1. DEFENSIVE (backend_metal.mm, coli_metal_register): re-registering a live
base overwrote s.buf and resset_add'ed the new wrapper without removing
the OLD one from the residency set -- ARC drops our reference but the set
retains the object and keeps its pages resident: a leak and a
set/g_slabs divergence. The found branch now stashes the replaced
wrapper under g_slab_mtx and, after dropping the lock (round-1 invariant:
no Metal call under the slab lock), resset_remove(old)s it before
resset_add(b); identical old==b early-outs both set operations.
Invariant defended, stated in the comment: residency-set membership
mirrors g_slabs exactly. No in-tree caller re-registers a live base
today (audited) -- closed defensively.
2. DOC (SUMMARY.md): the moe_submit lifecycle bullet still said
resset_flush commits "under g_slab_mtx" -- stale text from before the
round-1 mutex split; the code takes g_resset_mtx and never touches the
slab lock there. Parenthetical corrected; register bullet updated to
describe the re-register hygiene from item 1.
expert_load's fslab-OOM unwind freed s->slab via compat_aligned_free without
coli_metal_unregister -- a pre-existing gap on main/dev (validator-confirmed
during E4) that leaves a stale g_slabs entry over freed memory, letting
resolve() hand the GPU a dangling pointer. Under COLI_METAL_RESSET=1 the
exposure is strictly longer-lived: the wrapped buffer would stay a permanent
residency-set member over the freed pages instead of stock's transient
last-command-buffer window. Ported from e4/metal-heap validator fix 6753225,
adapted to dev's non-heap code shape (no coli_metal_heap_free wrapper --
plain unregister-before-free).
The uring_load_add analog (E4 audit round-2 insurance) is deliberately not
carried: that arm is #ifdef __linux__-gated while COLI_METAL is macOS-only,
so it is dead code on every real build target, and unlike E4 this branch has
no allocation-path reason to touch the function.
Reachable only through allocation failure mid-load; verified by inspection
and clean builds (no OOM-injection harness in tree). SUMMARY.md item 10
updated to "carried on this branch".
1. REQUIRED (backend_metal.mm): no Metal call runs under g_slab_mtx anymore.
The set mutations (addAllocation/removeAllocation/commit) and the dirty
flag moved to a dedicated g_resset_mtx; coli_metal_register/unregister do
their g_slabs bookkeeping under g_slab_mtx exactly as stock, then call
resset_add/resset_remove after dropping it (still before returning, so
unregister's remove+commit still lands before the caller frees the host
memory). The original shape -- commit under the slab lock the parallel OMP
loader threads contend on -- was structurally identical to the mutex-over-
live-Metal-call bug E4's audit round 2 fixed, the leading suspect for its
replicated +12s expert-disk regression. The register->flush->resolve
happens-before argument survives the split: resset_add completes inside
coli_metal_register before it returns, the engine cannot dispatch an
expert before its load returns, and add/flush are serialized by
g_resset_mtx (comment at resset_add). The two mutexes are never held
together, so no lock-order hazard exists.
2. REQUIRED (backend_metal.mm, backend_metal.h, glm.c): the resset_flush
cost in moe_submit -- deliberately outside the g_t_setup window so the
A/B harness counters keep their meaning -- was invisible. New
g_t_resset_flush accumulator timed around the flush, exported via
coli_metal_resset_stats(), printed by profile_print as a separate
gate-on-only "METAL-RESSET: flush" line (mirrors E4's METAL-HEAP line;
the METAL: line the harness parses keeps its exact format; stock output
byte-identical -- the stats call returns 0 with the gate off and the
whole block sits in the pre-existing #ifdef COLI_METAL arm). Register-
side add/remove costs land in the existing t_ewait window; comment at
resset_add says so instead of a second counter.
3. REQUIRED (SUMMARY.md; the moe_submit commit message was already reworded
in place, pre-push): corrected two false attributions -- the hazard-
tracking and thread-safety statements were credited to the SDK header,
which is silent on both; the actual source is Apple's online
MTLResidencySet class reference and residency-set adoption guide
(fetched 2026-07-18). Note: the scaffolding commit's message was checked
and contains no such claim, so it was left untouched.
4. DOC (SUMMARY.md): the pre-existing fslab OOM-unwind bug (glm.c ~1868-73,
frees s->slab without unregister) has a strictly longer-lived exposure
under E5 (permanent set member vs stock's transient per-CB declaration).
Out of scope here; the upstream PR built from E5 must carry the one-line
fix (reference: E4 branch commit 6753225).
The one seam the mechanism history actually implicates: moe_submit's `use`
vector (resolved expert weight/scale slabs) is the only useResource: loop
whose length scales with LRU cache size. With COLI_METAL_RESSET=1, skip that
loop entirely -- the queue-attached MTLResidencySet already guarantees those
buffers are resident -- after resset_flush() commits any pending adds from a
loader burst. Every other useResource: call site (bind_gemv's weight/scale
buffers, attn_decode/layer_decode's Lb/Rb/kvbW/kvbS/inB/pnB/rwB/rbB,
coli_metal_gemm's wb/sb) is left unconditionally unchanged: none of them
scale with cache size, and Lb/Rb carry real GPU-side write traffic ordered by
existing explicit memoryBarrierWithScope: calls, not by useResource:'s hazard
tracking -- narrowing the blast radius rather than removing useResource:
uniformly. Full reasoning, the hazard-tracking tradeoff (residency sets don't
support hazard tracking, per Apple's MTLResidencySet developer documentation
and residency-set adoption guide; the SDK header itself is silent on the
topic), and every judgment call in UNCERTAINTIES: see SUMMARY.md.
Gate off is unaffected: g_resset_enabled is false, so the useResource: loop
runs exactly as before.
monotophic