rmap: add exclusively owned pages to the newest anon_vma

The recent anon_vma fixes cause many anonymous pages to end up
in the parent process anon_vma, even when the page is exclusively
owned by the current process.

Adding exclusively owned anonymous pages to the top anon_vma
reduces rmap scanning overhead, especially in workloads with
forking servers.

This patch adds a parameter to __page_set_anon_rmap that can
be used to indicate whether or not the added page is exclusively
owned by the current process.

Pages added through page_add_new_anon_rmap are exclusively
owned by the current process, and can be added to the top
anon_vma.

Pages added through page_add_anon_rmap can be either shared
or exclusively owned, so we do the conservative thing and
add it to the oldest anon_vma.

A next step would be to add the exclusive parameter to
page_add_anon_rmap, to be used from functions where we do
know for sure whether a page is exclusively owned.

Signed-off-by: Rik van Riel <[email protected]>
Reviewed-by: Johannes Weiner <[email protected]>
Lightly-tested-by: Borislav Petkov <[email protected]>
Reviewed-by: Minchan Kim <[email protected]>
[ Edited to look nicer  - Linus ]
Signed-off-by: Linus Torvalds <[email protected]>

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