ZERO PAGE again v2

Hi, this is ZERO_PAGE mapping revival patch v2.

ZERO PAGE was removed in 2.6.24 (=> http://lkml.org/lkml/2007/10/9/112)
and I had no objections.

In these days, at user support jobs, I noticed a few of customers
are making use of ZERO_PAGE intentionally...brutal mmap and scan, etc. 
(For example, scanning big sparse table and save the contents.)

They are using RHEL4-5(before 2.6.18) then they don't notice that ZERO_PAGE
is gone, yet.
yes, I can say  "ZERO PAGE is gone" to them in next generation distro.

Recently, a question comes to lkml (http://lkml.org/lkml/2009/6/4/383

Maybe there are some users of ZERO_PAGE other than my customers.
So, can't we use ZERO_PAGE again ?

IIUC, the problem of ZERO_PAGE was
  - reference count cache ping-pong
  - complicated handling.
  - the behavior page-fault-twice can make applications slow.

This patch is a trial to de-refcounted ZERO_PAGE.

This includes 4 patches.
[1/4] introduce pte_zero() at el.
[2/4] use ZERO_PAGE for READ fault in anonymous mapping.
[3/4] corner cases, get_user_pages()
[4/4] introduce get_user_pages_nozero().

I feel these patches needs to be clearer but includes almost all
messes we have to handle at using ZERO_PAGE again.

What I feel now is
 a. technically, we can do because we did.
 b. Considering maintenance, code's beauty etc.. ZERO_PAGE adds messes.
 c. Very big benefits for some (a few?) users but no benefits to usual programs.
 
 There are trade-off between b. and c.
 
Any comments are welcome.
-Kame







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