memory-hotplug: common APIs to support page tables hot-remove

When memory is removed, the corresponding pagetables should alse be
removed.  This patch introduces some common APIs to support vmemmap
pagetable and x86_64 architecture direct mapping pagetable removing.

All pages of virtual mapping in removed memory cannot be freed if some
pages used as PGD/PUD include not only removed memory but also other
memory.  So this patch uses the following way to check whether a page
can be freed or not.

1) When removing memory, the page structs of the removed memory are
   filled with 0FD.

2) All page structs are filled with 0xFD on PT/PMD, PT/PMD can be
   cleared.  In this case, the page used as PT/PMD can be freed.

For direct mapping pages, update direct_pages_count[level] when we freed
their pagetables.  And do not free the pages again because they were
freed when offlining.

For vmemmap pages, free the pages and their pagetables.

For larger pages, do not split them into smaller ones because there is
no way to know if the larger page has been split.  As a result, there is
no way to decide when to split.  We deal the larger pages in the
following way:

1) For direct mapped pages, all the pages were freed when they were
   offlined.  And since menmory offline is done section by section, all
   the memory ranges being removed are aligned to PAGE_SIZE.  So only need
   to deal with unaligned pages when freeing vmemmap pages.

2) For vmemmap pages being used to store page_struct, if part of the
   larger page is still in use, just fill the unused part with 0xFD.  And
   when the whole page is fulfilled with 0xFD, then free the larger page.

[akpm@linux-foundation.org: fix typo in comment]
[tangchen@cn.fujitsu.com: do not calculate direct mapping pages when freeing vmemmap pagetables]
[tangchen@cn.fujitsu.com: do not free direct mapping pages twice]
[tangchen@cn.fujitsu.com: do not free page split from hugepage one by one]
[tangchen@cn.fujitsu.com: do not split pages when freeing pagetable pages]
[akpm@linux-foundation.org: use pmd_page_vaddr()]
[akpm@linux-foundation.org: fix used-uninitialised bug]
Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Signed-off-by: Jianguo Wu <wujianguo@huawei.com>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Jiang Liu <jiang.liu@huawei.com>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Wu Jianguo <wujianguo@huawei.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>