Re: [PATCH RFC 27/28] mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios

[Muchun Song <muchun.song@linux.dev>]

> On May 20, 2025, at 19:27, Harry Yoo <harry.yoo@oracle.com> wrote:
> 
> On Tue, Apr 15, 2025 at 10:45:31AM +0800, Muchun Song wrote:
>> Pagecache pages are charged at allocation time and hold a reference
>> to the original memory cgroup until reclaimed. Depending on memory
>> pressure, page sharing patterns between different cgroups and cgroup
>> creation/destruction rates, many dying memory cgroups can be pinned
>> by pagecache pages, reducing page reclaim efficiency and wasting
>> memory. Converting LRU folios and most other raw memory cgroup pins
>> to the object cgroup direction can fix this long-living problem.
>> 
>> Finally, folio->memcg_data of LRU folios and kmem folios will always
>> point to an object cgroup pointer. The folio->memcg_data of slab
>> folios will point to an vector of object cgroups.
>> 
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> include/linux/memcontrol.h |  78 +++++--------
>> mm/huge_memory.c           |  33 ++++++
>> mm/memcontrol-v1.c         |  15 ++-
>> mm/memcontrol.c            | 228 +++++++++++++++++++++++++------------
>> 4 files changed, 222 insertions(+), 132 deletions(-)
> 
> [...]
> 
>> +static void lruvec_reparent_lru(struct lruvec *src, struct lruvec *dst,
>> + 				enum lru_list lru)
>> +{
>> + 	int zid;
>> + 	struct mem_cgroup_per_node *mz_src, *mz_dst;
>> +
>> + 	mz_src = container_of(src, struct mem_cgroup_per_node, lruvec);
>> + 	mz_dst = container_of(dst, struct mem_cgroup_per_node, lruvec);
>> +	
>> + 	if (lru != LRU_UNEVICTABLE)
>> + 	list_splice_tail_init(&src->lists[lru], &dst->lists[lru]);
>> +
>> + 	for (zid = 0; zid < MAX_NR_ZONES; zid++) {
>> + 		mz_dst->lru_zone_size[zid][lru] += mz_src->lru_zone_size[zid][lru];
>> + 		mz_src->lru_zone_size[zid][lru] = 0;
>> + 	}
>> +}
> 
> I think this function should also update memcg and lruvec stats of
> parent memcg? Or is it intentional?

Hi Harry,

No. Do not need. Because the statistics are accounted hierarchically.

Thanks.

> 
> -- 
> Cheers,
> Harry / Hyeonggon