Claude review: drm/ttm: Use common ancestor of evictor and evictee as limit pool

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

Patch Review

This correctly implements recursive protection semantics for sibling cgroup competition. The commit message explanation is excellent.

The self-eviction guard is important and well-commented:

```c
+	if (evict_walk->alloc_state->only_evict_unprotected &&
+			bo->resource->css == evict_walk->alloc_state->charge_pool)
+		return 0;
```

Without this, `dmem_cgroup_state_evict_valuable` would allow a cgroup to evict from itself (since a cgroup is always allowed to self-evict), which would defeat the purpose of `only_evict_unprotected`.

The common ancestor fallback logic is correct:

```c
+	if (!limit_pool)
+		limit_pool = dmem_cgroup_common_ancestor(bo->resource->css,
+							 evict_walk->alloc_state->charge_pool);
```

The extensive comment explaining why the common ancestor matters for `effective_protection` calculation is very helpful.

**One observation**: The `below_min`/`below_low` checks in Patch 5 use `NULL` (global root) while eviction decisions in this patch use the common ancestor. These are at different logical levels - the global check determines the "policy" (should we attempt aggressive eviction?) while the per-candidate check determines the "mechanism" (is this specific buffer evictable?). This design makes sense.

No blocking issues with this patch.

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