Claude review: drm_buddy: fix power-of-2 rounding errs

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

Patch Review

**Problem analysis**: Correct. `roundup_pow_of_two()` and `rounddown_pow_of_two()` both use `unsigned long` internally (confirmed at `linux/include/linux/log2.h:55-67` — `1UL << fls_long(n - 1)` and `1UL << (fls_long(n) - 1)`). On 32-bit arches, `unsigned long` is 32 bits, so any `u64` value > 4GB is silently truncated.

**Rounddown fix** (line 306):
```c
-	modify_size = rounddown_pow_of_two(size);
+	modify_size = 1ULL << ilog2(size);
```
This is correct. `ilog2()` handles `u64` properly, and `1ULL << ilog2(size)` is equivalent to `rounddown_pow_of_two(size)` for non-zero, non-power-of-two inputs. For inputs that are already powers of two, both return the same value.

**Roundup fix** (line 315):
```c
-	size = roundup_pow_of_two(size);
+	size = 1ULL << (ilog2(size - 1) + 1);
```
**Bug**: This has an edge case problem. When `size` is already a power of two (e.g., `size = 4`), `ilog2(size - 1)` = `ilog2(3)` = 1, so the result is `1ULL << 2` = 4, which is correct. When `size = 1`, `ilog2(0)` is undefined behavior — `ilog2(0)` returns `-1` on most implementations, so `1ULL << 0` = 1, which happens to work but relies on undefined behavior for `ilog2(0)`.

However, given the calling context (this is inside `drm_buddy_alloc_blocks` with contiguous allocation), `size` should never be 0 or 1 in practice (it's a memory allocation size that's been validated), so this is likely fine in practice.

**Suggestion**: The commit message mentions "I'll use it to implement safe rounding" which is a bit informal for a commit message — first person and future tense. Consider also adding a `Fixes:` tag — the original `rounddown_pow_of_two()` / `roundup_pow_of_two()` calls have been present since the initial drm_buddy code was merged, and this is a legitimate bug on 32-bit architectures. Also, the kernel now has `roundup_pow_of_two_u64()` / `rounddown_pow_of_two_u64()` in some trees — it might be worth checking if those helpers exist or are more appropriate than hand-rolling the `ilog2` pattern.

**Missing Cc: stable**: For a genuine 32-bit architecture bug, a `Cc: stable@vger.kernel.org` tag would be appropriate.

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