Re: [PATCH 3/3] tcm/fileio: Add UNMAP / Block DISCARD support

[Boaz Harrosh <bharrosh@panasas.com>]

On 09/28/2010 12:51 AM, Nicholas A. Bellinger wrote:
> From: Nicholas Bellinger <nab@linux-iscsi.org>
> 
> This patch adds UNMAP emulation support using Block layer DISCARD in fd_emulate_scsi_cdb()
> -> transport_generic_unmap().  This includes the use of blk_queue_discard() in
> fd_create_virtdevice() to determine when to set DEV_ATTRIB(dev)->emulate_tpe=1 to
> signal to TCM Core to perform the necessary control CDB emulation for TPE=1 / UNMAP.
> 
> Note this patch igrab() + iput() when accessing struct block_device via
> I_BDEV(file->f_mapping->host) that is required for transport_generic_unmap()
> within fd_emulate_scsi_cdb().
> 
> Also note that when (S_ISBLK(inode->i_mode) is NULL, the TPE=1 and UNMAP emulation
> are disabled for non BD struct file I/O.
> 
> Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
> ---
>  drivers/target/target_core_file.c |   53 ++++++++++++++++++++++++++++++++++++-
>  1 files changed, 52 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/target/target_core_file.c b/drivers/target/target_core_file.c
> index 93d3f63..3b91556 100644
> --- a/drivers/target/target_core_file.c
> +++ b/drivers/target/target_core_file.c
> @@ -235,6 +235,17 @@ static struct se_device *fd_create_virtdevice(
>  
>  	fd_dev->fd_dev_id = fd_host->fd_host_dev_id_count++;
>  	fd_dev->fd_queue_depth = dev->queue_depth;
> +	/*
> +	 * Check for QUEUE_FLAG_DISCARD and enable TCM TPE emulation
> +	 * if present for struct block_device backend
> +	 */
> +	if (S_ISBLK(inode->i_mode)) {
> +		if (blk_queue_discard(bdev_get_queue(inode->i_bdev))) {
> +			DEV_ATTRIB(dev)->emulate_tpe = 1;
> +			printk(KERN_INFO "FILEIO: Enabling BLOCK Discard"
> +				" and TPE=1 emulation\n");
> +		}
> +	}
>  
>  	printk(KERN_INFO "CORE_FILE[%u] - Added TCM FILEIO Device ID: %u at %s,"
>  		" %llu total bytes\n", fd_host->fd_host_id, fd_dev->fd_dev_id,
> @@ -388,9 +399,13 @@ static int fd_emulate_read_cap16(struct se_task *task)
>   */
>  static int fd_emulate_scsi_cdb(struct se_task *task)
>  {
> -	int ret;
>  	struct se_cmd *cmd = TASK_CMD(task);
> +	struct fd_dev *fd_dev = (struct fd_dev *) task->se_dev->dev_ptr;

No need to typecast a void pointer

>  	struct fd_request *fd_req = (struct fd_request *) task->transport_req;
> +	struct file *f = fd_dev->fd_file;
> +	struct inode *i;
> +	struct block_device *bd;
> +	int ret;
>  
>  	switch (fd_req->fd_scsi_cdb[0]) {
>  	case INQUIRY:
> @@ -433,6 +448,42 @@ static int fd_emulate_scsi_cdb(struct se_task *task)
>  		if (ret < 0)
>  			return ret;
>  		break;
> +	case UNMAP:
> +		i = igrab(f->f_mapping->host);
> +		if (!(i)) {

You must have a reference on your fileio backend file, right?
This could be a BUG_ON()

> +			printk(KERN_ERR "FILEIO: Unable to locate inode for"
> +					" backend for UNMAP\n");
> +			return PYX_TRANSPORT_LU_COMM_FAILURE;
> +		}
> +		/*
> +		 * Currently for struct file w/o a struct block_device
> +		 * backend we return a success..
> +		 */	
> +		if (!(S_ISBLK(i->i_mode))) {

I did not see in the first patch that you do this check when actually sending
the VPD page. I only saw that it is enabled if set from configfs. (Sorry for
the noise if I missed it). So you might want to call a target specific function
when sending the VPD page. This way this will never happen right?

> +			printk(KERN_WARNING "Ignoring UNMAP for non BD"
> +					" backend for struct file\n");

TODO: Lots of extent based filesystems could enjoy if you punch an hole
      in the file. (Which will also send a proper discard).
      So how to punch an hole in a file via vfs?

> +			iput(i);
> +			return PYX_TRANSPORT_LU_COMM_FAILURE;
> +		}
> +		bd = I_BDEV(f->f_mapping->host);
> +		if (!(bd)) {
> +			printk(KERN_ERR "FILEIO: Unable to locate struct"
> +					" block_device for UNMAP\n");
> +			iput(i);
> +			return PYX_TRANSPORT_LU_COMM_FAILURE;
> +		}
> +		/*
> +		 * Now call the transport_generic_unmap() -> blkdev_issue_discard()
> +		 * wrapper to translate SCSI UNMAP into Linux/BLOCK discards on
> +		 * LBA+Range descriptors in the UNMAP write paylaod.
> +		 */
> +		ret = transport_generic_unmap(cmd, bd);
> +		if (ret < 0) {
> +			iput(i);
> +			return ret;
> +		}
> +		iput(i);
> +		break;
>  	case ALLOW_MEDIUM_REMOVAL:
>  	case ERASE:
>  	case REZERO_UNIT:

Boaz