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

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Date:	Wed, 13 Oct 2010 13:19:20 +0200
From:	Christoph Hellwig <hch@....de>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Mike Christie <michaelc@...wisc.edu>,
	Hannes Reinecke <hare@...e.de>,
	James Bottomley <James.Bottomley@...e.de>,
	Boaz Harrosh <bharrosh@...asas.com>,
	Jens Axboe <axboe@...nel.dk>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Douglas Gilbert <dgilbert@...erlog.com>,
	Richard Sharpe <realrichardsharpe@...il.com>
Subject: Re: [PATCH 4/5] tcm: Unify UNMAP and WRITE_SAME w/ UNMAP=1 subsystem plugin handling

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

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.
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