linux-kernel - Re: LVM snapshot broke between 4.14 and 4.16

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Date:   Thu, 2 Aug 2018 09:41:34 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     wgh@...lan.ru
Cc:     Ilya Dryomov <idryomov@...il.com>, Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sagi Grimberg <sagi@...mberg.me>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com
Subject: Re: LVM snapshot broke between 4.14 and 4.16

On Thu, Aug 2, 2018 at 8:16 AM WGH <wgh@...lan.ru> 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
> 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.

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?

                 Linus