Tlsf align overhead reduction

[han-jiang277]

Current (absorb padding) — 4 KiB-aligned, 4 KiB payload, GRANULARITY = 32 (64-bit)
 block_start (G-aligned)                         aligned_payload
 │                                               │
 ▼                                               ▼
 ┌──────┬───────── dead padding ─────────┬──────┬──────── PAYLOAD ────────┐
 │BlkHdr│        ~4080 B (lost)          │ Pad  │         4096 B           │
 └──────┴───────────────────────────────┴──────┴─────────────────────────┘
 └───────────────── used block ≈ 8192 B (retained) ──────────────────────┘


Proposed (reclaim leading gap)
 block_start          U_start = aligned_payload - G      aligned_payload
 │                    │                                  │
 ▼                    ▼                                  ▼
 ┌──── FREE  L ───────┬──────┬──────┬──── PAYLOAD ───────┬─── FREE T ───┐
 │  ~4064 B (reused)  │BlkHdr│ Pad  │      4096 B         │  remainder   │
 └────────────────────┴──────┴──────┴────────────────────┴──────────────┘
 phys chain:  prev ← L ← U(used) ← T ← next      (all prev_phys fixed up)
 retained for this alloc ≈ 4128 B   (was ≈ 8192 B)

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[han-jiang277]

build_prs

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Job is started, see https://github.com/vivoblueos/kernel/actions/runs/27002826313.

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❌ Job failed. Failed jobs: build_and_check_boards (failure), see https://github.com/vivoblueos/kernel/actions/runs/27002826313.

[han-jiang277]

build_prs

[github-actions]

Job is started, see https://github.com/vivoblueos/kernel/actions/runs/27003660272.

[han-jiang277]

Two reasons block.rs doesn't need code changes:

1. get_overhead_and_size — search size stays the same by design

This function computes the search size — how large a free block to look for. After reclamation, we still need to find a block of that size because the gap must physically exist before we can carve it. The search size is a mathematical floor, not the retained size. Changing it would break the allocator.

2. used_block_hdr_for_allocation — header location protocol unchanged

For align >= GRANULARITY, it reads the UsedBlockPad pointer stored immediately before the payload. After reclamation:

U_start = aligned_payload - GRANULARITY
UsedBlockPad sits at aligned_payload - sizeof(UsedBlockPad) = aligned_payload - GRANULARITY/2
The pad stores a pointer back to U_start
This is identical to the non-reclamation path — the pad is always placed just before the payload, and always points to the header. The relative geometry between payload, pad, and header is unchanged; only the header's absolute position in the pool moved forward. free/deallocate just follows the pad pointer and finds the right header regardless.

So block.rs needed only a comment update (provenance note), not a code change.

[github-actions]

✅ All jobs completed successfully, see https://github.com/vivoblueos/kernel/actions/runs/27003660272.

[xuchang-vivo]

Look good