linux-kernel - Re: [PATCH 1/6] null_blk: allow write zeores on non-membacked

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Date:   Wed, 5 Oct 2022 13:54:44 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Chaitanya Kulkarni <kch@...dia.com>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     axboe@...nel.dk, johannes.thumshirn@....com, bvanassche@....org,
        ming.lei@...hat.com, shinichiro.kawasaki@....com,
        vincent.fu@...sung.com, yukuai3@...wei.com
Subject: Re: [PATCH 1/6] null_blk: allow write zeores on non-membacked

On 10/5/22 12:16, Chaitanya Kulkarni wrote:
> Add a helper function to enable the REQ_OP_WRITE_ZEROES operations
> when null_blk is configured with the non-membacked operations.
> 
> 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@...dia.com>
> ---
>  drivers/block/null_blk/main.c     | 13 +++++++++++++
>  drivers/block/null_blk/null_blk.h |  1 +
>  2 files changed, 14 insertions(+)
> 
> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
> index 1f154f92f4c2..fc3e883f7b84 100644
> --- a/drivers/block/null_blk/main.c
> +++ b/drivers/block/null_blk/main.c
> @@ -209,6 +209,10 @@ static bool g_discard;
>  module_param_named(discard, g_discard, bool, 0444);
>  MODULE_PARM_DESC(discard, "Support discard operations (requires memory-backed null_blk device). Default: false");
>  
> +static bool g_write_zeroes;
> +module_param_named(write_zeroes, g_write_zeroes, bool, 0444);
> +MODULE_PARM_DESC(write_zeroes, "Support write-zeores operations. Default: false");

Why not make this a number of sectors representing the maximum size of a
write zero command (blk_queue_max_write_zeroes_sectors()) ? That would
allow exercising split write zeros BIOs.

> +
>  static unsigned long g_cache_size;
>  module_param_named(cache_size, g_cache_size, ulong, 0444);
>  MODULE_PARM_DESC(mbps, "Cache size in MiB for memory-backed device. Default: 0 (none)");
> @@ -678,6 +682,7 @@ static struct nullb_device *null_alloc_dev(void)
>  	dev->blocking = g_blocking;
>  	dev->memory_backed = g_memory_backed;
>  	dev->discard = g_discard;
> +	dev->write_zeroes = g_write_zeroes;
>  	dev->cache_size = g_cache_size;
>  	dev->mbps = g_mbps;
>  	dev->use_per_node_hctx = g_use_per_node_hctx;
> @@ -1800,6 +1805,13 @@ 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);
> +}
> +
>  static const struct block_device_operations null_bio_ops = {
>  	.owner		= THIS_MODULE,
>  	.submit_bio	= null_submit_bio,
> @@ -2111,6 +2123,7 @@ 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 */

-- 
Damien Le Moal
Western Digital Research