struct nullb_page *null_radix_tree_insert(struct nullb *nullb, u64 idx,
> struct nullb_page *t_page, bool is_cache)
> {
> @@ -1186,6 +1211,27 @@ blk_status_t null_handle_discard(struct nullb_device *dev,
> return BLK_STS_OK;
> }
>
> +static blk_status_t null_handle_write_zeroes(struct nullb_device *dev,
> + sector_t sector, sector_t nr_sectors)
> +{
> + unsigned int bytes_left = nr_sectors << 9;
> + struct nullb *nullb = dev->nullb;
> + size_t curr_bytes;
> +
> + spin_lock_irq(&nullb->lock);
> + while (bytes_left > 0) {
> + curr_bytes = min_t(size_t, bytes_left, nullb->dev->blocksize);
> + nr_sectors = curr_bytes >> SECTOR_SHIFT;
> + null_zero_sector(nullb->dev, sector, nr_sectors, false);
> + if (null_cache_active(nullb))
> + null_zero_sector(nullb->dev, sector, nr_sectors, true);
> + sector += nr_sectors;
> + bytes_left -= curr_bytes;
> + }
> + spin_unlock_irq(&nullb->lock);
> + return BLK_STS_OK;
> +}
> +
> static int null_handle_flush(struct nullb *nullb)
> {
> int err;
> @@ -1352,6 +1398,9 @@ static inline blk_status_t null_handle_memory_backed(struct nullb_cmd *cmd,
> if (op == REQ_OP_DISCARD)
> return null_handle_discard(dev, sector, nr_sectors);
>
> + if (op == REQ_OP_WRITE_ZEROES)
> + return null_handle_write_zeroes(dev, sector, nr_sectors);
> +
> if (dev->queue_mode == NULL_Q_BIO)
> err = null_handle_bio(cmd);
> else
> @@ -1800,6 +1849,13 @@ static void null_config_discard(struct nullb *nullb)
> blk_queue_max_discard_sectors(nullb->q, UINT_MAX >> 9);
> }
>
> +static void null_config_write_zeroes(struct nullb *nullb)
> +{
> + if (!nullb->dev->write_zeroes)
> + return;
> + blk_queue_max_write_zeroes_sectors(nullb->q, UINT_MAX >> 9);

Just comment: this value UINT_MAX >> 9 sounds a bit weird, but probably ok. This
value was introduced by commit 306eb6b4ad4f ("nullb: support discard") to call
blk_queue_max_discard_sectors(). I guess you chose the same value for write
zeroes.

> +}
> +
> static const struct block_device_operations null_bio_ops = {
> .owner = THIS_MODULE,
> .submit_bio = null_submit_bio,
> @@ -2111,6 +2167,7 @@ static int null_add_dev(struct nullb_device *dev)
> blk_queue_virt_boundary(nullb->q, PAGE_SIZE - 1);
>
> null_config_discard(nullb);
> + null_config_write_zeroes(nullb);
>
> if (config_item_name(&dev->item)) {
> /* Use configfs dir name as the device name */
> diff --git a/drivers/block/null_blk/null_blk.h b/drivers/block/null_blk/null_blk.h
> index 94ff68052b1e..2c0c9c29158f 100644
> --- a/drivers/block/null_blk/null_blk.h
> +++ b/drivers/block/null_blk/null_blk.h
> @@ -111,6 +111,7 @@ struct nullb_device {
> bool power; /* power on/off the device */
> bool memory_backed; /* if data is stored in memory */
> bool discard; /* if support discard */
> + bool write_zeroes; /* if support write_zeroes */
> bool zoned; /* if device is zoned */
> bool virt_boundary; /* virtual boundary on/off for the device */
> bool no_sched; /* no IO scheduler for the device */
> --
> 2.29.0
>

--
Shin'ichiro Kawasaki