[kvinwang]

Option	Effect	Opt-in
READ_ONLY_THP_FOR_FS=y	Read-only file mappings (program/library text) can be collapsed into huge pages. Without it, code runs from 4 KiB pages, and every iTLB miss pays the two-dimensional (guest + EPT) page walk	MADV_HUGEPAGE / MADV_COLLAPSE, or THP always
LRU_GEN=y	Multi-Gen LRU, built in but off (LRU_GEN_ENABLED unset)	echo y > /sys/kernel/mm/lru_gen/enabled
TCP_CONG_BBR=y	Built in, so a container can select it in its own netns without loading a module. Default stays cubic	net.ipv4.tcp_congestion_control=bbr
DEFERRED_STRUCT_PAGE_INIT=y	Struct pages are initialised by parallel kthreads after SMP bring-up	always; it only moves boot work

All four are added to both backends and asserted in parity.json.

Testing:

• make os-image passes, and the built .config has all four options.
• A/B in a QEMU/KVM guest (non-TD, Xeon Gold 6530 host), using this config with and without the four options:
  • MADV_COLLAPSE on a read-only executable mapping of a 64 MiB ext4 file: EINVAL → 0, FilePmdMapped: 65536 kB.
  • lru_gen/enabled absent → 0x0000, and switches to 0x0007.
  • bbr is available and selectable.
  • With 256 GiB and 64 vCPUs, the kernel reaches userspace in 0.80–0.90 s instead of 2.54–2.67 s.
• TDX guest (dev image, 32 vCPUs, 128 GiB, ZFS data disk, KMS and gateway on):
  • Boots; node 0 deferred pages initialised in 23ms. Run /init at 7.23–7.24 s vs 7.44–8.15 s for next without this PR.
  • MADV_COLLAPSE with data checks before and after collapse, and after a write through a new O_WRONLY open, on ZFS (64 MiB–1 GiB), ext4, the squashfs rootfs and an overlay2-on-ZFS file in an unprivileged container: all consistent. On ZFS (no large folios) the first writable open drops the collapsed folios; ext4 keeps them.
  • The khugepaged path (MADV_HUGEPAGE) on ZFS collapses as well.
  • 300 rounds of four MADV_COLLAPSE loops racing a writer on one ZFS file (~26k collapses): no mapping/pread mismatch, no kernel warnings, zpool scrub clean.
  • An unprivileged container sets bbr in its netns; the host stays on cubic. lru_gen toggles 0x0000 ↔ 0x0007.