Claude review: cgroup/dmem: introduce a peak file

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

Patch Review

**Approach:** The patch adds a `get_resource_peak()` getter that reads `pool->cnt.watermark`, a `dmem_cgroup_region_peak_show()` display function, and a cftype entry — all following the exact same pattern as `get_resource_current()` / `dmem_cgroup_region_current_show()`.

**Positive:**

1. The `READ_ONCE` usage in `get_resource_peak()` is correct and consistent with how `page_counter_charge()` in `mm/page_counter.c` uses `WRITE_ONCE` to update the watermark. This is the same deliberate-racy-but-acceptable pattern documented in the page counter code.

2. The null check (`pool ? ... : 0`) mirrors `get_resource_current()` exactly.

3. `CFTYPE_NOT_ON_ROOT` is the correct flag — this matches `memory.peak` in the memory controller and is consistent with how `dmem.min`, `dmem.low`, and `dmem.max` are configured.

**Observations:**

1. **`current` lacks `CFTYPE_NOT_ON_ROOT` but `peak` has it.** Looking at the existing tree, `dmem.current` does *not* set `CFTYPE_NOT_ON_ROOT`, so it appears on root. The new `dmem.peak` does set `CFTYPE_NOT_ON_ROOT`, so it won't appear on root. This is consistent with how `memory.current` (no flag) and `memory.peak` (has `CFTYPE_NOT_ON_ROOT`) work in `mm/memcontrol.c`, so the asymmetry is intentional and correct.

2. **No write/reset support.** The commit message and documentation say "read-only for now," which is fine for an initial version. The `memory.peak` controller has grown `peak_open`/`peak_release`/`memory_peak_write` for per-fd local watermark tracking and reset support. If that's planned as a follow-up, it might be worth mentioning in the commit message for future reference, but this is a nit — the v1→v2 changelog already documents the deliberate decision to start read-only.

3. **Documentation placement is correct.** The `dmem.peak` entry is inserted between `dmem.max/min/low` and `dmem.capacity`, which is a natural location grouping the per-cgroup files together before the root-only `dmem.capacity`.

4. **Watermark initialization.** The `page_counter_init()` function zeroes `usage` and sets `max` but does not explicitly initialize `watermark` or `local_watermark`. Since `page_counter` structs in `dmem_cgroup_pool_state` are allocated with `kzalloc` (which zero-initializes), the watermark starts at 0, which is correct for a "max seen so far" semantic. This is fine but worth noting for anyone reading the patch.

**No functional issues found.** The patch is a straightforward, correct extension of the existing dmem cgroup interface.

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