Re: [PATCH 2/2] mm: memcg/slab: Don't create unfreeable slab

[Vlastimil Babka]

On 5/2/21 8:07 PM, Waiman Long wrote:
> The obj_cgroup array (memcg_data) embedded in the page structure is
> allocated at the first instance an accounted memory allocation happens.
> With the right size object, it is possible that the allocated obj_cgroup
> array comes from the same slab that requires memory accounting. If this
> happens, the slab will never become empty again as there is at least one
> object left (the obj_cgroup array) in the slab.
> 
> With instructmentation code added to detect this situation, I got 76
> hits on the kmalloc-192 slab when booting up a test kernel on a VM.
> So this can really happen.
> 
> To avoid the creation of these unfreeable slabs, a check is added to
> memcg_alloc_page_obj_cgroups() to detect that and double the size
> of the array in case it happens to make sure that it comes from a
> different kmemcache.
> 
> This change, however, does not completely eliminate the presence
> of unfreeable slabs which can still happen if a circular obj_cgroup
> array dependency is formed.

Hm this looks like only a half fix then.
I'm afraid the proper fix is for kmemcg to create own set of caches for the
arrays. It would also solve the recursive kfree() issue.

> Fixes: 286e04b8ed7a ("mm: memcg/slab: allocate obj_cgroups for non-root slab pages")
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  mm/memcontrol.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b0695d3aa530..44852ac048c3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2876,12 +2876,24 @@ int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
>  	 */
>  	objects = max(objs_per_slab_page(s, page),
>  		      (int)(sizeof(struct rcu_head)/sizeof(void *)));
> -
> +retry:
>  	vec = kcalloc_node(objects, sizeof(struct obj_cgroup *), gfp,
>  			   page_to_nid(page));
>  	if (!vec)
>  		return -ENOMEM;
>  
> +	/*
> +	 * The allocated vector should not come from the same slab.
> +	 * Otherwise, this slab will never become empty. Double the size
> +	 * in this case to make sure that the vector comes from a different
> +	 * kmemcache.
> +	 */
> +	if (unlikely(virt_to_head_page(vec) == page)) {
> +		kfree(vec);
> +		objects *= 2;
> +		goto retry;
> +	}
> +
>  	memcg_data = (unsigned long) vec | MEMCG_DATA_OBJCGS;
>  	if (new_page) {
>  		/*
>