Claude review: cgroup/dmem: implement dmem.high soft limit via prioritized eviction

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Overall Series Review

Subject: cgroup/dmem: implement dmem.high soft limit via prioritized eviction
Author: Qiliang Yuan <realwujing@gmail.com>
Patches: 2
Reviewed: 2026-06-04T14:56:52.172403

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This is a single-patch series (v6) adding a `dmem.high` soft limit to the dmem cgroup v2 controller. The concept is sound: rather than only having a hard `max` limit that causes allocation failures, a `high` soft limit causes prioritized eviction of buffer objects belonging to over-limit cgroups. The implementation adds a 3-pass eviction strategy in TTM and a `try_high` parameter to `dmem_cgroup_state_evict_valuable()`.

The patch has gone through 6 revisions, fixing real bugs (NULL dereferences, deadlock risks, stub behavior). The core design -- prioritized eviction via a first pass targeting over-high cgroups -- is a reasonable approach. However, there are several issues ranging from correctness concerns to minor nits.

**Key concerns:**
1. The `try_high` pass respects `dmem.min` but ignores `dmem.low` -- this creates an inconsistency where the high-priority eviction pass is more aggressive than the standard pass with respect to low-watermark protection.
2. The parent-chain walk for the `try_high` check traverses `page_counter` parents, which correspond to cgroup hierarchy parents. This is correct but the walk has no bound and could be expensive in deeply nested hierarchies (minor concern).
3. The `limit_pool == test_pool` + `try_high` path does NOT check `dmem.min` protection, unlike the general `try_high` path -- this is asymmetric.

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