Message-ID: <20120729015428.GA16643@bbox>
Date:	Sun, 29 Jul 2012 10:54:28 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCH 0/4] promote zcache from staging

On Fri, Jul 27, 2012 at 02:42:14PM -0700, Dan Magenheimer wrote:
> > From: Konrad Rzeszutek Wilk [mailto:konrad@...nok.org]
> > Sent: Friday, July 27, 2012 3:00 PM
> > Subject: Re: [PATCH 0/4] promote zcache from staging
> > 
> > On Fri, Jul 27, 2012 at 12:21:50PM -0700, Dan Magenheimer wrote:
> > > > From: Seth Jennings [mailto:sjenning@...ux.vnet.ibm.com]
> > > > Subject: [PATCH 0/4] promote zcache from staging
> > > >
> > > > zcache is the remaining piece of code required to support in-kernel
> > > > memory compression.  The other two features, cleancache and frontswap,
> > > > have been promoted to mainline in 3.0 and 3.5.  This patchset
> > > > promotes zcache from the staging tree to mainline.
> > > >
> > > > Based on the level of activity and contributions we're seeing from a
> > > > diverse set of people and interests, I think zcache has matured to the
> > > > point where it makes sense to promote this out of staging.
> > >
> > > Hi Seth --
> > >
> > > Per offline communication, I'd like to see this delayed for three
> > > reasons:
> > >
> > > 1) I've completely rewritten zcache and will post the rewrite soon.
> > >    The redesigned code fixes many of the weaknesses in zcache that
> > >    makes it (IMHO) unsuitable for an enterprise distro.  (Some of
> > >    these previously discussed in linux-mm [1].)
> > > 2) zcache is truly mm (memory management) code and the fact that
> > >    it is in drivers at all was purely for logistical reasons
> > >    (e.g. the only in-tree "staging" is in the drivers directory).
> > >    My rewrite promotes it to (a subdirectory of) mm where IMHO it
> > >    belongs.
> > > 3) Ramster heavily duplicates code from zcache.  My rewrite resolves
> > >    this.  My soon-to-be-post also places the re-factored ramster
> > >    in mm, though with some minor work zcache could go in mm and
> > >    ramster could stay in staging.
> > >
> > > Let's have this discussion, but unless the community decides
> > > otherwise, please consider this a NACK.
> 
> Hi Konrad --
>  
> > Hold on, that is rather unfair. The zcache has been in staging
> > for quite some time - your code has not been posted. Part of
> > "unstaging" a driver is for folks to review the code - and you
> > just said "No, mine is better" without showing your goods.
> 
> Sorry, I'm not trying to be unfair.  However, I don't see the point
> of promoting zcache out of staging unless it is intended to be used
> by real users in a real distro.  There's been a lot of discussion,
> onlist and offlist, about what needs to be fixed in zcache and not
> much visible progress on fixing it.  But fixing it is where I've spent
> most of my time over the last couple of months.
> 
> If IBM or some other company or distro is eager to ship and support
> zcache in its current form, I agree that "promote now, improve later"
> is a fine approach.  But promoting zcache out of staging simply because
> there is urgency to promote zsmalloc+zram out of staging doesn't
> seem wise.  At a minimum, it distracts reviewers/effort from what IMHO
> is required to turn zcache into an enterprise-ready kernel feature.
> 
> I can post my "goods" anytime.  In its current form it is better
> than the zcache in staging (and, please remember, I wrote both so
> I think I am in a good position to compare the two).
> I have been waiting until I think the new zcache is feature complete
> before asking for review, especially since the newest features
> should demonstrate clearly why the rewrite is necessary and
> beneficial.  But I can post* my current bits if people don't
> believe they exist and/or don't mind reviewing non-final code.
> (* Or I can put them in a publicly available git tree.)
> 
> > There is a third option - which is to continue the promotion
> > of zcache from staging, get reviews, work on them ,etc, and
> > alongside of that you can work on fixing up (or ripping out)
> > zcache1 with zcache2 components as they make sense. Or even
> > having two of them - an enterprise and an embedded version
> > that will eventually get merged together. There is nothing
> > wrong with modifying a driver once it has left staging.
> 
> Minchan and Seth can correct me if I am wrong, but I believe
> zram+zsmalloc, not zcache, is the target solution for embedded.

NOT ture. Some embedded devices use zcache but it's not original
zcache but modificated one.
Anyway, although embedded people use modified zcache, I am biased to Dan.
I admit I don't spend lots of time to look zcache but as looking the
code, it wasn't good shape and even had a bug found during code review
and I felt strongly we should clean up it for promoting it to mm/.
So I would like to wait Dan's posting if you guys are not urgent.
(And I am not sure akpm allow it with current shape of zcache code.)
But the concern is about adding new feature. I guess there might be some
debate for long time and it can prevent promoting again.
I think It's not what Seth want.
I hope Dan doesn't mix clean up series and new feature series and
post clean up series as soon as possible so let's clean up first and
try to promote it and later, adding new feature or changing algorithm
is desirable.


> The limitations of zsmalloc aren't an issue for zram but they are
> for zcache, and this deficiency was one of the catalysts for the
> rewrite.  The issues are explained in more detail in [1],
> but if any point isn't clear, I'd be happy to explain further.
> 
> However, I have limited time for this right now and I'd prefer
> to spend it finishing the code. :-}
> 
> So, as I said, I am still a NACK, but if there are good reasons
> to duplicate effort and pursue the "third option", let's discuss
> them.
> 
> Thanks,
> Dan
> 
> [1] http://marc.info/?t=133886706700002&r=1&w=2
> 
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-- 
Kind regards,
Minchan Kim
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