Re: [PATCH v2] dm: Fix alignment stacking on partitioned devices

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Jan 08 2010 at  1:41pm -0500,
Martin K. Petersen <martin.petersen@oracle.com> wrote:

> >>>>> "Mike" == Mike Snitzer <snitzer@redhat.com> writes:
> 
> Mike,
> 
> Mike> Seems I took the time to add a comment whose FIXME doesn't ring
> Mike> many bells now!  But ignoring that, the comment before the FIXME
> Mike> is making a veiled reference to userspace having consumed
> Mike> alignment_offset.
> 
> I read and understood the comment.  But even then explicitly zeroing out
> those two values didn't make sense (because if user space did in fact do
> the right thing they'd always be zero).
> 
> I know that the DM utilities take care of business.  And that DM devices
> are special because they are always set up by user space and not a
> kernel discovery process.
> 
> But since the code *is* in place to validate things I'm not so keen on
> you clearing fields that have been calculated and have a meaning.  For
> me it masked a case where the DM utilities did the wrong thing (because
> they were old).
> 
> With the Enterprise Linux hat on it is easy for us to specify that you
> must use this version of the kernel and the device mapper utilities.
> But reality is that lots of people are running upstream kernels on
> distributions with older userland.  And some distributions get things
> wrong, ship broken bits, etc.
> 
> It's great that new DM utils will transparently adjust the starting
> offset.  That's the way it's supposed to work.  No arguments there.
> 
> My main concern is making sure that we never get into a case where we
> run with misaligned components without indicating that there is a
> problem.  Ever!  Regardless of which DM utils might be in place.
> 
> We have the power to get that right.  All the pieces are in place.  I'd
> simply like us to stop making assumptions about user space always doing
> the right thing.

Yeap, I already agreed that those 2 lines should be removed (in the
p.s. of my previous mail).  They really aren't serving any purpose and
their justification was/is tenuous at best.

I'll get the fix, for 2.6.33, to Alasdair shortly.

Mike