Re: [PATCH v2] make dm and dm-crypt forward cgroup context (was: dm-crypt parallelization patches)

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Apr 16 2013 at  1:24pm -0400,
Tejun Heo <tj@kernel.org> wrote:

> Hey,
> 
> On Mon, Apr 15, 2013 at 09:02:06AM -0400, Mikulas Patocka wrote:
> > The patch is not bug-prone, because we already must make sure that the 
> > cloned bio has shorter lifetime than the master bio - so the patch doesn't 
> > introduce any new possibilities to make bugs.
> 
> The whole world isn't composed of only your code.  As I said
> repeatedly, you're introducing an API which is misleading and can
> easily cause subtle bugs which are very difficult to reproduce.
> 
> Imagine it being used to tag a metatdata or checksum update bio being
> sent down while processing another bio and used to "clone" the context
> of the original bio.  It'll work most of the time even if the original
> bio gets completed first but it'll break when it gets really unlucky -
> e.g. racing with other operations which can put the base css ref, and
> it'll be hellish to reproduce and everyone would have to pay for your
> silly hack.
> 
> > > Do the two really look the same to you?  The page refs are much more
> > > expensive, mostly contained in and the main focus of dm.  ioc/css refs
> > > aren't that expensive to begin with, css refcnting is widely scattered
> > 
> > ioc is per-task, so it is likely to be cached (but there are processors 
> > that have slow atomic operations even on cached data - on Pentium 4 it 
> > takes about 100 cycles). But css is shared between tasks and produces the 
> > cache ping-pong effect.
> 
> For $DIETY's sake, how many times do I have to tell you to use per-cpu
> reference count?  Why do I have to repeat the same story over and over
> again?  What part of "make the reference count per-cpu" don't you get?
> It's not a complicated message.
> 
> At this point, I can't even understand why or what the hell you're
> arguing.  There's a clearly better way to do it and you're just
> repeating yourself like a broken record that your hack in itself isn't
> broken.
> 
> So, if you wanna continue that way for whatever reason, you have my
> firm nack and I'm outta this thread.
> 
> Bye bye.

Hey Tejun,

I see you nack and raise you with: please reconsider in the near term.

Your point about not wanting to introduce a generic block interface that
isn't "safe" for all users noted.  But as Mikulas has repeatedly said DM
does _not_ ever need to do the refcounting.  So it seems a bit absurd to
introduce the requirement that DM should stand up an interface that uses
percpu.  That is a fair amount of churn that DM will never have a need
to take advantage of.

So why not introduce __bio_copy_association(bio1, bio2) and add a BUG_ON
in it if bio2 isn't a clone of bio1?

When there is a need for async IO to have more scalable refcounting that
would be the time to introduce bio_copy_association that uses per-cpu
refcounting (and yes we could then even nuke __bio_copy_association).

It just seems to me a bit burdensome to ask Mikulas to add this
infrastructure when DM really doesn't need it at all.  But again I do
understand your desire for others to be stearing the kernel where it
needs to be to benefit future use-cases.  But I think in general it best
to introduce complexity when there is an actual need.

Your insights are amazingly helpful and I think it is unfortunate that
this refcounting issue overshadowed the positive advancements of
dm-crypt scaling.  I'm just looking to see if we can carry on with a
temporary intermediate step with __bio_copy_association.

Thanks,
Mike