find_next_best_node() picks the next-closest node when building a
fallback list, and hardcodes N_MEMORY as the set it picks from.

Refactor it into find_next_best_node_in(), which takes the candidate
set explicitly.

This makes the existing behaviour explicit at both mm/memory-tiers.c
call sites - they select demotion targets in fallback order from
N_MEMORY - and lets callers narrow that set.

Then extract the per-node construction loop out of build_zonelists()
into build_node_zonelist(), parameterised on the candidate nodemask
and on the destination zonelist index.

Together these allow a zonelist to be built over a candidate set other
than N_MEMORY, into a zonelist other than FALLBACK, and iterated in
fallback order over a caller-defined subset.

These are prerequisites for generating a private node zonelist (nodes
unreachable by default), but are otherwise general improvements to
the existing interfaces so I'm proposing them separately.

No functional change intended - purely refactor commits.

Verified on x86_64:

  - find_next_best_node_in() compiles to the same 319 bytes as
    find_next_best_node().
  - Both mm/memory-tiers.c callers grow a single instruction - the
    added argument.
  - Boot-time fallback orders are identical on a 4-node guest with an
    asymmetric distance matrix.

Gregory Price (2):
  mm: refactor find_next_best_node to find_next_best_node_in
  mm/page_alloc: refactor build_node_zonelist() out of build_zonelists()

 mm/internal.h     |  6 ++++--
 mm/memory-tiers.c |  7 +++---
 mm/page_alloc.c   | 55 +++++++++++++++++++++++++++++++----------------
 3 files changed, 45 insertions(+), 23 deletions(-)

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