linux-kernel - Re: [PATCH] [ConfigFS]: Allow symbolic links from a SysFS struct kobject source.

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Date:	Fri, 17 Oct 2008 12:48:57 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	Greg KH <greg@...ah.com>, LKML <linux-kernel@...r.kernel.org>,
	Linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	"Linux-iSCSI.org Target Dev" 
	<linux-iscsi-target-dev@...glegroups.com>,
	SCST-Devel <scst-devel@...ts.sourceforge.net>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrew Morton <akpm@...l.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] [ConfigFS]: Allow symbolic links from a SysFS struct
	kobject source.

On Fri, Oct 17, 2008 at 01:22:18AM -0700, Nicholas A. Bellinger wrote:
> On Fri, 2008-10-17 at 00:44 -0700, Greg KH wrote:
> > What is the problem you are attempting to solve here?
> > 
> 
> So, the generic target_core_mod engine that lives
> under /sys/kernel/config/target/core needs a method to locate struct
> block_device and struct scsi_device for access via bio_submit() and
> scsi_execute_() respectively.
> 
> Originally, target_core_mod used key echoed through configfs attributes
> like so:
> 
> export TARGET=/sys/kernel/config/target/core/
> 
> # Create $STORAGE_OBJECT of type Linux/BLOCK in generic target_core_mod
> mkdir -p $TARGET/iblock_0/lvm_test0
> # OLD METHOD to reference struct block_device
> echo iblock_major=254,iblock_minor=2 > $TARGET/iblock_0/lvm_test0/control
> echo 1 > $TARGET/iblock_0/lvm_test0/enable
> # NEW METHOD using sysfs ->configfs symlinks to reference struct block_device
> ln -s /sys/block/dm-2 $TARGET/iblock_0/lvm_test0/dm-2

	Pass an open file descriptor.  In bash(1):

    exec 3<>/dev/dm-7
    echo 3 >$TARGET/iblock_0/lvm_test0/control  # I'd call it 'fd'

In kernel, in the ->store() function of 'control':

	p = (char *)page;
	fd = simple_strtol(p, &p, 0);
	filp = fget(fd);
	inode = igrab(filp->f_mapping->host);
	dev = I_BDEV(filp->f_mapping->host);
	blkdev_get(dev, FMODE_WRITE | FMODE_READ, 0);

Error handling is left up to you (validate the strtol, fd range, ISBLK,
etc).  This assumes you want the struct block_device and want to pin it
in memory.  I assume you do.

Joel

-- 

Life's Little Instruction Book #450

	"Don't be afraid to say, 'I need help.'"

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@...cle.com
Phone: (650) 506-8127
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