linux-kernel - Re: [PATCH] Modify loop device to be able to manage partitions of the disk image

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Message-Id: <200803062057.50181.nickpiggin@yahoo.com.au>
Date:	Thu, 6 Mar 2008 20:57:50 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Laurent Vivier <Laurent.Vivier@...l.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Modify loop device to be able to manage partitions of the disk image

On Thursday 06 March 2008 20:43, Laurent Vivier wrote:
> This patch allows to use loop device with partitionned disk image.
>
> Original behavior of loop is not modified.
>
> A new parameter is introduced to define how many partition we want to be
> able to manage per loop device. This parameter is "max_part".
>
> For instance, to manage 63 partitions / loop device, we will do:
> # modprobe max_part=63
> # ls -l /dev/loop?*
> brw-rw---- 1 root disk 7,   0 2008-03-05 14:55 /dev/loop0
> brw-rw---- 1 root disk 7,  64 2008-03-05 14:55 /dev/loop1
> brw-rw---- 1 root disk 7, 128 2008-03-05 14:55 /dev/loop2
> brw-rw---- 1 root disk 7, 192 2008-03-05 14:55 /dev/loop3
> brw-rw---- 1 root disk 7, 256 2008-03-05 14:55 /dev/loop4
> brw-rw---- 1 root disk 7, 320 2008-03-05 14:55 /dev/loop5
> brw-rw---- 1 root disk 7, 384 2008-03-05 14:55 /dev/loop6
> brw-rw---- 1 root disk 7, 448 2008-03-05 14:55 /dev/loop7
>
> And to attach a raw partitionned disk image, the original losetup is used:
>
> # losetup -f etch.img
> # ls -l /dev/loop?*
> brw-rw---- 1 root disk 7,   0 2008-03-05 14:55 /dev/loop0
> brw-rw---- 1 root disk 7,   1 2008-03-05 14:57 /dev/loop0p1
> brw-rw---- 1 root disk 7,   2 2008-03-05 14:57 /dev/loop0p2
> brw-rw---- 1 root disk 7,   5 2008-03-05 14:57 /dev/loop0p5
> brw-rw---- 1 root disk 7,  64 2008-03-05 14:55 /dev/loop1
> brw-rw---- 1 root disk 7, 128 2008-03-05 14:55 /dev/loop2
> brw-rw---- 1 root disk 7, 192 2008-03-05 14:55 /dev/loop3
> brw-rw---- 1 root disk 7, 256 2008-03-05 14:55 /dev/loop4
> brw-rw---- 1 root disk 7, 320 2008-03-05 14:55 /dev/loop5
> brw-rw---- 1 root disk 7, 384 2008-03-05 14:55 /dev/loop6
> brw-rw---- 1 root disk 7, 448 2008-03-05 14:55 /dev/loop7
> # mount /dev/loop0p1 /mnt
> # ls /mnt
> bench  cdrom  home        lib         mnt   root     srv  usr
> bin    dev    initrd      lost+found  opt   sbin     sys  var
> boot   etc    initrd.img  media       proc  selinux  tmp  vmlinuz
> # umount /mnt
> # losetup -d /dev/loop0
>
> Of course, the same behavior can be done using kpartx on a loop device,
> but modifying loop avoids to stack several layers of block device (loop +
> device mapper), this is a very light modification (40% of modifications
> are to manage the new parameter). Moreover all partition tables known
> by the kernel are managed (kpartx implements only a subset).

Do you think we do something similar for drivers/block/brd.c too? I'd
like to try to maintain parity between them where possible...

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