- stack::retain_range: pure checked span fn (retain page-up = zap less;
None when retain covers the reserve, so the 64 KiB default config never
pays a syscall) + 6 adversarial units mirroring shrink_range's.
- Stack::recycle_zap: advisory MADV_DONTNEED of [usable_base, top-RETAIN);
stack is unowned at the call site, synchronous eager zap races nothing.
- recycle_stack: zap OFF-LOCK before pool admission (acquire_stack's
no-syscall-under-the-pool-lock invariant); rare cap-overflow pays a
wasted zap ahead of munmap, accepted over a second lock round-trip.
- pub const RECYCLE_RETAIN = 64 KiB beside the shrink knobs, ratified-as-
constant rationale in doc.
- tests/stack_recycle.rs: mincore-based exact-zero-resident assert over
the zap span. smaps was tried first and over-counts: a neighboring rw
anon VMA can merge flush against the stack top (observed once under the
full-suite run); the PROT_NONE guard pins the usable base exactly.
FLAGGED (Claude-solo calls):
- RFC §6 'above the bottom RETAIN' is direction-ambiguous in address
terms; implemented as retain the ENTRY end (highest addresses, the
pages the next actor faults first), zap the cold deep span below.
- Const named RECYCLE_RETAIN (RFC says RETAIN) to sit beside SHRINK_*.
smarm-agent