hwm: AtomicUsize lands beside sp on the slot: the single context-save
site min-updates it (one branch + at most one Relaxed store into the
line the sp store just dirtied), install resets it to the fresh top.
Advisory by construction — correctness never depends on it. The mod-doc
ordering chain gains a line: hwm piggybacks the existing
Relaxed-store-before-Release pattern and adds no edges.
Shrink hook in the YieldIntent::Park arm only, before the park_return
Release transition — the owned window (obligation 1's assert-comment at
the site): after the sp store, before Parked is published, scheduler on
its own stack, actor saved and unstealable. It runs on both arms of the
park_return race (a consumed unpark flag means one wasted-but-harmless
madvise). The preempt/yield path deliberately never checks: §4's
bounded, self-healing leak under saturation, when syscalls are least
affordable.
SHRINK_THRESHOLD = 256 KiB and SHRINK_COOLDOWN = 64 parks are pub
constants with the ratified doc rationale, not Config fields. The freed
span is shrink_range(hwm, sp, page): whole pages of [hwm, sp − 1-page
redzone), rounded inward, checked arithmetic — adversarial inputs
collapse to None (obligation 2). MADV_FREE marks lazily; the kernel's
reclaim-under-pressure IS the hysteresis, cancel-on-write is the safety
net. parks_since_shrink + shrink_count ride the slot for the cooldown
and the future introspect surface.
Tests: 7 adversarial shrink_range units (inverted/empty spans, redzone
underflow, unaligned ends, sp-crossing sweep); integration — 8 MiB
reserve, ~3 MiB spike sampled via yield-at-depth, parks gated on
introspected Parked state past the cooldown, then ≥ 2 MiB LazyFree
asserted inside the stack's smaps range with live data intact; and the
inverse guard — a shallow never-spiking actor ends at exactly 0
LazyFree (also proves the parser isn't vacuously zero via the first
test).
smarm-agent