Hi,
> > Before trying the sparsemem approach, I tried a radix tree per node,

quoted text
> > per zone and I seemed to actually get some performance

> > improvement.(1.5% (noise maybe))

> >

> > But please do see and review (tested on my x86_64 box with unixbench

> > and some other simple tests)

> >

> > v4..v3

> > 1. Use a radix tree per node, per zone

> >

> > v3...v2

> > 1. Convert flags to unsigned long

> > 2. Move page_cgroup->lock to a bit spin lock in flags

> >

> > v2...v1

> >

> > 1. Fix a small bug, don't call radix_tree_preload_end(), if preload fails

> >

> > This is a rewrite of a patch I had written long back to remove struct page

> > (I shared the patches with Kamezawa, but never posted them anywhere else).

> > I spent the weekend, cleaning them up for 2.6.27-rc5-mmotm (29 Aug 2008).

> >

> > I've tested the patches on an x86_64 box, I've run a simple test running

> > under the memory control group and the same test running concurrently under

> > two different groups (and creating pressure within their groups).

> >

> > Advantages of the patch

> >

> > 1. It removes the extra pointer in struct page

> >

> > Disadvantages

> >

> > 1. Radix tree lookup is not an O(1) operation, once the page is known

> >    getting to the page_cgroup (pc) is a little more expensive now.

>

> Why are we doing this?  I can guess, but I'd rather not have to.

I think this design is just temporary and the goal is to pre-allocate

all page_cgroups at boot time if it isn't disabled.
But I think each memory model type should have its own way of managing

its page_cgroup arrays as doing for its struct page arrays.

It would be better rather than the sparsemem approach he said.
> a) It's slower.

quoted text
>

> b) It uses even more memory worst-case.

>

> c) It uses less memory best-case.

>

> someone somewhere decided that (Aa + Bb) / Cc < 1.0.  What are the values

> of A, B and C and where did they come from? ;)

>

> (IOW, your changelog is in the category "sucky", along with 90% of the others)

>

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