[PATCH 0/2] ZERO PAGE again v3.

[KAMEZAWA Hiroyuki]

After v2 discussion, I felt that "Go" sign can be given if implemetaion is neat
and tiny and overhead seems very small. Here is v3.

In this version,

 - use pte_special() in vm_normal_page()
   All ZERO_PAGE check will go down to vm_normal_page() and check is done here.
   Some new flags in follow_page() and get_user_pages().

 - per arch use-zero-page config is added.
   IIUC, archs which have _PAGE_SPECIAL is only x86, powerpc, s390.
   Because this patch make use of pte_special() check, config to use zero page
   is added and you can turn it off if necessary.
   I this patch, only x86 is turned on which I can test.

Any comments are welcome. 

Thanks,
-Kame

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