Claude review: cgroup/dmem: Add reclaim callback for lowering max below current usage

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

Patch Review

**Verdict: Good with minor observations.**

**Core design change — `xchg` replacing `page_counter_set_max`:**

```c
+	xchg(&pool->cnt.max, (unsigned long)val);
```

The original `page_counter_set_max()` refuses to set max below current usage (returns `-EBUSY`). The new code unconditionally sets max via `xchg`, which preserves the full memory barrier semantics while allowing max < usage. This is the right approach: it throttles concurrent allocations immediately while reclaim runs. Well motivated.

**Retry loop termination on partial progress:**

```c
+	for (int retries = 5; retries > 0; retries--) {
+		...
+		ret = region->reclaim(pool, usage - val, region->reclaim_priv);
+		...
+		if (ret)
+			break;
```

The reclaim callback (in patch 3) returns `-ENOSPC` when partial progress is made. This causes the retry loop to break even if some memory was freed. A more aggressive approach would be to retry on partial progress (similar to how the memory cgroup controller distinguishes `-EAGAIN` for "retry" from hard errors). However, since `page_counter_read()` reflects the actual usage and the max is already set to throttle new allocations, this best-effort approach is defensible and matches the stated design intent.

**rwsem protection looks correct:**

- `down_write` in `dmem_cgroup_unregister_region()` ensures all in-flight reclaim callbacks complete before clearing the callback pointer.
- `down_read_interruptible` in `set_resource_max()` allows concurrent reclaim and handles signals gracefully.
- `WRITE_ONCE`/`READ_ONCE` pairs on `region->reclaim` are properly placed.

**Unused `nonblock` parameter in `set_resource_min`/`set_resource_low`:**

```c
 static void
-set_resource_min(struct dmem_cgroup_pool_state *pool, u64 val)
+set_resource_min(struct dmem_cgroup_pool_state *pool, u64 val, bool nonblock)
```

These functions accept `nonblock` but ignore it, keeping the function pointer signature uniform for `dmemcg_limit_write()`. This is clean.

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Generated by Claude Code Patch Reviewer