On 08/02/2018 04:31 PM, Ilya Dryomov wrote: > 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 After further investigation, this was fixed on lvm2 side (userspace) in https://sourceware.org/git/?p=lvm2.git;a=commit;h=a6fdb9d9d70f51c49ad11a87ab4243344e6701a3 (snapshot: keep COW writable for read-only volumes). So I guess that's it. Time to poke my distribution package maintainers to bump the package version.
