Re: [PATCH] loopdev: sync capacity after setting it

Karel Zak
2013-03-18T09:27:06Z

On Sun, Mar 17, 2013 at 10:18:28AM -0400, Jeff Mahoney wrote:
> On 03/15/2013 05:04 PM, Jeff Mahoney wrote:
> >I recently tried to mount an hfsplus file system from an image file with
> >a partition table by using the loop offset and sizelimit options to specify
> >the location of the file system.
> >
> >hfsplus stores some metadata at a set offset from the end of the partition,
> >so it's sensitive to the device size reported by the kernel.
> >
> >It worked with this:
> ># losetup -r -o 32k --sizelimit=102400000 <loopdev> <imagefile>
> ># mount -r -t hfsplus <loopdev> <mountpoint>
> >
> >But failed with this:
> ># mount -r -oloop,offset=32k,sizelimit=102400000 <imagefile> <mountpoint>
> >
> ># losetup -a
> >/dev/loop0: [0089]:2 (<imagefile>), offset 32768, sizelimit 102400000
> >/dev/loop1: [0089]:2 (<imagefile>), offset 32768, sizelimit 102400000
> >
> >/proc/partitions shows the correct number of blocks to match the sizelimit.
> >
> >But if I set a breakpoint in mount before the mount syscall, I could see:
> ># blockdev --getsize64 /dev/loop[01]
> >102400000
> >102432768
> >
> >The kernel loop driver will set the gendisk capacity of the device at
> >LOOP_SET_STATUS64 but won't sync it to the block device until one of two
> >conditions are met: All open file descriptors referring to the device are
> >closed (and it will sync when re-opened) or if the LOOP_SET_CAPACITY ioctl
> >is called to sync it. Since mount opens the device and passes it directly
> >to the mount syscall after LOOP_SET_STATUS64 without closing and reopening
> >it, the sizelimit argument is effectively ignroed. The capacity needs to
> >be synced immediately for it to work as expected.
> >
> >This patch adds the LOOP_SET_CAPACITY call to loopctx_setup_device since
> >the device isn't yet released to the user, so it's safe to sync the capacity
> >immediately.
>
> It turns out this ioctl wasn't introduced until 2.6.30. I'll fix that up and
> resend tomorrow.

Do you mean #ifdef LOOP_SET_CAPACITY? I can fix the patch manually.

(It seem that we already use this ioctl in code without any extra
care about old kernel and nobody complains :-)

Karel

--
Karel Zak <kzak@redhat.com>