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

[Martin K. Petersen]

>>>>> "Mike" == Mike Snitzer <snitzer at 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.

-- 
Martin K. Petersen	Oracle Linux Engineering