Non-fixed MAP_SHARED file-backed mmap allocations whose result landed mid 2 MiB block forced hvf_apply_file_overlay_quiesced to split the containing HVF stage-2 segment at both ends. Back-to-back memfd-style allocations burned segments at roughly two per mmap and could approach GUEST_MAX_HVF_SEGMENTS (256). The earlier multi-segment overlay fix handles correctness; this change reduces the segment-table churn.

find_free_gap and find_free_gap_inner gain a per-call alignment argument. sys_mmap passes BLOCK_2MIB when the allocation actually qualifies for the host overlay fast path (MAP_SHARED, host-page-aligned offset, writable backer) and host_page_size_cached() otherwise. The backing fd is now opened before the gap finder so overlay_fd_writable can gate the alignment decision; every failure path closes the ref.

Alignment is a best-effort placement preference, not a Linux-visible constraint:

• The cached gap hint advances by host-page granularity so a small MAP_PRIVATE 4 KiB allocation can still occupy the trailing slack of the 2 MiB block.
• When no 2 MiB-aligned gap is available, sys_mmap retries with host-page alignment instead of returning -ENOMEM.
• For non-zero addr hints the path probes the exact host-page-aligned hint window first, falls back to 2 MiB alignment in the wider hint range, then to host-page alignment. A guest that pinpoints a free address still gets the address it asked for.

mremap stays at host-page alignment because the destination does not reinstall a file overlay.

Close #94

Summary by cubic

Aligns non-fixed MAP_SHARED file-backed mmap to 2 MiB when eligible to reduce HVF stage-2 segment fragmentation and avoid segment exhaustion. Falls back to host-page alignment when needed and preserves exact hints; resolves #94.

• Refactors
  • Gap finder now accepts an alignment; sys_mmap uses 2 MiB for eligible MAP_SHARED allocations and retries at host-page if no 2 MiB gap exists; mremap stays host-page aligned.
  • Open the backing fd before placement to check writability and gate 2 MiB alignment; close on all failure paths.
  • Hint handling: probe the exact host-page-aligned window first, then try 2 MiB within the hint range, then host-page; gap hints still advance by host page so small MAP_PRIVATE mappings can use trailing slack. Added tests for hint fallback and back-to-back MAP_SHARED memfd mappings.

^{Written for commit d8b8952. Summary will update on new commits.}