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G/T34DITOF3FWNHLigWyZJituc16tnqoUPSBgwX9dGINdKbM= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Oct 28, 2025 at 09:58:19PM +0800, Qi Zheng wrote: > From: Muchun Song > > Memory cgroup functions such as get_mem_cgroup_from_folio() and > get_mem_cgroup_from_mm() return a valid memory cgroup pointer, > even for the root memory cgroup. In contrast, the situation for > object cgroups has been different. > > Previously, the root object cgroup couldn't be returned because > it didn't exist. Now that a valid root object cgroup exists, for > the sake of consistency, it's necessary to align the behavior of > object-cgroup-related operations with that of memory cgroup APIs. > > Signed-off-by: Muchun Song > Signed-off-by: Qi Zheng > --- > include/linux/memcontrol.h | 29 +++++++++++++++++------- > mm/memcontrol.c | 45 ++++++++++++++++++++------------------ > mm/percpu.c | 2 +- > 3 files changed, 46 insertions(+), 30 deletions(-) > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index 6185d8399a54e..9fdbd4970021d 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -332,6 +332,7 @@ struct mem_cgroup { > #define MEMCG_CHARGE_BATCH 64U > > extern struct mem_cgroup *root_mem_cgroup; > +extern struct obj_cgroup *root_obj_cgroup; > > enum page_memcg_data_flags { > /* page->memcg_data is a pointer to an slabobj_ext vector */ > @@ -549,6 +550,11 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg) > return (memcg == root_mem_cgroup); > } > > +static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg) > +{ > + return objcg == root_obj_cgroup; > +} After reparenting, an objcg may satisfy objcg->memcg == root_mem_cgroup while objcg != root_obj_cgroup. Should they be considered as root objcgs? > static inline bool mem_cgroup_disabled(void) > { > return !cgroup_subsys_enabled(memory_cgrp_subsys); > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 2afd7f99ca101..d484b632c790f 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2871,7 +2865,7 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order) > int ret = 0; > > objcg = current_obj_cgroup(); > - if (objcg) { > + if (!obj_cgroup_is_root(objcg)) { Now that we support the page and slab allocators support allocating memory in NMI contexts (on some archs), current_obj_cgroup() can return NULL if (IS_ENABLED(CONFIG_MEMCG_NMI_UNSAFE) && in_nmi()) returns true (then it leads to a NULL-pointer-deref bug). But IIUC this is applied to kmem charging only (as they use this_cpu ops for stats update), and we don't have to apply the same restriction to charging LRU pages with objcg. Maybe Shakeel has more insight on this. Link: https://lore.kernel.org/all/20250519063142.111219-1-shakeel.butt@linux.dev -- Cheers, Harry / Hyeonggon