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

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

Patch Review

**Assessment: Correct accounting, clean implementation.**

The scoreboard is maintained at all six state-transition points:

| Site | Operation | Correct? |
|------|-----------|----------|
| `mark_free()` | `free_scoreboard[order]++` | Yes — block enters FREE |
| `mark_allocated()` | `free_scoreboard[order]--` | Yes — block leaves FREE |
| `mark_split()` | `free_scoreboard[order]--` | Yes — block leaves FREE |
| `__gpu_buddy_free()` coalesce loop | `free_scoreboard[buddy_order]--` | Yes — buddy removed from tree |
| `__force_merge()` | `free_scoreboard[order]--` | Yes — block removed from tree |
| Four err_undo sites | `free_scoreboard[order]--` | Yes — block removed from tree |

The coalesce loop in `__gpu_buddy_free` only decrements for the buddy (not the block), because the block was already accounted for by the caller (either the err_undo decrement or `mark_free`→`mark_allocated` transition). The final `mark_free(mm, block)` at the bottom of `__gpu_buddy_free` then increments the scoreboard for the coalesced (higher-order) block. This is correct.

**Memory management** is handled properly:
- `gpu_buddy_init()`: `kcalloc` with proper cleanup label `out_free_scoreboard` on the error path
- `gpu_buddy_fini()`: `kfree(mm->free_scoreboard)`

**Minor nit on the header comment:**

```c
+	/*
+	 * Per-order free block scoreboard: free_scoreboard[order] holds the
+	 * number of blocks of that order currently in the free state.
+	 * Incremented in mark_free(), decremented wherever rbtree_remove() is
+	 * called on a free block.
+	 */
```

This is a good comment for an internal field. But the "decremented wherever rbtree_remove() is called on a free block" formulation is slightly imprecise — it's also decremented in `mark_split()` which does its own `rbtree_remove` internally. The wording is fine in practice though; it captures the invariant well enough.

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Generated by Claude Code Patch Reviewer