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

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Message-Id: <1224274036.5556.358.camel@haakon2.linux-iscsi.org>
Date:	Fri, 17 Oct 2008 13:07:16 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Joel Becker <Joel.Becker@...cle.com>
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, 2008-10-17 at 12:48 -0700, Joel Becker wrote:
> 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.
> 

Btw, ideally I would like to do the same type of thing for accessing
drivers/scsi for both TYPE_DISK and non TYPE_DISK..  I will have a look
and see if something is possible to access struct scsi_device in all
device type cases..

--nab

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