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Date:   Fri, 25 May 2018 01:37:10 +0200
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Michal Hocko <mhocko@kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Kirill Tkhai <ktkhai@virtuozzo.com>, peterz@infradead.org,
        viro@zeniv.linux.org.uk, mingo@kernel.org,
        paulmck@linux.vnet.ibm.com, keescook@chromium.org, riel@redhat.com,
        tglx@linutronix.de, kirill.shutemov@linux.intel.com,
        marcos.souza.org@gmail.com, hoeun.ryu@gmail.com,
        pasha.tatashin@oracle.com, gs051095@gmail.com, dhowells@redhat.com,
        rppt@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        Tejun Heo <tj@kernel.org>, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH 0/2] mm->owner to mm->memcg fixes
Message-ID: <20180524233710.GA2993@andrea>
References: <20180504145435.GA26573@redhat.com>
 <87y3gzfmjt.fsf@xmission.com>
 <20180504162209.GB26573@redhat.com>
 <871serfk77.fsf@xmission.com>
 <87tvrncoyc.fsf_-_@xmission.com>
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 <20180522125757.GL20020@dhcp22.suse.cz>
 <87wovu889o.fsf@xmission.com>
 <20180524111002.GB20441@dhcp22.suse.cz>
 <20180524141635.c99b7025a73a709e179f92a2@linux-foundation.org>
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On Thu, May 24, 2018 at 02:16:35PM -0700, Andrew Morton wrote:
> On Thu, 24 May 2018 13:10:02 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> 
> > I would really prefer and appreciate a repost with all the fixes folded
> > in.
> 
> [1/2]
> 
> From: "Eric W. Biederman" <ebiederm@xmission.com>
> Subject: memcg: replace mm->owner with mm->memcg
> 
> Recently it was reported that mm_update_next_owner could get into cases
> where it was executing its fallback for_each_process part of the loop and
> thus taking up a lot of time.

Reference?


> 
> To deal with this replace mm->owner with mm->memcg.  This just reduces the
> complexity of everything.

"the complexity of everything"?


> As much as possible I have maintained the
> current semantics.

"As much as possible"?


> There are two siginificant exceptions.

s/siginificant/significant


> During fork
> the memcg of the process calling fork is charged rather than init_css_set.
> During memory cgroup migration the charges are migrated not if the
> process is the owner of the mm, but if the process being migrated has the
> same memory cgroup as the mm.
> 
> I believe it was a bug

It was a bug or not??


> if init_css_set is charged for memory activity
> during fork, and the old behavior was simply a consequence of the new task
> not having tsk->cgroup not initialized to it's proper cgroup.
> 
> During cgroup migration only thread group leaders are allowed to migrate. 
> Which means in practice there should only be one.

"in practice there should"??


> Linux tasks created
> with CLONE_VM are the only exception, but the common cases are already
> ruled out.  Processes created with vfork have a suspended parent and can
> do nothing but call exec so they should never show up.  Threads of the
> same cgroup are not the thread group leader so also should not show up. 
> That leaves the old LinuxThreads library which is probably out of use by

"probably"???


> now, and someone doing something very creative with cgroups,

"very creative"?


> and rolling
> their own threads with CLONE_VM.  So in practice I don't think

"in practice I don't think"??

  Andrea


> the
> difference charge migration will affect anyone.
> 
> To ensure that mm->memcg is updated appropriately I have implemented
> cgroup "attach" and "fork" methods.  This ensures that at those points the
> mm pointed to the task has the appropriate memory cgroup.
> 
> For simplicity instead of introducing a new mm lock I simply use exchange
> on the pointer where the mm->memcg is updated to get atomic updates.
> 
> Looking at the history effectively this change is a revert.  The reason
> given for adding mm->owner is so that multiple cgroups can be attached to
> the same mm.  In the last 8 years a second user of mm->owner has not
> appeared.  A feature that has never used, makes the code more complicated
> and has horrible worst case performance should go.
> 
> [ebiederm@xmission.com: update to work when !CONFIG_MMU]
>   Link: http://lkml.kernel.org/r/87lgczcox0.fsf_-_@xmission.com
> [ebiederm@xmission.com: close race between migration and installing bprm->mm as mm]
>   Link: http://lkml.kernel.org/r/87fu37cow4.fsf_-_@xmission.com
> Link: http://lkml.kernel.org/r/87lgd1zww0.fsf_-_@xmission.com
> Fixes: cf475ad28ac3 ("cgroups: add an owner to the mm_struct")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> Reported-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  fs/exec.c                  |    3 -
>  include/linux/memcontrol.h |   16 +++++-
>  include/linux/mm_types.h   |   12 ----
>  include/linux/sched/mm.h   |    8 ---
>  kernel/exit.c              |   89 -----------------------------------
>  kernel/fork.c              |   17 +++++-
>  mm/debug.c                 |    4 -
>  mm/memcontrol.c            |   81 +++++++++++++++++++++++--------
>  8 files changed, 93 insertions(+), 137 deletions(-)
> 
> diff -puN fs/exec.c~memcg-replace-mm-owner-with-mm-memcg fs/exec.c
> --- a/fs/exec.c~memcg-replace-mm-owner-with-mm-memcg
> +++ a/fs/exec.c
> @@ -1040,11 +1040,12 @@ static int exec_mmap(struct mm_struct *m
>  		up_read(&old_mm->mmap_sem);
>  		BUG_ON(active_mm != old_mm);
>  		setmax_mm_hiwater_rss(&tsk->signal->maxrss, old_mm);
> -		mm_update_next_owner(old_mm);
>  		mmput(old_mm);
>  		return 0;
>  	}
>  	mmdrop(active_mm);
> +	/* The tsk may have migrated before the new mm was attached */
> +	mm_sync_memcg_from_task(tsk);
>  	return 0;
>  }
>  
> diff -puN include/linux/memcontrol.h~memcg-replace-mm-owner-with-mm-memcg include/linux/memcontrol.h
> --- a/include/linux/memcontrol.h~memcg-replace-mm-owner-with-mm-memcg
> +++ a/include/linux/memcontrol.h
> @@ -345,7 +345,6 @@ out:
>  struct lruvec *mem_cgroup_page_lruvec(struct page *, struct pglist_data *);
>  
>  bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg);
> -struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);
>  
>  struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm);
>  
> @@ -408,6 +407,9 @@ static inline bool mem_cgroup_is_descend
>  	return cgroup_is_descendant(memcg->css.cgroup, root->css.cgroup);
>  }
>  
> +void mm_update_memcg(struct mm_struct *mm, struct mem_cgroup *new);
> +void mm_sync_memcg_from_task(struct task_struct *tsk);
> +
>  static inline bool mm_match_cgroup(struct mm_struct *mm,
>  				   struct mem_cgroup *memcg)
>  {
> @@ -415,7 +417,7 @@ static inline bool mm_match_cgroup(struc
>  	bool match = false;
>  
>  	rcu_read_lock();
> -	task_memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> +	task_memcg = rcu_dereference(mm->memcg);
>  	if (task_memcg)
>  		match = mem_cgroup_is_descendant(task_memcg, memcg);
>  	rcu_read_unlock();
> @@ -699,7 +701,7 @@ static inline void count_memcg_event_mm(
>  		return;
>  
>  	rcu_read_lock();
> -	memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> +	memcg = rcu_dereference(mm->memcg);
>  	if (likely(memcg)) {
>  		count_memcg_events(memcg, idx, 1);
>  		if (idx == OOM_KILL)
> @@ -787,6 +789,14 @@ static inline struct lruvec *mem_cgroup_
>  	return &pgdat->lruvec;
>  }
>  
> +static inline void mm_update_memcg(struct mm_struct *mm, struct mem_cgroup *new)
> +{
> +}
> +
> +static inline void mm_sync_memcg_from_task(struct task_struct *tsk)
> +{
> +}
> +
>  static inline bool mm_match_cgroup(struct mm_struct *mm,
>  		struct mem_cgroup *memcg)
>  {
> diff -puN include/linux/mm_types.h~memcg-replace-mm-owner-with-mm-memcg include/linux/mm_types.h
> --- a/include/linux/mm_types.h~memcg-replace-mm-owner-with-mm-memcg
> +++ a/include/linux/mm_types.h
> @@ -445,17 +445,7 @@ struct mm_struct {
>  	struct kioctx_table __rcu	*ioctx_table;
>  #endif
>  #ifdef CONFIG_MEMCG
> -	/*
> -	 * "owner" points to a task that is regarded as the canonical
> -	 * user/owner of this mm. All of the following must be true in
> -	 * order for it to be changed:
> -	 *
> -	 * current == mm->owner
> -	 * current->mm != mm
> -	 * new_owner->mm == mm
> -	 * new_owner->alloc_lock is held
> -	 */
> -	struct task_struct __rcu *owner;
> +	struct mem_cgroup __rcu	*memcg;
>  #endif
>  	struct user_namespace *user_ns;
>  
> diff -puN include/linux/sched/mm.h~memcg-replace-mm-owner-with-mm-memcg include/linux/sched/mm.h
> --- a/include/linux/sched/mm.h~memcg-replace-mm-owner-with-mm-memcg
> +++ a/include/linux/sched/mm.h
> @@ -95,14 +95,6 @@ extern struct mm_struct *mm_access(struc
>  /* Remove the current tasks stale references to the old mm_struct */
>  extern void mm_release(struct task_struct *, struct mm_struct *);
>  
> -#ifdef CONFIG_MEMCG
> -extern void mm_update_next_owner(struct mm_struct *mm);
> -#else
> -static inline void mm_update_next_owner(struct mm_struct *mm)
> -{
> -}
> -#endif /* CONFIG_MEMCG */
> -
>  #ifdef CONFIG_MMU
>  extern void arch_pick_mmap_layout(struct mm_struct *mm,
>  				  struct rlimit *rlim_stack);
> diff -puN kernel/exit.c~memcg-replace-mm-owner-with-mm-memcg kernel/exit.c
> --- a/kernel/exit.c~memcg-replace-mm-owner-with-mm-memcg
> +++ a/kernel/exit.c
> @@ -399,94 +399,6 @@ kill_orphaned_pgrp(struct task_struct *t
>  	}
>  }
>  
> -#ifdef CONFIG_MEMCG
> -/*
> - * A task is exiting.   If it owned this mm, find a new owner for the mm.
> - */
> -void mm_update_next_owner(struct mm_struct *mm)
> -{
> -	struct task_struct *c, *g, *p = current;
> -
> -retry:
> -	/*
> -	 * If the exiting or execing task is not the owner, it's
> -	 * someone else's problem.
> -	 */
> -	if (mm->owner != p)
> -		return;
> -	/*
> -	 * The current owner is exiting/execing and there are no other
> -	 * candidates.  Do not leave the mm pointing to a possibly
> -	 * freed task structure.
> -	 */
> -	if (atomic_read(&mm->mm_users) <= 1) {
> -		mm->owner = NULL;
> -		return;
> -	}
> -
> -	read_lock(&tasklist_lock);
> -	/*
> -	 * Search in the children
> -	 */
> -	list_for_each_entry(c, &p->children, sibling) {
> -		if (c->mm == mm)
> -			goto assign_new_owner;
> -	}
> -
> -	/*
> -	 * Search in the siblings
> -	 */
> -	list_for_each_entry(c, &p->real_parent->children, sibling) {
> -		if (c->mm == mm)
> -			goto assign_new_owner;
> -	}
> -
> -	/*
> -	 * Search through everything else, we should not get here often.
> -	 */
> -	for_each_process(g) {
> -		if (g->flags & PF_KTHREAD)
> -			continue;
> -		for_each_thread(g, c) {
> -			if (c->mm == mm)
> -				goto assign_new_owner;
> -			if (c->mm)
> -				break;
> -		}
> -	}
> -	read_unlock(&tasklist_lock);
> -	/*
> -	 * We found no owner yet mm_users > 1: this implies that we are
> -	 * most likely racing with swapoff (try_to_unuse()) or /proc or
> -	 * ptrace or page migration (get_task_mm()).  Mark owner as NULL.
> -	 */
> -	mm->owner = NULL;
> -	return;
> -
> -assign_new_owner:
> -	BUG_ON(c == p);
> -	get_task_struct(c);
> -	/*
> -	 * The task_lock protects c->mm from changing.
> -	 * We always want mm->owner->mm == mm
> -	 */
> -	task_lock(c);
> -	/*
> -	 * Delay read_unlock() till we have the task_lock()
> -	 * to ensure that c does not slip away underneath us
> -	 */
> -	read_unlock(&tasklist_lock);
> -	if (c->mm != mm) {
> -		task_unlock(c);
> -		put_task_struct(c);
> -		goto retry;
> -	}
> -	mm->owner = c;
> -	task_unlock(c);
> -	put_task_struct(c);
> -}
> -#endif /* CONFIG_MEMCG */
> -
>  /*
>   * Turn us into a lazy TLB process if we
>   * aren't already..
> @@ -540,7 +452,6 @@ static void exit_mm(void)
>  	up_read(&mm->mmap_sem);
>  	enter_lazy_tlb(mm, current);
>  	task_unlock(current);
> -	mm_update_next_owner(mm);
>  	mmput(mm);
>  	if (test_thread_flag(TIF_MEMDIE))
>  		exit_oom_victim();
> diff -puN kernel/fork.c~memcg-replace-mm-owner-with-mm-memcg kernel/fork.c
> --- a/kernel/fork.c~memcg-replace-mm-owner-with-mm-memcg
> +++ a/kernel/fork.c
> @@ -878,10 +878,19 @@ static void mm_init_aio(struct mm_struct
>  #endif
>  }
>  
> -static void mm_init_owner(struct mm_struct *mm, struct task_struct *p)
> +static void mm_init_memcg(struct mm_struct *mm)
>  {
>  #ifdef CONFIG_MEMCG
> -	mm->owner = p;
> +	struct cgroup_subsys_state *css;
> +
> +	/* Ensure mm->memcg is initialized */
> +	mm->memcg = NULL;
> +
> +	rcu_read_lock();
> +	css = task_css(current, memory_cgrp_id);
> +	if (css && css_tryget(css))
> +		mm_update_memcg(mm, mem_cgroup_from_css(css));
> +	rcu_read_unlock();
>  #endif
>  }
>  
> @@ -912,7 +921,7 @@ static struct mm_struct *mm_init(struct
>  	spin_lock_init(&mm->arg_lock);
>  	mm_init_cpumask(mm);
>  	mm_init_aio(mm);
> -	mm_init_owner(mm, p);
> +	mm_init_memcg(mm);
>  	RCU_INIT_POINTER(mm->exe_file, NULL);
>  	mmu_notifier_mm_init(mm);
>  	hmm_mm_init(mm);
> @@ -942,6 +951,7 @@ static struct mm_struct *mm_init(struct
>  fail_nocontext:
>  	mm_free_pgd(mm);
>  fail_nopgd:
> +	mm_update_memcg(mm, NULL);
>  	free_mm(mm);
>  	return NULL;
>  }
> @@ -979,6 +989,7 @@ static inline void __mmput(struct mm_str
>  	}
>  	if (mm->binfmt)
>  		module_put(mm->binfmt->module);
> +	mm_update_memcg(mm, NULL);
>  	mmdrop(mm);
>  }
>  
> diff -puN mm/debug.c~memcg-replace-mm-owner-with-mm-memcg mm/debug.c
> --- a/mm/debug.c~memcg-replace-mm-owner-with-mm-memcg
> +++ a/mm/debug.c
> @@ -116,7 +116,7 @@ void dump_mm(const struct mm_struct *mm)
>  		"ioctx_table %px\n"
>  #endif
>  #ifdef CONFIG_MEMCG
> -		"owner %px "
> +		"memcg %px "
>  #endif
>  		"exe_file %px\n"
>  #ifdef CONFIG_MMU_NOTIFIER
> @@ -147,7 +147,7 @@ void dump_mm(const struct mm_struct *mm)
>  		mm->ioctx_table,
>  #endif
>  #ifdef CONFIG_MEMCG
> -		mm->owner,
> +		mm->memcg,
>  #endif
>  		mm->exe_file,
>  #ifdef CONFIG_MMU_NOTIFIER
> diff -puN mm/memcontrol.c~memcg-replace-mm-owner-with-mm-memcg mm/memcontrol.c
> --- a/mm/memcontrol.c~memcg-replace-mm-owner-with-mm-memcg
> +++ a/mm/memcontrol.c
> @@ -664,20 +664,6 @@ static void memcg_check_events(struct me
>  	}
>  }
>  
> -struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
> -{
> -	/*
> -	 * mm_update_next_owner() may clear mm->owner to NULL
> -	 * if it races with swapoff, page migration, etc.
> -	 * So this can be called with p == NULL.
> -	 */
> -	if (unlikely(!p))
> -		return NULL;
> -
> -	return mem_cgroup_from_css(task_css(p, memory_cgrp_id));
> -}
> -EXPORT_SYMBOL(mem_cgroup_from_task);
> -
>  struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
>  {
>  	struct mem_cgroup *memcg = NULL;
> @@ -692,7 +678,7 @@ struct mem_cgroup *get_mem_cgroup_from_m
>  		if (unlikely(!mm))
>  			memcg = root_mem_cgroup;
>  		else {
> -			memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> +			memcg = rcu_dereference(mm->memcg);
>  			if (unlikely(!memcg))
>  				memcg = root_mem_cgroup;
>  		}
> @@ -1025,7 +1011,7 @@ bool task_in_mem_cgroup(struct task_stru
>  		 * killed to prevent needlessly killing additional tasks.
>  		 */
>  		rcu_read_lock();
> -		task_memcg = mem_cgroup_from_task(task);
> +		task_memcg = mem_cgroup_from_css(task_css(task, memory_cgrp_id));
>  		css_get(&task_memcg->css);
>  		rcu_read_unlock();
>  	}
> @@ -4850,15 +4836,16 @@ static int mem_cgroup_can_attach(struct
>  	if (!move_flags)
>  		return 0;
>  
> -	from = mem_cgroup_from_task(p);
> +	from = mem_cgroup_from_css(task_css(p, memory_cgrp_id));
>  
>  	VM_BUG_ON(from == memcg);
>  
>  	mm = get_task_mm(p);
>  	if (!mm)
>  		return 0;
> -	/* We move charges only when we move a owner of the mm */
> -	if (mm->owner == p) {
> +
> +	/* We move charges except for creative uses of CLONE_VM */
> +	if (mm->memcg == from) {
>  		VM_BUG_ON(mc.from);
>  		VM_BUG_ON(mc.to);
>  		VM_BUG_ON(mc.precharge);
> @@ -5058,6 +5045,58 @@ static void mem_cgroup_move_task(void)
>  }
>  #endif
>  
> +/**
> + * mm_update_memcg - Update the memory cgroup of a mm_struct
> + * @mm: mm struct
> + * @new: new memory cgroup value
> + *
> + * Called whenever mm->memcg needs to change.   Consumes a reference
> + * to new (unless new is NULL).   The reference to the old memory
> + * cgroup is decreased.
> + */
> +void mm_update_memcg(struct mm_struct *mm, struct mem_cgroup *new)
> +{
> +	/* This is the only place where mm->memcg is changed */
> +	struct mem_cgroup *old;
> +
> +	old = xchg(&mm->memcg, new);
> +	if (old)
> +		css_put(&old->css);
> +}
> +
> +static void task_update_memcg(struct task_struct *tsk, struct mem_cgroup *new)
> +{
> +	struct mm_struct *mm;
> +	task_lock(tsk);
> +	mm = tsk->mm;
> +	if (mm && !(tsk->flags & PF_KTHREAD))
> +		mm_update_memcg(mm, new);
> +	task_unlock(tsk);
> +}
> +
> +static void mem_cgroup_attach(struct cgroup_taskset *tset)
> +{
> +	struct cgroup_subsys_state *css;
> +	struct task_struct *tsk;
> +
> +	cgroup_taskset_for_each(tsk, css, tset) {
> +		struct mem_cgroup *new = mem_cgroup_from_css(css);
> +		css_get(css);
> +		task_update_memcg(tsk, new);
> +	}
> +}
> +
> +void mm_sync_memcg_from_task(struct task_struct *tsk)
> +{
> +	struct cgroup_subsys_state *css;
> +
> +	rcu_read_lock();
> +	css = task_css(tsk, memory_cgrp_id);
> +	if (css && css_tryget(css))
> +		task_update_memcg(tsk, mem_cgroup_from_css(css));
> +	rcu_read_unlock();
> +}
> +
>  /*
>   * Cgroup retains root cgroups across [un]mount cycles making it necessary
>   * to verify whether we're attached to the default hierarchy on each mount
> @@ -5358,8 +5397,10 @@ struct cgroup_subsys memory_cgrp_subsys
>  	.css_free = mem_cgroup_css_free,
>  	.css_reset = mem_cgroup_css_reset,
>  	.can_attach = mem_cgroup_can_attach,
> +	.attach = mem_cgroup_attach,
>  	.cancel_attach = mem_cgroup_cancel_attach,
>  	.post_attach = mem_cgroup_move_task,
> +	.fork = mm_sync_memcg_from_task,
>  	.bind = mem_cgroup_bind,
>  	.dfl_cftypes = memory_files,
>  	.legacy_cftypes = mem_cgroup_legacy_files,
> @@ -5846,7 +5887,7 @@ void mem_cgroup_sk_alloc(struct sock *sk
>  	}
>  
>  	rcu_read_lock();
> -	memcg = mem_cgroup_from_task(current);
> +	memcg = mem_cgroup_from_css(task_css(current, memory_cgrp_id));
>  	if (memcg == root_mem_cgroup)
>  		goto out;
>  	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !memcg->tcpmem_active)
> _
>