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Date: Tue, 2 Mar 2021 12:11:06 +0800
From: Muchun Song <songmuchun@...edance.com>
To: Roman Gushchin <guro@...com>
Cc: Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Shakeel Butt <shakeelb@...gle.com>,
LKML <linux-kernel@...r.kernel.org>,
Linux Memory Management List <linux-mm@...ck.org>,
Xiongchun duan <duanxiongchun@...edance.com>
Subject: Re: [External] Re: [PATCH 5/5] mm: memcontrol: use object cgroup for
remote memory cgroup charging
On Tue, Mar 2, 2021 at 9:29 AM Roman Gushchin <guro@...com> wrote:
>
> On Mon, Mar 01, 2021 at 02:22:27PM +0800, Muchun Song wrote:
> > We spent a lot of energy to make slab accounting do not hold a refcount
> > to memory cgroup, so the dying cgroup can be freed as soon as possible
> > on cgroup offlined.
> >
> > But some users of remote memory cgroup charging (e.g. bpf and fsnotify)
> > hold a refcount to memory cgroup for charging to it later. Actually,
> > the slab core use obj_cgroup APIs for memory cgroup charing, so we can
> > hold a refcount to obj_cgroup instead of memory cgroup. In this case,
> > the infrastructure of remote meory charging also do not hold a refcount
> > to memory cgroup.
>
> -cc all except mm folks
>
> Same here, let's not switch the remote charging infra to objcg to save
> an ability to use it for user pages. If we have a real problem with bpf/...,
> let's solve it case by case.
How about rename current->active_memcg to current->active_memcg_data?
If the lowest bit of active_memcg_data is one, it points to an object cgroup.
Otherwise, it points to a memoy cgroup. Just like page->memcg_data.
In this case, we can charge objects by using obj_cgroup APIs.
Just my thought.
Thanks.
>
> Thanks!
>
> >
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > ---
> > fs/buffer.c | 10 ++++--
> > fs/notify/fanotify/fanotify.c | 6 ++--
> > fs/notify/fanotify/fanotify_user.c | 2 +-
> > fs/notify/group.c | 3 +-
> > fs/notify/inotify/inotify_fsnotify.c | 8 ++---
> > fs/notify/inotify/inotify_user.c | 2 +-
> > include/linux/bpf.h | 2 +-
> > include/linux/fsnotify_backend.h | 2 +-
> > include/linux/memcontrol.h | 15 ++++++++
> > include/linux/sched.h | 4 +--
> > include/linux/sched/mm.h | 28 +++++++--------
> > kernel/bpf/syscall.c | 35 +++++++++----------
> > kernel/fork.c | 2 +-
> > mm/memcontrol.c | 66 ++++++++++++++++++++++++++++--------
> > 14 files changed, 121 insertions(+), 64 deletions(-)
> >
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index 591547779dbd..cc99fcf66368 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -842,14 +842,16 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
> > struct buffer_head *bh, *head;
> > gfp_t gfp = GFP_NOFS | __GFP_ACCOUNT;
> > long offset;
> > - struct mem_cgroup *memcg, *old_memcg;
> > + struct mem_cgroup *memcg;
> > + struct obj_cgroup *objcg, *old_objcg;
> >
> > if (retry)
> > gfp |= __GFP_NOFAIL;
> >
> > /* The page lock pins the memcg */
> > memcg = page_memcg(page);
> > - old_memcg = set_active_memcg(memcg);
> > + objcg = get_obj_cgroup_from_mem_cgroup(memcg);
> > + old_objcg = set_active_obj_cgroup(objcg);
> >
> > head = NULL;
> > offset = PAGE_SIZE;
> > @@ -868,7 +870,9 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
> > set_bh_page(bh, page, offset);
> > }
> > out:
> > - set_active_memcg(old_memcg);
> > + set_active_obj_cgroup(old_objcg);
> > + if (objcg)
> > + obj_cgroup_put(objcg);
> > return head;
> > /*
> > * In case anything failed, we just free everything we got.
> > diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
> > index 1192c9953620..04d24acfffc7 100644
> > --- a/fs/notify/fanotify/fanotify.c
> > +++ b/fs/notify/fanotify/fanotify.c
> > @@ -530,7 +530,7 @@ static struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
> > struct inode *dirid = fanotify_dfid_inode(mask, data, data_type, dir);
> > const struct path *path = fsnotify_data_path(data, data_type);
> > unsigned int fid_mode = FAN_GROUP_FLAG(group, FANOTIFY_FID_BITS);
> > - struct mem_cgroup *old_memcg;
> > + struct obj_cgroup *old_objcg;
> > struct inode *child = NULL;
> > bool name_event = false;
> >
> > @@ -580,7 +580,7 @@ static struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
> > gfp |= __GFP_RETRY_MAYFAIL;
> >
> > /* Whoever is interested in the event, pays for the allocation. */
> > - old_memcg = set_active_memcg(group->memcg);
> > + old_objcg = set_active_obj_cgroup(group->objcg);
> >
> > if (fanotify_is_perm_event(mask)) {
> > event = fanotify_alloc_perm_event(path, gfp);
> > @@ -608,7 +608,7 @@ static struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
> > event->pid = get_pid(task_tgid(current));
> >
> > out:
> > - set_active_memcg(old_memcg);
> > + set_active_obj_cgroup(old_objcg);
> > return event;
> > }
> >
> > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> > index 9e0c1afac8bd..055ca36d4e0e 100644
> > --- a/fs/notify/fanotify/fanotify_user.c
> > +++ b/fs/notify/fanotify/fanotify_user.c
> > @@ -985,7 +985,7 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
> > group->fanotify_data.user = user;
> > group->fanotify_data.flags = flags;
> > atomic_inc(&user->fanotify_listeners);
> > - group->memcg = get_mem_cgroup_from_mm(current->mm);
> > + group->objcg = get_obj_cgroup_from_current();
> >
> > group->overflow_event = fanotify_alloc_overflow_event();
> > if (unlikely(!group->overflow_event)) {
> > diff --git a/fs/notify/group.c b/fs/notify/group.c
> > index ffd723ffe46d..fac46b92c16f 100644
> > --- a/fs/notify/group.c
> > +++ b/fs/notify/group.c
> > @@ -24,7 +24,8 @@ static void fsnotify_final_destroy_group(struct fsnotify_group *group)
> > if (group->ops->free_group_priv)
> > group->ops->free_group_priv(group);
> >
> > - mem_cgroup_put(group->memcg);
> > + if (group->objcg)
> > + obj_cgroup_put(group->objcg);
> > mutex_destroy(&group->mark_mutex);
> >
> > kfree(group);
> > diff --git a/fs/notify/inotify/inotify_fsnotify.c b/fs/notify/inotify/inotify_fsnotify.c
> > index 1901d799909b..20835554819a 100644
> > --- a/fs/notify/inotify/inotify_fsnotify.c
> > +++ b/fs/notify/inotify/inotify_fsnotify.c
> > @@ -66,7 +66,7 @@ int inotify_handle_inode_event(struct fsnotify_mark *inode_mark, u32 mask,
> > int ret;
> > int len = 0;
> > int alloc_len = sizeof(struct inotify_event_info);
> > - struct mem_cgroup *old_memcg;
> > + struct obj_cgroup *old_objcg;
> >
> > if (name) {
> > len = name->len;
> > @@ -81,12 +81,12 @@ int inotify_handle_inode_event(struct fsnotify_mark *inode_mark, u32 mask,
> >
> > /*
> > * Whoever is interested in the event, pays for the allocation. Do not
> > - * trigger OOM killer in the target monitoring memcg as it may have
> > + * trigger OOM killer in the target monitoring objcg as it may have
> > * security repercussion.
> > */
> > - old_memcg = set_active_memcg(group->memcg);
> > + old_objcg = set_active_obj_cgroup(group->objcg);
> > event = kmalloc(alloc_len, GFP_KERNEL_ACCOUNT | __GFP_RETRY_MAYFAIL);
> > - set_active_memcg(old_memcg);
> > + set_active_obj_cgroup(old_objcg);
> >
> > if (unlikely(!event)) {
> > /*
> > diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
> > index c71be4fb7dc5..5b4de477fcac 100644
> > --- a/fs/notify/inotify/inotify_user.c
> > +++ b/fs/notify/inotify/inotify_user.c
> > @@ -649,7 +649,7 @@ static struct fsnotify_group *inotify_new_group(unsigned int max_events)
> > oevent->name_len = 0;
> >
> > group->max_events = max_events;
> > - group->memcg = get_mem_cgroup_from_mm(current->mm);
> > + group->objcg = get_obj_cgroup_from_current();
> >
> > spin_lock_init(&group->inotify_data.idr_lock);
> > idr_init(&group->inotify_data.idr);
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index cccaef1088ea..b6894e3cd095 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -158,7 +158,7 @@ struct bpf_map {
> > u32 btf_value_type_id;
> > struct btf *btf;
> > #ifdef CONFIG_MEMCG_KMEM
> > - struct mem_cgroup *memcg;
> > + struct obj_cgroup *objcg;
> > #endif
> > char name[BPF_OBJ_NAME_LEN];
> > u32 btf_vmlinux_value_type_id;
> > diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
> > index e5409b83e731..d0303f634da6 100644
> > --- a/include/linux/fsnotify_backend.h
> > +++ b/include/linux/fsnotify_backend.h
> > @@ -220,7 +220,7 @@ struct fsnotify_group {
> > * notification list is too
> > * full */
> >
> > - struct mem_cgroup *memcg; /* memcg to charge allocations */
> > + struct obj_cgroup *objcg; /* objcg to charge allocations */
> >
> > /* groups can define private fields here or use the void *private */
> > union {
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 27043478220f..96e63ec7274a 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -1642,6 +1642,7 @@ static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
> > int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order);
> > void __memcg_kmem_uncharge_page(struct page *page, int order);
> >
> > +struct obj_cgroup *get_obj_cgroup_from_mem_cgroup(struct mem_cgroup *memcg);
> > struct obj_cgroup *get_obj_cgroup_from_current(void);
> >
> > int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size);
> > @@ -1692,6 +1693,20 @@ static inline int memcg_cache_id(struct mem_cgroup *memcg)
> > struct mem_cgroup *mem_cgroup_from_obj(void *p);
> >
> > #else
> > +static inline
> > +struct obj_cgroup *get_obj_cgroup_from_mem_cgroup(struct mem_cgroup *memcg)
> > +{
> > + return NULL;
> > +}
> > +
> > +static inline struct obj_cgroup *get_obj_cgroup_from_current(void)
> > +{
> > + return NULL;
> > +}
> > +
> > +static inline void obj_cgroup_put(struct obj_cgroup *objcg)
> > +{
> > +}
> >
> > static inline int memcg_kmem_charge_page(struct page *page, gfp_t gfp,
> > int order)
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index c2d488eddf85..75d5b571edcb 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1317,8 +1317,8 @@ struct task_struct {
> > #endif
> >
> > #ifdef CONFIG_MEMCG_KMEM
> > - /* Used by memcontrol for targeted memcg charge: */
> > - struct mem_cgroup *active_memcg;
> > + /* Used by memcontrol for targeted object cgroup charge: */
> > + struct obj_cgroup *active_objcg;
> > #endif
> >
> > #ifdef CONFIG_BLK_CGROUP
> > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> > index 64a72975270e..e713f4290914 100644
> > --- a/include/linux/sched/mm.h
> > +++ b/include/linux/sched/mm.h
> > @@ -295,36 +295,34 @@ static inline void memalloc_nocma_restore(unsigned int flags)
> > #endif
> >
> > #ifdef CONFIG_MEMCG_KMEM
> > -DECLARE_PER_CPU(struct mem_cgroup *, int_active_memcg);
> > +DECLARE_PER_CPU(struct obj_cgroup *, int_active_objcg);
> > /**
> > - * set_active_memcg - Starts the remote memcg charging scope.
> > - * @memcg: memcg to charge.
> > + * set_active_obj_cgroup - Starts the remote object cgroup charging scope.
> > + * @objcg: object cgroup to charge.
> > *
> > - * This function marks the beginning of the remote memcg charging scope. All the
> > - * __GFP_ACCOUNT allocations till the end of the scope will be charged to the
> > - * given memcg.
> > + * This function marks the beginning of the remote object cgroup charging scope.
> > + * All the __GFP_ACCOUNT allocations till the end of the scope will be charged
> > + * to the given object cgroup.
> > *
> > * NOTE: This function can nest. Users must save the return value and
> > * reset the previous value after their own charging scope is over.
> > */
> > -static inline struct mem_cgroup *
> > -set_active_memcg(struct mem_cgroup *memcg)
> > +static inline struct obj_cgroup *set_active_obj_cgroup(struct obj_cgroup *objcg)
> > {
> > - struct mem_cgroup *old;
> > + struct obj_cgroup *old;
> >
> > if (in_interrupt()) {
> > - old = this_cpu_read(int_active_memcg);
> > - this_cpu_write(int_active_memcg, memcg);
> > + old = this_cpu_read(int_active_objcg);
> > + this_cpu_write(int_active_objcg, objcg);
> > } else {
> > - old = current->active_memcg;
> > - current->active_memcg = memcg;
> > + old = current->active_objcg;
> > + current->active_objcg = objcg;
> > }
> >
> > return old;
> > }
> > #else
> > -static inline struct mem_cgroup *
> > -set_active_memcg(struct mem_cgroup *memcg)
> > +static inline struct obj_cgroup *set_active_obj_cgroup(struct obj_cgroup *objcg)
> > {
> > return NULL;
> > }
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index c859bc46d06c..1b078eddf083 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -390,37 +390,38 @@ void bpf_map_free_id(struct bpf_map *map, bool do_idr_lock)
> > }
> >
> > #ifdef CONFIG_MEMCG_KMEM
> > -static void bpf_map_save_memcg(struct bpf_map *map)
> > +static void bpf_map_save_objcg(struct bpf_map *map)
> > {
> > - map->memcg = get_mem_cgroup_from_mm(current->mm);
> > + map->objcg = get_obj_cgroup_from_current();
> > }
> >
> > -static void bpf_map_release_memcg(struct bpf_map *map)
> > +static void bpf_map_release_objcg(struct bpf_map *map)
> > {
> > - mem_cgroup_put(map->memcg);
> > + if (map->objcg)
> > + obj_cgroup_put(map->objcg);
> > }
> >
> > void *bpf_map_kmalloc_node(const struct bpf_map *map, size_t size, gfp_t flags,
> > int node)
> > {
> > - struct mem_cgroup *old_memcg;
> > + struct obj_cgroup *old_objcg;
> > void *ptr;
> >
> > - old_memcg = set_active_memcg(map->memcg);
> > + old_objcg = set_active_obj_cgroup(map->objcg);
> > ptr = kmalloc_node(size, flags | __GFP_ACCOUNT, node);
> > - set_active_memcg(old_memcg);
> > + set_active_obj_cgroup(old_objcg);
> >
> > return ptr;
> > }
> >
> > void *bpf_map_kzalloc(const struct bpf_map *map, size_t size, gfp_t flags)
> > {
> > - struct mem_cgroup *old_memcg;
> > + struct obj_cgroup *old_objcg;
> > void *ptr;
> >
> > - old_memcg = set_active_memcg(map->memcg);
> > + old_objcg = set_active_obj_cgroup(map->objcg);
> > ptr = kzalloc(size, flags | __GFP_ACCOUNT);
> > - set_active_memcg(old_memcg);
> > + set_active_obj_cgroup(old_objcg);
> >
> > return ptr;
> > }
> > @@ -428,22 +429,22 @@ void *bpf_map_kzalloc(const struct bpf_map *map, size_t size, gfp_t flags)
> > void __percpu *bpf_map_alloc_percpu(const struct bpf_map *map, size_t size,
> > size_t align, gfp_t flags)
> > {
> > - struct mem_cgroup *old_memcg;
> > + struct obj_cgroup *old_objcg;
> > void __percpu *ptr;
> >
> > - old_memcg = set_active_memcg(map->memcg);
> > + old_objcg = set_active_obj_cgroup(map->objcg);
> > ptr = __alloc_percpu_gfp(size, align, flags | __GFP_ACCOUNT);
> > - set_active_memcg(old_memcg);
> > + set_active_obj_cgroup(old_objcg);
> >
> > return ptr;
> > }
> >
> > #else
> > -static void bpf_map_save_memcg(struct bpf_map *map)
> > +static void bpf_map_save_objcg(struct bpf_map *map)
> > {
> > }
> >
> > -static void bpf_map_release_memcg(struct bpf_map *map)
> > +static void bpf_map_release_objcg(struct bpf_map *map)
> > {
> > }
> > #endif
> > @@ -454,7 +455,7 @@ static void bpf_map_free_deferred(struct work_struct *work)
> > struct bpf_map *map = container_of(work, struct bpf_map, work);
> >
> > security_bpf_map_free(map);
> > - bpf_map_release_memcg(map);
> > + bpf_map_release_objcg(map);
> > /* implementation dependent freeing */
> > map->ops->map_free(map);
> > }
> > @@ -877,7 +878,7 @@ static int map_create(union bpf_attr *attr)
> > if (err)
> > goto free_map_sec;
> >
> > - bpf_map_save_memcg(map);
> > + bpf_map_save_objcg(map);
> >
> > err = bpf_map_new_fd(map, f_flags);
> > if (err < 0) {
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index d66718bc82d5..5a800916ad8d 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -943,7 +943,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
> > #endif
> >
> > #ifdef CONFIG_MEMCG_KMEM
> > - tsk->active_memcg = NULL;
> > + tsk->active_objcg = NULL;
> > #endif
> > return tsk;
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 092dc4588b43..024a0f377eb7 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -77,8 +77,8 @@ EXPORT_SYMBOL(memory_cgrp_subsys);
> > struct mem_cgroup *root_mem_cgroup __read_mostly;
> >
> > #ifdef CONFIG_MEMCG_KMEM
> > -/* Active memory cgroup to use from an interrupt context */
> > -DEFINE_PER_CPU(struct mem_cgroup *, int_active_memcg);
> > +/* Active object cgroup to use from an interrupt context */
> > +DEFINE_PER_CPU(struct obj_cgroup *, int_active_objcg);
> > #endif
> >
> > /* Socket memory accounting disabled? */
> > @@ -1057,18 +1057,18 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
> > EXPORT_SYMBOL(get_mem_cgroup_from_mm);
> >
> > #ifdef CONFIG_MEMCG_KMEM
> > -static __always_inline struct mem_cgroup *active_memcg(void)
> > +static __always_inline struct obj_cgroup *active_obj_cgroup(void)
> > {
> > if (in_interrupt())
> > - return this_cpu_read(int_active_memcg);
> > + return this_cpu_read(int_active_objcg);
> > else
> > - return current->active_memcg;
> > + return current->active_objcg;
> > }
> >
> > static __always_inline bool memcg_kmem_bypass(void)
> > {
> > /* Allow remote memcg charging from any context. */
> > - if (unlikely(active_memcg()))
> > + if (unlikely(active_obj_cgroup()))
> > return false;
> >
> > /* Memcg to charge can't be determined. */
> > @@ -2971,26 +2971,47 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
> > return page_memcg_check(page);
> > }
> >
> > -__always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
> > +__always_inline
> > +struct obj_cgroup *get_obj_cgroup_from_mem_cgroup(struct mem_cgroup *memcg)
> > {
> > struct obj_cgroup *objcg = NULL;
> > +
> > + rcu_read_lock();
> > + for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
> > + objcg = rcu_dereference(memcg->objcg);
> > + if (objcg && obj_cgroup_tryget(objcg))
> > + break;
> > + objcg = NULL;
> > + }
> > + rcu_read_unlock();
> > +
> > + return objcg;
> > +}
> > +
> > +__always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
> > +{
> > + struct obj_cgroup *objcg;
> > struct mem_cgroup *memcg;
> >
> > if (memcg_kmem_bypass())
> > return NULL;
> >
> > rcu_read_lock();
> > - if (unlikely(active_memcg()))
> > - memcg = active_memcg();
> > - else
> > - memcg = mem_cgroup_from_task(current);
> > + objcg = active_obj_cgroup();
> > + if (unlikely(objcg)) {
> > + /* remote object cgroup must hold a reference. */
> > + obj_cgroup_get(objcg);
> > + goto out;
> > + }
> >
> > + memcg = mem_cgroup_from_task(current);
> > for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg)) {
> > objcg = rcu_dereference(memcg->objcg);
> > if (objcg && obj_cgroup_tryget(objcg))
> > break;
> > objcg = NULL;
> > }
> > +out:
> > rcu_read_unlock();
> >
> > return objcg;
> > @@ -5296,16 +5317,33 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
> > return ERR_PTR(error);
> > }
> >
> > +#ifdef CONFIG_MEMCG_KMEM
> > +static inline struct obj_cgroup *memcg_obj_cgroup(struct mem_cgroup *memcg)
> > +{
> > + return memcg ? memcg->objcg : NULL;
> > +}
> > +#else
> > +static inline struct obj_cgroup *memcg_obj_cgroup(struct mem_cgroup *memcg)
> > +{
> > + return NULL;
> > +}
> > +#endif
> > +
> > static struct cgroup_subsys_state * __ref
> > mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
> > {
> > struct mem_cgroup *parent = mem_cgroup_from_css(parent_css);
> > - struct mem_cgroup *memcg, *old_memcg;
> > + struct mem_cgroup *memcg;
> > + struct obj_cgroup *old_objcg;
> > long error = -ENOMEM;
> >
> > - old_memcg = set_active_memcg(parent);
> > + /*
> > + * The @parent cannot be offlined, so @parent->objcg cannot be freed
> > + * under us.
> > + */
> > + old_objcg = set_active_obj_cgroup(memcg_obj_cgroup(parent));
> > memcg = mem_cgroup_alloc();
> > - set_active_memcg(old_memcg);
> > + set_active_obj_cgroup(old_objcg);
> > if (IS_ERR(memcg))
> > return ERR_CAST(memcg);
> >
> > --
> > 2.11.0
> >
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