Re: LVM snapshot broke between 4.14 and 4.16

[Ilya Dryomov]

On Thu, Aug 2, 2018 at 2:26 PM WGH <wgh@xxxxxxxxx> wrote:
>
> (I originally reported this problem here:
> https://bugzilla.kernel.org/show_bug.cgi?id=200439)
>
> When I updated from 4.14 to 4.16, my LVM snapshotting script broke for
> no apparent reason.
>
> My script has the following line, and it fails like this:
> + lvcreate --size 5G --snapshot --name snap0 --permission r
> /dev/mapper/vg0-lvol_rootfs
>   device-mapper: create ioctl on
> vg0-snap0-cowLVM-sDdIeh9cecWdaNyRfZC31mxgfwTa4sOeHMJXVOykGVRtfP6Aii7IHvwS066AOLOM-cow
> failed: Device or resource busy
>   Failed to lock logical volume vg0/lvol_rootfs.
>   Aborting. Manual intervention required.
>
> At the same time, some errors appear in dmesg as well:
> [   26.145279] generic_make_request: Trying to write to read-only
> block-device dm-3 (partno 0)
> [   26.145288] device-mapper: persistent snapshot: write_header failed
> [   26.145847] device-mapper: table: 253:4: snapshot: Failed to read
> snapshot metadata
> [   26.145851] device-mapper: ioctl: error adding target to table
>
> I bisected the vanilla kernel, and the first bad commit is
> [721c7fc701c71f693307d274d2b346a1ecd4a534] block: fail op_is_write()
> requests to read-only partitions

Adding Mike and dm-devel.

>From a quick look, --permission r sets DM_READONLY_FLAG, which makes dm
mark the disk read-only with set_disk_ro(dm_disk(md), 1) in do_resume().
A bit later it tries to write to the disk from write_header():

  return chunk_io(ps, ps->header_area, 0, REQ_OP_WRITE, 0, 1);

Thanks,

                Ilya