Re: [PATCH V2 1/1] loop: introduce LO_FLAGS_NO_DEALLOC

["Darrick J. Wong" <djwong@kernel.org>]

On Sat, Aug 06, 2022 at 11:30:22PM +0800, Zhang Boyang wrote:
> Previously, for file-backed loop devices, REQ_OP_DISCARD and
> REQ_OP_WRITE_ZEROES (without REQ_NOUNMAP) are implemented using
> fallocate(FALLOC_FL_PUNCH_HOLE), which will cause the underlying file to
> be sparse and disk space freed. The users have no choice to prevent this
> this from happening.
> 
> This patch introduces LO_FLAGS_NO_DEALLOC. With this flag set,
> REQ_OP_DISCARD and REQ_OP_WRITE_ZEROES are forced to use
> fallocate(FALLOC_FL_ZERO_RANGE). The disk space of underlying file is
> kept allocated. This is useful if users, for example, want to use a
> preallocated file as the backing file.

Considering that discard isn't required to do anything, why not
echo 0 | sudo tee /sys/block/loopX/queue/discard_max_bytes ?

--D

> Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
> ---
>  drivers/block/loop.c      | 17 +++++++++++++++--
>  include/uapi/linux/loop.h | 15 +++++++++++----
>  2 files changed, 26 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 084f9b8a0ba3..36bd9906a154 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -483,11 +483,15 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
>  		 * write zeroes the range.  Otherwise, punch them out.
>  		 */
>  		return lo_fallocate(lo, rq, pos,
> -			(rq->cmd_flags & REQ_NOUNMAP) ?
> +			((rq->cmd_flags & REQ_NOUNMAP) ||
> +			 (lo->lo_flags & LO_FLAGS_NO_DEALLOC)) ?
>  				FALLOC_FL_ZERO_RANGE :
>  				FALLOC_FL_PUNCH_HOLE);
>  	case REQ_OP_DISCARD:
> -		return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);
> +		return lo_fallocate(lo, rq, pos,
> +			(lo->lo_flags & LO_FLAGS_NO_DEALLOC) ?
> +				FALLOC_FL_ZERO_RANGE :
> +				FALLOC_FL_PUNCH_HOLE);
>  	case REQ_OP_WRITE:
>  		if (cmd->use_aio)
>  			return lo_rw_aio(lo, cmd, pos, WRITE);
> @@ -719,12 +723,20 @@ static ssize_t loop_attr_dio_show(struct loop_device *lo, char *buf)
>  	return sysfs_emit(buf, "%s\n", dio ? "1" : "0");
>  }
>  
> +static ssize_t loop_attr_no_dealloc_show(struct loop_device *lo, char *buf)
> +{
> +	int no_dealloc = (lo->lo_flags & LO_FLAGS_NO_DEALLOC);
> +
> +	return sysfs_emit(buf, "%s\n", no_dealloc ? "1" : "0");
> +}
> +
>  LOOP_ATTR_RO(backing_file);
>  LOOP_ATTR_RO(offset);
>  LOOP_ATTR_RO(sizelimit);
>  LOOP_ATTR_RO(autoclear);
>  LOOP_ATTR_RO(partscan);
>  LOOP_ATTR_RO(dio);
> +LOOP_ATTR_RO(no_dealloc);
>  
>  static struct attribute *loop_attrs[] = {
>  	&loop_attr_backing_file.attr,
> @@ -733,6 +745,7 @@ static struct attribute *loop_attrs[] = {
>  	&loop_attr_autoclear.attr,
>  	&loop_attr_partscan.attr,
>  	&loop_attr_dio.attr,
> +	&loop_attr_no_dealloc.attr,
>  	NULL,
>  };
>  
> diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
> index 6f63527dd2ed..91a0a8b1f298 100644
> --- a/include/uapi/linux/loop.h
> +++ b/include/uapi/linux/loop.h
> @@ -18,17 +18,24 @@ enum {
>  	LO_FLAGS_AUTOCLEAR	= 4,
>  	LO_FLAGS_PARTSCAN	= 8,
>  	LO_FLAGS_DIRECT_IO	= 16,
> +	LO_FLAGS_NO_DEALLOC	= 32,
>  };
>  
>  /* LO_FLAGS that can be set using LOOP_SET_STATUS(64) */
> -#define LOOP_SET_STATUS_SETTABLE_FLAGS (LO_FLAGS_AUTOCLEAR | LO_FLAGS_PARTSCAN)
> +#define LOOP_SET_STATUS_SETTABLE_FLAGS (LO_FLAGS_AUTOCLEAR		\
> +					| LO_FLAGS_PARTSCAN		\
> +					| LO_FLAGS_NO_DEALLOC)
>  
>  /* LO_FLAGS that can be cleared using LOOP_SET_STATUS(64) */
> -#define LOOP_SET_STATUS_CLEARABLE_FLAGS (LO_FLAGS_AUTOCLEAR)
> +#define LOOP_SET_STATUS_CLEARABLE_FLAGS (LO_FLAGS_AUTOCLEAR		\
> +					 | LO_FLAGS_NO_DEALLOC)
>  
>  /* LO_FLAGS that can be set using LOOP_CONFIGURE */
> -#define LOOP_CONFIGURE_SETTABLE_FLAGS (LO_FLAGS_READ_ONLY | LO_FLAGS_AUTOCLEAR \
> -				       | LO_FLAGS_PARTSCAN | LO_FLAGS_DIRECT_IO)
> +#define LOOP_CONFIGURE_SETTABLE_FLAGS (LO_FLAGS_READ_ONLY		\
> +				       | LO_FLAGS_AUTOCLEAR		\
> +				       | LO_FLAGS_PARTSCAN		\
> +				       | LO_FLAGS_DIRECT_IO		\
> +				       | LO_FLAGS_NO_DEALLOC)
>  
>  #include <asm/posix_types.h>	/* for __kernel_old_dev_t */
>  #include <linux/types.h>	/* for __u64 */
> -- 
> 2.30.2
>