Claude review: gpu/buddy: Track per-order used blocks with a scoreboard

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Assessment: Correct, with one observation.**

The used scoreboard has fewer update sites than the free one because blocks can only become ALLOCATED through `mark_allocated()` and can only leave ALLOCATED through `mark_free()` or direct `gpu_block_free()` during coalescing.

| Site | Operation | Guard | Correct? |
|------|-----------|-------|----------|
| `mark_allocated()` | `used_scoreboard[order]++` | Always (block is becoming allocated) | Yes |
| `mark_free()` | `used_scoreboard[order]--` | `gpu_buddy_block_is_allocated(block)` | Yes |
| `__gpu_buddy_free()` coalesce | `used_scoreboard[order]--` | `gpu_buddy_block_is_allocated(block)` | Yes |

The guard in `mark_free()` is needed because `mark_free()` is called on blocks that were SPLIT (during `__force_merge` cleanup), not just ALLOCATED. The guard in `__gpu_buddy_free()` is needed because the block entering the coalesce loop can be either ALLOCATED (from `gpu_buddy_free_block`) or FREE (from err_undo / __force_merge paths).

I traced the full lifecycle of `gpu_buddy_free_block()` → `__gpu_buddy_free()`:
- If no coalescing: the decrement happens in `mark_free()` (at the bottom of `__gpu_buddy_free`) via the `is_allocated` guard. Correct.
- If coalescing happens: the decrement happens in the new code inside the coalesce loop body. After `gpu_block_free(block)`, `block = parent` (SPLIT state), so subsequent iterations and the final `mark_free()` do NOT double-decrement. Correct.

**Print format change:** The threshold for KiB vs MiB formatting changes from `if (free < SZ_1M)` to `if (block_size < SZ_1M)`. This is a good change — it makes the unit consistent per-order regardless of how many blocks exist, and ensures `free` and `used` values in the same line always use the same unit. But this is a user-visible output change in debugfs, so anyone scraping `/sys/kernel/debug/dri/0/tile0/vram_mm` with scripts may notice.

**Label naming nit:** `out_free_free_scoreboard` is a bit stutter-y. Something like `out_free_sb` or `out_scoreboard` would read better, but this is purely cosmetic.

**Type choice:** Both scoreboards use `u64`. Given that the maximum number of blocks at any order is bounded by `size / chunk_size` (which is also `u64`), this is appropriate and overflow-safe.

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Generated by Claude Code Patch Reviewer