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: 1
Reviewed: 2026-06-04T15:03:19.411766

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This is a single-patch v5 series adding a `dmem.high` soft limit to the dmem cgroup v2 controller. The concept is sound: when a hard `max` limit is hit and eviction is triggered, BOs from cgroups exceeding their `dmem.high` are targeted first via a new "Pass 1" in `ttm_bo_evict_alloc()`. This provides a useful middle ground between no limit and hard failure.

The patch has gone through significant iteration (v1→v5) addressing NULL-pointer issues, deadlock risks, and control flow problems. The v5 changes are clearly motivated by real bugs found in v4. However, several issues remain:

**Key concerns:**
1. **Missing `elow` check in the `try_high` path** — The try_high block respects `emin` but silently ignores `elow`, meaning BOs protected by `dmem.low` can be evicted during the high-priority pass.
2. **No documentation** — The patch adds a new cgroupfs control file but provides no documentation update (e.g., `Documentation/admin-guide/cgroup-v2.rst` or similar).
3. **The `try_high` path for `limit_pool == test_pool` skips `emin` protection** — When a cgroup hits its own limit and the same pool is being tested, the code only checks `used > high` but does not check against `emin`, unlike the cross-cgroup path.
4. **The 3-pass strategy has a subtle interaction** — Pass 1 uses blocking locks when a ticket is available, but Pass 2 always uses trylock. If Pass 1 fails to evict anything (all over-high BOs were locked and no ticket), and Pass 2 also fails to evict, the code falls through to Pass 3 which is the existing properly-locked path. This seems correct but could benefit from a comment about the fallback chain.

**Minor issues:**
- The cover letter text in the commit message body is duplicative with the commit message itself.
- Typo in the v2 changelog: "above-elow" (line 199).

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