Re: [PATCH v7 6/6] loop: Add support for REQ_ALLOCATE

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Feb 13, 2020 at 10:39:35AM +0300, Kirill Tkhai wrote:
> Support for new modifier of REQ_OP_WRITE_ZEROES command.
> This results in allocation extents in backing file instead
> of actual blocks zeroing.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> Reviewed-by: Bob Liu <bob.liu@oracle.com>
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  drivers/block/loop.c |   20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 739b372a5112..0704167a5aaa 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -581,6 +581,16 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
>  	return 0;
>  }
>  

Urgh, I meant "copy the comment directly to here", e.g.

/*
 * Convert REQ_OP_WRITE_ZEROES modifiers into fallocate mode
 *
 * If the caller requires allocation, select that mode.  If the caller
 * doesn't want deallocation, call zeroout to write zeroes the range.
 * Otherwise, punch out the blocks.
 */

The goal here is to reinforce the notion that FL_ZERO_RANGE is how we
achieve nounmap zeroing...

--D

> +static unsigned int write_zeroes_to_fallocate_mode(unsigned int flags)
> +{
> +	if (flags & REQ_ALLOCATE)
> +		return 0;
> +	if (flags & REQ_NOUNMAP)
> +		return FALLOC_FL_ZERO_RANGE;
> +	return FALLOC_FL_PUNCH_HOLE;
> +}
> +
>  static int do_req_filebacked(struct loop_device *lo, struct request *rq)
>  {
>  	struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
> @@ -599,14 +609,8 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
>  	case REQ_OP_FLUSH:
>  		return lo_req_flush(lo, rq);
>  	case REQ_OP_WRITE_ZEROES:
> -		/*
> -		 * If the caller doesn't want deallocation, call zeroout to
> -		 * write zeroes the range.  Otherwise, punch them out.
> -		 */
>  		return lo_fallocate(lo, rq, pos,
> -			(rq->cmd_flags & REQ_NOUNMAP) ?
> -				FALLOC_FL_ZERO_RANGE :
> -				FALLOC_FL_PUNCH_HOLE);
> +			write_zeroes_to_fallocate_mode(rq->cmd_flags));
>  	case REQ_OP_DISCARD:
>  		return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);
>  	case REQ_OP_WRITE:
> @@ -877,6 +881,7 @@ static void loop_config_discard(struct loop_device *lo)
>  		q->limits.discard_alignment = 0;
>  		blk_queue_max_discard_sectors(q, 0);
>  		blk_queue_max_write_zeroes_sectors(q, 0);
> +		blk_queue_max_allocate_sectors(q, 0);
>  		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
>  		return;
>  	}
> @@ -886,6 +891,7 @@ static void loop_config_discard(struct loop_device *lo)
>  
>  	blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
>  	blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
> +	blk_queue_max_allocate_sectors(q, UINT_MAX >> 9);
>  	blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
>  }
>  
> 
>