Claude review: drm/amdkfd: Add userptr batch allocation UAPI structures

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

Patch Review

**Overview:** Adds UAPI structures for batch allocation.

**Issues:**

1. **ABI Compatibility Concern - Critical:**
```c
+ * @mmap_offset: for CPU-mapping the allocation by mmapping a render node
+ *               for userptrs this is overloaded to specify the CPU address
++ *               for batch userptr (KFD_IOC_ALLOC_MEM_FLAGS_USERPTR_BATCH),
++ *               this should point to a kfd_ioctl_userptr_ranges_data structure
```
Overloading `mmap_offset` to be a pointer in batch mode is problematic:
- Creates type confusion (u64 vs pointer)
- Different semantics on 32-bit vs 64-bit systems
- Makes it impossible to detect misuse at compile time

**Recommendation:** Consider adding a new field or using a union to make this type-safe.

2. **Potential Denial of Service:**
```c
+struct kfd_ioctl_userptr_ranges_data {
+	__u32 num_ranges;	/* to KFD */
```
No documented limit on `num_ranges` could allow userspace to allocate arbitrary amounts of kernel memory. Should define `KFD_MAX_USERPTR_RANGES` (e.g., 4096) to prevent abuse.

3. **Missing Documentation:**
- No maximum limit documented for `num_ranges`
- No explanation of what happens if ranges overlap or have gaps
- No specification of required ordering (ascending addresses?)

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Generated by Claude Code Patch Reviewer