Re: [PATCH 4/5] tcm: Unify UNMAP and WRITE_SAME w/ UNMAP=1 subsystem plugin handling

["Nicholas A. Bellinger" <nab@linux-iscsi.org>]

On Wed, 2010-10-13 at 13:19 +0200, Christoph Hellwig wrote:
> > +static int iblock_do_discard(struct se_task *task, enum blk_discard_type type)
> > +{
> > +	struct iblock_dev *ibd = task->se_dev->dev_ptr;
> > +	struct block_device *bd = ibd->ibd_bd;
> > +	struct se_cmd *cmd = TASK_CMD(task);
> > +
> > +	if (type == DISCARD_UNMAP)
> > +		return transport_generic_unmap(cmd, bd);	
> > +	else if (type == DISCARD_WRITE_SAME_UNMAP)
> > +		return iblock_emulate_write_same_unmap(task);	
> > +	else {
> > +		printk(KERN_ERR "Unsupported discard_type_t: %d\n", type);
> > +		return -ENOSYS;
> > +	}
> > +
> > +	return -ENOSYS;
> > +}
> > +
> 
> I don't think the discard code is quite, nor nicely structured.
> 
> The parsing of the WRITE SAME and UNMAP CDBs is something the generic
> CDB parsing code should do,

Ok, so you are thinking about a seperate transport_emulate_write_same()
and transport_emulate_unmap() called from
transport_emulate_control_cdb(), right..?

>  and just give a range of lists of lba/len
> pairs to the ->discard method in the backed.

Yes, these are already available from the passed struct
se_task->task_lba and ->task_size values.

>  Also your generic
> discard helpers aren't actually generic - they require a block device
> and thus should be only in iblock.c.  While your hack in the file
> backend to use it if we're using a block device as backing file
> works it's rather gross.  Having the file backend general enough to
> work with a block devices is fine, but adding special hacks that
> only work on block device while having a fully working bio based backed
> is a bit gross.

Yes, so the problem of trying to make this code generic (eg: outside of
TCM subsystem plugins) is that blk_issue_discard() takes struct
block_device, which means we the subsystem plugin has to locate struct
block_device inside of non generic cide.

So, then the main issue becomes FILEIO + block level discard and how to
issue an blk_issue_discard() from struct fileio in the most sane way.
If there is no sane way then I will just drop this bit, or just do the
file level 'hole punch' that you are speaking about.

> Btw, at least on XFS you can implement discard using
> hole punch operations, although that can lead to quite bad fragmentation
> in cases.  Just as block-level discards can lead to quite bad
> performance - I'd suggest to not enable them by default.
> 

Ok, I will disable these by default for IBLOCK and FILEIO, and require
an explict override from user with the emulate_tpu and emulate_tpws
values in /sys/kernel/config/target/core/$HBA/$DEV/attrib/

> One other thing I noticed is that you use igrab a lot.  In general
> drivers have absolutely no need for a igrab.  If you have a reference
> to the file behind an inode you keep the inode in core and there's no
> need at all to grab a second reference to it.

The igrab() and iput() usage in FILEIO code are used for locating the
struct block_device for blk_issue_discard().  If this is the case then I
will remove these from FILEIO.

Thanks!

--nab