From ea74172be27d19d72a72647d7b1ef131662d1ded Mon Sep 17 00:00:00 2001 From: Bingfang Guo Date: Mon, 21 Sep 2026 14:16:25 +0800 Subject: [PATCH] memcg: base swap charge accounting on memcgid root status A private ID currently pins its original memcg, so testing whether the ID belongs to root is equivalent to testing whether the resolved memcg is root. That equivalence will no longer hold when IDs refer to objcgs. A non-root ID may resolve to root after reparenting, but its swap entries still carry counter charges inherited by root. Skipping their uncharge based on the resolved memcg would leave those charges behind. Use the private ID's root status to decide whether a swap entry carries a counter charge. A root-ID entry carries none, while a non-root-ID entry must release its charge even if its current accounting memcg has become root. This patch adds a new helper to check if the memcgid equals to that of the root memcg. For swap uncharging and v2 swap charging, simply decide whether to charge/uncharge memsw or swap counter based on the swap memcgid is root or not. For v1 swapout, don't recharge the memsw counter, just cancel the charge if the swap memcg ID refers to the root memcg. This makes memory and swap charging have similar semantics: one relys on the objcg's root status, and the other relys on the memcgid's root status. Link: https://lore.kernel.org/20260921-bingfangguo-memcgid-rework-v2-2-6c0637dc0edb@tencent.com Signed-off-by: Bingfang Guo Signed-off-by: Andrew Morton Acked-by: Muchun Song Cc: Johannes Weiner Cc: Michal Hocko Cc: Roman Gushchin Cc: Shakeel Butt Cc: Dave Chinner Cc: Qi Zheng Cc: Kairui Song Cc: Barry Song Cc: Axel Rasmussen Cc: Yuanchu Xie Cc: Wei Xu Cc: David Hildenbrand Cc: Lorenzo Stoakes Cc: Bingfang Guo --- mm/memcontrol-v1.c | 16 ++++++++-------- mm/memcontrol-v1.h | 5 +++++ mm/memcontrol.c | 4 ++-- 3 files changed, 15 insertions(+), 10 deletions(-) diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c index bf2c7d53b01b..ed015fdd9512 100644 --- a/mm/memcontrol-v1.c +++ b/mm/memcontrol-v1.c @@ -271,6 +271,7 @@ void __memcg1_swapout(struct folio *folio, struct swap_cluster_info *ci) struct mem_cgroup *memcg, *swap_memcg; struct obj_cgroup *objcg; unsigned int nr_entries; + unsigned short private_id; VM_WARN_ON_ONCE_FOLIO(!folio_test_swapcache(folio), folio); VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio); @@ -293,25 +294,24 @@ void __memcg1_swapout(struct folio *folio, struct swap_cluster_info *ci) /* * In case the memcg owning these pages has been offlined and doesn't * have an ID allocated to it anymore, charge the closest online - * ancestor for the swap instead and transfer the memory+swap charge. + * ancestor for the swap instead and cancel the memory+swap charge + * if the ID refers to the root memcg. */ nr_entries = folio_nr_pages(folio); swap_memcg = mem_cgroup_private_id_get_online(memcg, nr_entries); + private_id = mem_cgroup_private_id(swap_memcg); mod_memcg_state(swap_memcg, MEMCG_SWAP, nr_entries); - __swap_cgroup_set(ci, swp_cluster_offset(folio->swap), nr_entries, - mem_cgroup_private_id(swap_memcg)); + __swap_cgroup_set(ci, swp_cluster_offset(folio->swap), nr_entries, private_id); folio_unqueue_deferred_split(folio); folio->memcg_data = 0; - if (!obj_cgroup_is_root(objcg)) + if (!obj_cgroup_is_root(objcg)) { page_counter_uncharge(&memcg->memory, nr_entries); - if (memcg != swap_memcg) { - if (!mem_cgroup_is_root(swap_memcg)) - page_counter_charge(&swap_memcg->memsw, nr_entries); - page_counter_uncharge(&memcg->memsw, nr_entries); + if (mem_cgroup_private_id_is_root(private_id)) + page_counter_uncharge(&memcg->memsw, nr_entries); } /* diff --git a/mm/memcontrol-v1.h b/mm/memcontrol-v1.h index 0952b2a783e5..7286456125db 100644 --- a/mm/memcontrol-v1.h +++ b/mm/memcontrol-v1.h @@ -22,6 +22,11 @@ void drain_all_stock(struct mem_cgroup *root_memcg); int memory_stat_show(struct seq_file *m, void *v); +static inline bool mem_cgroup_private_id_is_root(unsigned short id) +{ + return id == mem_cgroup_private_id(root_mem_cgroup); +} + struct mem_cgroup *mem_cgroup_private_id_get_online(struct mem_cgroup *memcg, unsigned int n); diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 059eaca173bb..c43b09178512 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5975,7 +5975,7 @@ int __mem_cgroup_try_charge_swap(struct folio *folio) /* memcg is pined by memcg ID. */ private_id = mem_cgroup_private_id(memcg); - if (!mem_cgroup_is_root(memcg) && + if (!mem_cgroup_private_id_is_root(private_id) && !page_counter_try_charge(&memcg->swap, nr_pages, &counter)) { memcg_memory_event(memcg, MEMCG_SWAP_MAX); memcg_memory_event(memcg, MEMCG_SWAP_FAIL); @@ -6004,7 +6004,7 @@ void __mem_cgroup_uncharge_swap(unsigned short id, unsigned int nr_pages) rcu_read_lock(); memcg = mem_cgroup_from_private_id(id); if (memcg) { - if (!mem_cgroup_is_root(memcg)) { + if (!mem_cgroup_private_id_is_root(id)) { if (do_memsw_account()) page_counter_uncharge(&memcg->memsw, nr_pages); else