[copybara-service]

Re-observe stale hugepage-backed trackers during the background treatment pass.

Previously, HugePageUnbackedTrackerTreatment skipped every tracker whose cached residency state reported that it was hugepage-backed, so a first observation or a successful collapse stood until a page release cleared the bit. A hugepage that the kernel later split through compaction or swap was neither recorded as split nor reconsidered for collapse.

Each pass now also selects up to 512 trackers whose hugepage-backed records are older than the five-minute record interval, ordered oldest record first so passes round-robin across large heaps instead of starving the tail. Candidate selection buffers up to twice that limit and compacts the buffer with std::nth_element when full, raising a timestamp cutoff so selection remains amortized constant time per tracker while holding the pageheap lock.

After dropping the lock, the pass queries pageflags for the selected trackers. Trackers that remain hugepage-backed get a fresh record timestamp, while up to 128 trackers found split per pass have their full residency bitmaps recorded and are treated for collapse and subrelease alongside regular candidates.

Re-observation is gated by Parameters::reobserve_hugepage_backed() and is disabled by default.

Co-authored-by: Chris Kennelly ckennelly@google.com

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