mm: disable top-tier fallback to reclaim on proactive reclaim

Reclaiming directly from top tier nodes breaks the aging pipeline of
memory tiers.  If we have a RAM -> CXL -> storage hierarchy, we should
demote from RAM to CXL and from CXL to storage.  If we reclaim a page from
RAM, it means we 'demote' it directly from RAM to storage, bypassing
potentially a huge amount of pages colder than it in CXL.

However disabling reclaim from top tier nodes entirely would cause ooms in
edge scenarios where lower tier memory is unreclaimable for whatever
reason, e.g.  memory being mlocked() or too hot to reclaim.  In these
cases we would rather the job run with a performance regression rather
than it oom altogether.

However, we can disable reclaim from top tier nodes for proactive reclaim.
That reclaim is not real memory pressure, and we don't have any cause to
be breaking the aging pipeline.

[[email protected]: restore comment layout, per Ying Huang]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mina Almasry <[email protected]>
Reviewed-by: "Huang, Ying" <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Wei Xu <[email protected]>
Cc: Yosry Ahmed <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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