mm/demotion: demote pages according to allocation fallback order

Currently, a higher tier node can only be demoted to selected nodes on the
next lower tier as defined by the demotion path.  This strict demotion
order does not work in all use cases (e.g.  some use cases may want to
allow cross-socket demotion to another node in the same demotion tier as a
fallback when the preferred demotion node is out of space).  This demotion
order is also inconsistent with the page allocation fallback order when
all the nodes in a higher tier are out of space: The page allocation can
fall back to any node from any lower tier, whereas the demotion order
doesn't allow that currently.

This patch adds support to get all the allowed demotion targets for a
memory tier.  demote_page_list() function is now modified to utilize this
allowed node mask as the fallback allocation mask.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jagdish Gediya <[email protected]>
Signed-off-by: Aneesh Kumar K.V <[email protected]>
Reviewed-by: "Huang, Ying" <[email protected]>
Acked-by: Wei Xu <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Bharata B Rao <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Hesham Almatary <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: SeongJae Park <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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