vm_contig_pg_clean color-queue validation is provably wrong: reclaims only color-0 pages, skipping 1023/1024 page queues — contigmalloc reclaim flush is ~99.9% ineffective

Summary

After busying a page, vm_contig_pg_clean re-validates with if (m->queue - m->pc != queue). Pages are always enqueued on PQ_base + m->pc with m->pc &= PQ_L2_MASK, so for any page on queue PQ_INACTIVE+q, m->queue - m->pc == PQ_INACTIVE — which equals queue (PQ_INACTIVE + q) only when q==0. The intended check was m->queue != queue (correctly asserted at :176). Net effect: both callers loop q=0..1023 but every queue with q>0 is walked page-by-page (spinlocks + busy_try + wakeup per page) with ZERO pages reclaimed — only color-0 pages are ever cleaned. Large contigmalloc allocations (DRM/agp/netmap/nvmm) fail far more often than designed; each failed pass still performs the full slow walk of every page in every queue ("very, very slow" per the code's own comment), stalling all contigmalloc callers three times including M_NOWAIT ones — a local availability amplification for any user-reachable contigmalloc path (netmap/GPU ioctls). Fix: m->queue != queue (one-line, row diff).

Timeline

• 2026-09-02 Discovered during pass-2 audit of vm_contig.c (GLM 5.3).