[6.6][Feature] mm/mglru: add per-memcg lru_gen_idle_stats interface · Pull Request !7621 · anolis/cloud-kernel

bugzilla: https://bugzilla.openanolis.cn/show_bug.cgi?id=36700

When MGLRU is enabled, kidled refuses to run (the two are mutually exclusive), so there is currently no per-memcg cold/hot signal on an MGLRU system: kidled's memory.idle_page_stats is empty and MGLRU only exposes the raw generation dump via memory.lru_gen, which also has a write side that triggers real aging/eviction. This adds a read-only cgroup file memory.lru_gen_idle_stats that reshapes the existing MGLRU generation data into a stable per-memcg cold/hot histogram in kB, with no write side effect and no aging.

## What it does

Adds a read-only `memory.lru_gen_idle_stats` (cgroup v2 and v1, under CONFIG_LRU_GEN) that aggregates the MGLRU generations across NUMA nodes into a single per-memcg view, aligning generations by their distance from max_seq (offset 0 = youngest/hottest, larger offsets = older/colder), and reports anon/file sizes in kB.

```
#  gen      age_ms      anon_kb      file_kb
     0        1200         4096        16384
     1       45000        65536       131072
     2      300000       262144       524288
```

It reuses the same lru_gen_folio fields as `lru_gen_print_memcg()` but, unlike that file, has no write side and produces no aging — purely a reshaping of existing data. Emits nothing when MGLRU is disabled; compiles to a no-op stub under !CONFIG_LRU_GEN.

## Why

Operators running MGLRU-enabled hosts currently have no clean per-memcg cold/hot signal: kidled is disabled by MGLRU, and the only MGLRU-native view (memory.lru_gen) is a raw per-node dump whose write path triggers real reclaim, so it is unsafe to wire into monitoring. This file gives a safe, read-only cold/hot histogram for capacity planning, noisy-neighbour diagnosis and pre-reclaim safety assessment.

## Implementation / KABI

- mm/vmscan.c: new lru_gen_idle_stats_show() (~78 lines), derived from lru_gen_print_memcg.
- mm/memcontrol.c: read-only cftype in the v2 and v1 memory controller sets (~15 lines).
- include/linux/mmzone.h: declaration + !CONFIG_LRU_GEN stub (~6 lines).

KABI-safe: additive read-only cftype reusing existing lru_gen_folio fields, no struct/page-flag change, no EXPORT removed.

## Test

Built and booted on a 6.6.102 KVM host with CONFIG_LRU_GEN=y. Single-node testable (neither kidled nor MGLRU needs NUMA tiering): create a memcg, run a mixed cold/hot anon workload (one region continuously touched = hot, one touched once then idle = cold), drive aging, and read memory.lru_gen_idle_stats — hot data appears in the youngest generation (offset 0, small age_ms), cold data ages into older offsets. Verified read-only (write rejected) and that /proc and memory.lru_gen are unaffected. Both CONFIG_LRU_GEN=y and =n build cleanly with no warnings; checkpatch --strict clean.

## Scope

This is an additive, read-only step toward unifying the kidled and MGLRU cold/hot views. It does not touch kidled's idle_page_stats contract, the page-flags layout, or the kidled/MGLRU mutual-exclusion interlock. Mapping MGLRU generations onto a richer per-type/slab kidled-equivalent histogram, and retiring kidled's full-PFN scan, are left as follow-ups.

类型	指派人员	状态
审查	zhongjiang winterddd xueshuai vxuyu baolin.wang	已完成 (0/0人)
测试	小龙	进行中 (0/1人)

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