Re: LVM snapshot broke between 4.14 and 4.16
From: Linus Torvalds
Date: Thu Aug 02 2018 - 12:41:47 EST
On Thu, Aug 2, 2018 at 8:16 AM WGH <wgh@xxxxxxxxx> wrote:
> On 08/02/2018 04:31 PM, Ilya Dryomov wrote:
> > 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
> (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.
That is *not* how kernel development is supposed to work. If your
script used to work, it should continue to work.
Why did it use to work despite that read-only flag? And what was it
that actually broke this?
We remain bug-for-bug compatible with older kernel versions when
people depend on the bugs. Unless the old bugs are security issues,
and even then we try to make it _look_ like we work the same way.
Or was it a user-space lvm tool that broke in the first place, and the
kernel release update was a red herring?