Re: [PATCH 1/3] cgroup: fix pre_destroy and semantics of css->refcnt

[Li Zefan]

KAMEZAWA Hiroyuki wrote:
> Now, final check of refcnt is done after pre_destroy(), so rmdir() can fail
> after pre_destroy().
> memcg set mem->obsolete to be 1 at pre_destroy and this is buggy..
> 
> Several ways to fix this can be considered. This is an idea.
> 

I don't see what's the difference with css_under_removal() in this patch and
cgroup_is_removed() which is currently available.

CGRP_REMOVED flag is set in cgroup_rmdir() when it's confirmed that rmdir can
be sucessfully performed.

So mem->obsolete can be replaced with:

bool mem_cgroup_is_obsolete(struct mem_cgroup *mem)
{
	return cgroup_is_removed(mem->css.cgroup);
}

Or am I missing something?

> Fortunately, the user of css_get()/css_put() is only memcg, now.
> And it seems assumption on css_ref in cgroup.c is a bit complicated.
> I'd like to reuse it.
> This patch changes this css->refcnt usage and action as following
> 	- css->refcnt is initialized to 1.
> 
> 	- after pre_destroy, before destroy(), try to drop css->refcnt to 0.
> 
> 	- css_tryget() is added. This only success when css->refcnt > 0.
> 
> 	- css_is_removed() is added. This checks css->refcnt == 0 and means
> 	  this cgroup is under destroy() or not.
> 
> 	- css_put() is changed not to call notify_on_release().
> 	  From documentation, notify_on_release() is called when there is no
> 	  tasks/children in cgroup. On implementation, notify_on_release is
> 	  not called if css->refcnt > 0.
> 	  This is problematic. memcg has css->refcnt by each page even when
> 	  there are no tasks. release handler will be never called.
> 	  But, now, rmdir()/pre_destroy() of memcg works well and checking
> 	  checking css->ref is not (and shouldn't be) necessary for notifying.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxx>
> 
> 
>  include/linux/cgroup.h |   21 +++++++++++++++++--
>  kernel/cgroup.c        |   53 +++++++++++++++++++++++++++++++++++--------------
>  mm/memcontrol.c        |   40 +++++++++++++++++++++++++-----------
>  3 files changed, 85 insertions(+), 29 deletions(-)
> 
> Index: mmotm-2.6.28-Nov29/include/linux/cgroup.h
> ===================================================================
> --- mmotm-2.6.28-Nov29.orig/include/linux/cgroup.h
> +++ mmotm-2.6.28-Nov29/include/linux/cgroup.h
> @@ -54,7 +54,9 @@ struct cgroup_subsys_state {
>  
>  	/* State maintained by the cgroup system to allow
>  	 * subsystems to be "busy". Should be accessed via css_get()
> -	 * and css_put() */
> +	 * and css_put(). If this value is 0, css is now under removal and
> +	 * destroy() will be called soon. (and there is no roll-back.)
> +	 */
>  
>  	atomic_t refcnt;
>  
> @@ -86,7 +88,22 @@ extern void __css_put(struct cgroup_subs
>  static inline void css_put(struct cgroup_subsys_state *css)
>  {
>  	if (!test_bit(CSS_ROOT, &css->flags))
> -		__css_put(css);
> +		atomic_dec(&css->refcnt);
> +}
> +
> +/* returns not-zero if success */
> +static inline int css_tryget(struct cgroup_subsys_state *css)
> +{
> +	if (!test_bit(CSS_ROOT, &css->flags))
> +		return atomic_inc_not_zero(&css->refcnt);
> +	return 1;
> +}
> +
> +static inline bool css_under_removal(struct cgroup_subsys_state *css)
> +{
> +	if (test_bit(CSS_ROOT, &css->flags))
> +		return false;
> +	return atomic_read(&css->refcnt) == 0;
>  }
>  
>  /* bits in struct cgroup flags field */
> Index: mmotm-2.6.28-Nov29/kernel/cgroup.c
> ===================================================================
> --- mmotm-2.6.28-Nov29.orig/kernel/cgroup.c
> +++ mmotm-2.6.28-Nov29/kernel/cgroup.c
> @@ -589,6 +589,32 @@ static void cgroup_call_pre_destroy(stru
>  	return;
>  }
>  
> +/*
> + * Try to set all subsys's refcnt to be 0.
> + * css->refcnt==0 means this subsys will be destroy()'d.
> + */
> +static bool cgroup_set_subsys_removed(struct cgroup *cgrp)
> +{
> +	struct cgroup_subsys *ss;
> +	struct cgroup_subsys_state *css, *tmp;
> +
> +	for_each_subsys(cgrp->root, ss) {
> +		css = cgrp->subsys[ss->subsys_id];
> +		if (!atomic_dec_and_test(&css->refcnt))
> +			goto rollback;
> +	}
> +	return true;
> +rollback:
> +	for_each_subsys(cgrp->root, ss) {
> +		tmp = cgrp->subsys[ss->subsys_id];
> +		atomic_inc(&tmp->refcnt);
> +		if (tmp == css)
> +			break;
> +	}
> +	return false;
> +}
> +
> +
>  static void cgroup_diput(struct dentry *dentry, struct inode *inode)
>  {
>  	/* is dentry a directory ? if so, kfree() associated cgroup */
> @@ -2310,7 +2336,7 @@ static void init_cgroup_css(struct cgrou
>  			       struct cgroup *cgrp)
>  {
>  	css->cgroup = cgrp;
> -	atomic_set(&css->refcnt, 0);
> +	atomic_set(&css->refcnt, 1);
>  	css->flags = 0;
>  	if (cgrp == dummytop)
>  		set_bit(CSS_ROOT, &css->flags);
> @@ -2438,7 +2464,7 @@ static int cgroup_has_css_refs(struct cg
>  		 * matter, since it can only happen if the cgroup
>  		 * has been deleted and hence no longer needs the
>  		 * release agent to be called anyway. */
> -		if (css && atomic_read(&css->refcnt))
> +		if (css && (atomic_read(&css->refcnt) > 1))
>  			return 1;
>  	}
>  	return 0;
> @@ -2465,7 +2491,8 @@ static int cgroup_rmdir(struct inode *un
>  
>  	/*
>  	 * Call pre_destroy handlers of subsys. Notify subsystems
> -	 * that rmdir() request comes.
> +	 * that rmdir() request comes. pre_destroy() is expected to drop all
> +	 * extra refcnt to css. (css->refcnt == 1)
>  	 */
>  	cgroup_call_pre_destroy(cgrp);
>  
> @@ -2479,8 +2506,15 @@ static int cgroup_rmdir(struct inode *un
>  		return -EBUSY;
>  	}
>  
> +	/* last check ! */
> +	if (!cgroup_set_subsys_removed(cgrp)) {
> +		mutex_unlock(&cgroup_mutex);
> +		return -EBUSY;
> +	}
> +
>  	spin_lock(&release_list_lock);
>  	set_bit(CGRP_REMOVED, &cgrp->flags);
> +
>  	if (!list_empty(&cgrp->release_list))
>  		list_del(&cgrp->release_list);
>  	spin_unlock(&release_list_lock);
> @@ -3003,7 +3037,7 @@ static void check_for_release(struct cgr
>  	/* All of these checks rely on RCU to keep the cgroup
>  	 * structure alive */
>  	if (cgroup_is_releasable(cgrp) && !atomic_read(&cgrp->count)
> -	    && list_empty(&cgrp->children) && !cgroup_has_css_refs(cgrp)) {
> +	    && list_empty(&cgrp->children)) {
>  		/* Control Group is currently removeable. If it's not
>  		 * already queued for a userspace notification, queue
>  		 * it now */
> @@ -3020,17 +3054,6 @@ static void check_for_release(struct cgr
>  	}
>  }
>  
> -void __css_put(struct cgroup_subsys_state *css)
> -{
> -	struct cgroup *cgrp = css->cgroup;
> -	rcu_read_lock();
> -	if (atomic_dec_and_test(&css->refcnt) && notify_on_release(cgrp)) {
> -		set_bit(CGRP_RELEASABLE, &cgrp->flags);
> -		check_for_release(cgrp);
> -	}
> -	rcu_read_unlock();
> -}
> -
>  /*
>   * Notify userspace when a cgroup is released, by running the
>   * configured release agent with the name of the cgroup (path
> Index: mmotm-2.6.28-Nov29/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.28-Nov29.orig/mm/memcontrol.c
> +++ mmotm-2.6.28-Nov29/mm/memcontrol.c
> @@ -154,7 +154,6 @@ struct mem_cgroup {
>  	 */
>  	bool use_hierarchy;
>  	unsigned long	last_oom_jiffies;
> -	int		obsolete;
>  	atomic_t	refcnt;
>  	/*
>  	 * statistics. This must be placed at the end of memcg.
> @@ -540,8 +539,14 @@ mem_cgroup_get_first_node(struct mem_cgr
>  {
>  	struct cgroup *cgroup;
>  	struct mem_cgroup *ret;
> -	bool obsolete = (root_mem->last_scanned_child &&
> -				root_mem->last_scanned_child->obsolete);
> +	struct mem_cgroup *last_scan = root_mem->last_scanned_child;
> +	bool obsolete = false;
> +
> +	if (last_scan) {
> +		if (css_under_removal(&last_scan->css))
> +			obsolete = true;
> +	} else
> +		obsolete = true;
>  
>  	/*
>  	 * Scan all children under the mem_cgroup mem
> @@ -598,7 +603,7 @@ static int mem_cgroup_hierarchical_recla
>  	next_mem = mem_cgroup_get_first_node(root_mem);
>  
>  	while (next_mem != root_mem) {
> -		if (next_mem->obsolete) {
> +		if (css_under_removal(&next_mem->css)) {
>  			mem_cgroup_put(next_mem);
>  			cgroup_lock();
>  			next_mem = mem_cgroup_get_first_node(root_mem);
> @@ -985,6 +990,7 @@ int mem_cgroup_try_charge_swapin(struct 
>  {
>  	struct mem_cgroup *mem;
>  	swp_entry_t     ent;
> +	int ret;
>  
>  	if (mem_cgroup_disabled())
>  		return 0;
> @@ -1003,10 +1009,18 @@ int mem_cgroup_try_charge_swapin(struct 
>  	ent.val = page_private(page);
>  
>  	mem = lookup_swap_cgroup(ent);
> -	if (!mem || mem->obsolete)
> +	/*
> +	 * Because we can't assume "mem" is alive now, use tryget() and
> +	 * drop extra count later
> +	 */
> +	if (!mem || !css_tryget(&mem->css))
>  		goto charge_cur_mm;
>  	*ptr = mem;
> -	return __mem_cgroup_try_charge(NULL, mask, ptr, true);
> +	ret = __mem_cgroup_try_charge(NULL, mask, ptr, true);
> +	/* drop extra count */
> +	css_put(&mem->css);
> +
> +	return ret;
>  charge_cur_mm:
>  	if (unlikely(!mm))
>  		mm = &init_mm;
> @@ -1037,14 +1051,16 @@ int mem_cgroup_cache_charge_swapin(struc
>  		ent.val = page_private(page);
>  		if (do_swap_account) {
>  			mem = lookup_swap_cgroup(ent);
> -			if (mem && mem->obsolete)
> +			if (mem && !css_tryget(&mem->css))
>  				mem = NULL;
>  			if (mem)
>  				mm = NULL;
>  		}
>  		ret = mem_cgroup_charge_common(page, mm, mask,
>  				MEM_CGROUP_CHARGE_TYPE_SHMEM, mem);
> -
> +		/* drop extra ref */
> +		if (mem)
> +			css_put(&mem->css);
>  		if (!ret && do_swap_account) {
>  			/* avoid double counting */
>  			mem = swap_cgroup_record(ent, NULL);
> @@ -1886,8 +1902,8 @@ static struct mem_cgroup *mem_cgroup_all
>   * the number of reference from swap_cgroup and free mem_cgroup when
>   * it goes down to 0.
>   *
> - * When mem_cgroup is destroyed, mem->obsolete will be set to 0 and
> - * entry which points to this memcg will be ignore at swapin.
> + * When mem_cgroup is destroyed, css_under_removal() is true and entry which
> + * points to this memcg will be ignore at swapin.
>   *
>   * Removal of cgroup itself succeeds regardless of refs from swap.
>   */
> @@ -1917,7 +1933,7 @@ static void mem_cgroup_get(struct mem_cg
>  static void mem_cgroup_put(struct mem_cgroup *mem)
>  {
>  	if (atomic_dec_and_test(&mem->refcnt)) {
> -		if (!mem->obsolete)
> +		if (!css_under_removal(&mem->css))
>  			return;
>  		mem_cgroup_free(mem);
>  	}
> @@ -1980,7 +1996,7 @@ static void mem_cgroup_pre_destroy(struc
>  					struct cgroup *cont)
>  {
>  	struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
> -	mem->obsolete = 1;
> +	/* dentry's mutex makes this safe. */
>  	mem_cgroup_force_empty(mem, false);
>  }
>  
> 
> 
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