Re: [PATCH] mm: use vmalloc fallback path for certain memcg allocations

[Michal Hocko <mhocko@suse.com>]

Thanks for CCing me Vlastimil

On Fri 02-12-16 09:44:23, Vlastimil Babka wrote:
> On 12/01/2016 02:16 AM, Anatoly Stepanov wrote:
> > As memcg array size can be up to:
> > sizeof(struct memcg_cache_array) + kmemcg_id * sizeof(void *);
> > 
> > where kmemcg_id can be up to MEMCG_CACHES_MAX_SIZE.
> > 
> > When a memcg instance count is large enough it can lead
> > to high order allocations up to order 7.

This is definitely not nice and worth fixing! I am just wondering
whether this is something you have encountered in the real life. Having
thousands of memcgs sounds quite crazy^Wscary to me. I am not at all
sure we are prepared for that and some controllers would have real
issues with it AFAIR.

> > The same story with memcg_lrus allocations.
> > So let's work this around by utilizing vmalloc fallback path.
> > 
> > Signed-off-by: Anatoly Stepanov <astepanov@cloudlinux.com>
> > ---
> >  include/linux/memcontrol.h | 16 ++++++++++++++++
> >  mm/list_lru.c              | 14 +++++++-------
> >  mm/slab_common.c           | 21 ++++++++++++++-------
> >  3 files changed, 37 insertions(+), 14 deletions(-)
> > 
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 61d20c1..a281622 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -29,6 +29,9 @@
> >  #include <linux/mmzone.h>
> >  #include <linux/writeback.h>
> >  #include <linux/page-flags.h>
> > +#include <linux/vmalloc.h>
> > +#include <linux/slab.h>
> > +#include <linux/mm.h>
> > 
> >  struct mem_cgroup;
> >  struct page;
> > @@ -878,4 +881,17 @@ static inline void memcg_kmem_update_page_stat(struct page *page,
> >  }
> >  #endif /* CONFIG_MEMCG && !CONFIG_SLOB */
> > 
> > +static inline void memcg_free(const void *ptr)
> > +{
> > +	is_vmalloc_addr(ptr) ? vfree(ptr) : kfree(ptr);
> > +}
> > +
> > +static inline void *memcg_alloc(size_t size)
> > +{
> > +	if (likely(size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER)))

you are mixing two different units here...

> > +		return kzalloc(size, GFP_KERNEL|__GFP_NORETRY);
> > +
> > +	return vzalloc(size);
> 
> That's not how I imagine a "fallback" to work. You should be trying
> kzalloc() and if that fails, call vzalloc(), not distinguish it by costly
> order check. Also IIRC __GFP_NORETRY can easily fail even for non-costly
> orders.

Completely agreed! This should be done simply by
	gfp_t gfp_mask = GFP_KERNEL;
	void *ret;
	
	/*
	 * Do not invoke OOM killer for larger requests as we can fall
	 * back to the vmalloc
	 */
	if (size > PAGE_SIZE)
		gfp_mask |= __GFP_NORETRY | __GFP_NOWARN;

	ret = kzalloc(size, gfp_mask);
	if (ret)
		return ret;
	return vzalloc(size);

I also do not like memcg_alloc helper name. It suggests we are
allocating a memcg while it is used for cache arrays and slab LRUS.
Anyway this pattern is quite widespread in the kernel so I would simply
suggest adding kvmalloc function instead.

> 
> > +}
> > +
> >  #endif /* _LINUX_MEMCONTROL_H */
> > diff --git a/mm/list_lru.c b/mm/list_lru.c
> > index 234676e..8f49339 100644
> > --- a/mm/list_lru.c
> > +++ b/mm/list_lru.c
> > @@ -327,12 +327,12 @@ static int memcg_init_list_lru_node(struct list_lru_node *nlru)
> >  {
> >  	int size = memcg_nr_cache_ids;
> > 
> > -	nlru->memcg_lrus = kmalloc(size * sizeof(void *), GFP_KERNEL);
> > -	if (!nlru->memcg_lrus)
> > +	nlru->memcg_lrus = memcg_alloc(size * sizeof(void *));
> > +	if (nlru->memcg_lrus == NULL)
> >  		return -ENOMEM;
> > 
> >  	if (__memcg_init_list_lru_node(nlru->memcg_lrus, 0, size)) {
> > -		kfree(nlru->memcg_lrus);
> > +		memcg_free(nlru->memcg_lrus);
> >  		return -ENOMEM;
> >  	}
> > 
> > @@ -342,7 +342,7 @@ static int memcg_init_list_lru_node(struct list_lru_node *nlru)
> >  static void memcg_destroy_list_lru_node(struct list_lru_node *nlru)
> >  {
> >  	__memcg_destroy_list_lru_node(nlru->memcg_lrus, 0, memcg_nr_cache_ids);
> > -	kfree(nlru->memcg_lrus);
> > +	memcg_free(nlru->memcg_lrus);
> >  }
> > 
> >  static int memcg_update_list_lru_node(struct list_lru_node *nlru,
> > @@ -353,12 +353,12 @@ static int memcg_update_list_lru_node(struct list_lru_node *nlru,
> >  	BUG_ON(old_size > new_size);
> > 
> >  	old = nlru->memcg_lrus;
> > -	new = kmalloc(new_size * sizeof(void *), GFP_KERNEL);
> > +	new = memcg_alloc(new_size * sizeof(void *));
> >  	if (!new)
> >  		return -ENOMEM;
> > 
> >  	if (__memcg_init_list_lru_node(new, old_size, new_size)) {
> > -		kfree(new);
> > +		memcg_free(new);
> >  		return -ENOMEM;
> >  	}
> > 
> > @@ -375,7 +375,7 @@ static int memcg_update_list_lru_node(struct list_lru_node *nlru,
> >  	nlru->memcg_lrus = new;
> >  	spin_unlock_irq(&nlru->lock);
> > 
> > -	kfree(old);
> > +	memcg_free(old);
> >  	return 0;
> >  }
> > 
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 329b038..19f8cb5 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -157,9 +157,8 @@ static int init_memcg_params(struct kmem_cache *s,
> >  	if (!memcg_nr_cache_ids)
> >  		return 0;
> > 
> > -	arr = kzalloc(sizeof(struct memcg_cache_array) +
> > -		      memcg_nr_cache_ids * sizeof(void *),
> > -		      GFP_KERNEL);
> > +	arr = memcg_alloc(sizeof(struct memcg_cache_array) +
> > +			memcg_nr_cache_ids * sizeof(void *));
> >  	if (!arr)
> >  		return -ENOMEM;
> > 
> > @@ -170,7 +169,15 @@ static int init_memcg_params(struct kmem_cache *s,
> >  static void destroy_memcg_params(struct kmem_cache *s)
> >  {
> >  	if (is_root_cache(s))
> > -		kfree(rcu_access_pointer(s->memcg_params.memcg_caches));
> > +		memcg_free(rcu_access_pointer(s->memcg_params.memcg_caches));
> > +}
> > +
> > +static void memcg_rcu_free(struct rcu_head *rcu)
> > +{
> > +	struct memcg_cache_array *arr;
> > +
> > +	arr = container_of(rcu, struct memcg_cache_array, rcu);
> > +	memcg_free(arr);
> >  }
> > 
> >  static int update_memcg_params(struct kmem_cache *s, int new_array_size)
> > @@ -180,8 +187,8 @@ static int update_memcg_params(struct kmem_cache *s, int new_array_size)
> >  	if (!is_root_cache(s))
> >  		return 0;
> > 
> > -	new = kzalloc(sizeof(struct memcg_cache_array) +
> > -		      new_array_size * sizeof(void *), GFP_KERNEL);
> > +	new = memcg_alloc(sizeof(struct memcg_cache_array) +
> > +				new_array_size * sizeof(void *));
> >  	if (!new)
> >  		return -ENOMEM;
> > 
> > @@ -193,7 +200,7 @@ static int update_memcg_params(struct kmem_cache *s, int new_array_size)
> > 
> >  	rcu_assign_pointer(s->memcg_params.memcg_caches, new);
> >  	if (old)
> > -		kfree_rcu(old, rcu);
> > +		call_rcu(&old->rcu, memcg_rcu_free);
> >  	return 0;
> >  }
> > 
> > 

-- 
Michal Hocko
SUSE Labs

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