[RFC] mm: add support for zsmalloc and zcache

[Dan Magenheimer]

> From: James Bottomley [mailto:James.Bottomley at HansenPartnership.com]
> Subject: Re: [RFC] mm: add support for zsmalloc and zcache

> On Sat, 2012-09-22 at 02:07 +0100, Mel Gorman wrote:
> > > The two proposals:
> > > A) Recreate all the work done for zcache2 as a proper sequence of
> > >    independent patches and apply them to zcache1. (Seth/Konrad)
> > > B) Add zsmalloc back in to zcache2 as an alternative allocator
> > >    for frontswap pages. (Dan)
> >
> > Throwing it out there but ....
> >
> > C) Merge both, but freeze zcache1 except for critical fixes. Only
> > allow
> >    future work on zcache2. Document limitations of zcache1 and
> >    workarounds until zcache2 is fully production ready.
> >
> Actually, there is a fourth option, which is the one we'd have usually
> used when staging wasn't around:  Throw the old code out as a successful
> prototype which showed the author how to do it better (i.e. flush it
> from staging) and start again from the new code which has all the
> benefits learned from the old code.
> 
> Staging isn't supposed to be some magical set of history that we have to
> adhere to no matter what (unlike the rest of the tree). It's supposed to
> be an accelerator to get stuff into the kernel and not become a
> hindrance to it.
> 
> There also seem to be a couple of process issues here that could do with
> sorting:  Firstly that rewrites on better reflection, while not common,
> are also not unusual so we need a mechanism for coping with them.  This
> is actually a serious process problem: everyone becomes so attached to
> the code they helped clean up that they're hugely unwilling to
> countenance a rewrite which would in their (probably correct) opinion
> have the cleanups start from ground zero again. Secondly, we've got a
> set of use cases and add ons which grew up around code in staging that
> act as a bit of a barrier to ABI/API evolution, even as they help to
> demonstrate the problems.
> 
> I think the first process issue really crystallises the problem we're
> having in staging:  we need to get the design approximately right before
> we start on the code cleanups.  What I think this means is that we start
> on the list where the people who understand the design issues reside
> then, when they're happy with the design, we can begin cleaning it up
> afterwards if necessary.  I don't think this is hard and fast: there is,
> of course, code so bad that even the experts can't penetrate it to see
> the design without having their eyes bleed but we should at least always
> try to begin with design.


Hi James --

I think you've hit the nail on the head, generalizing this interminable
debate into a process problem that needs to be solved more generally.
Thanks for your insight!

Dan