Re: [PATCH v2 16/28] mm: memcg/slab: allocate obj_cgroups for non-root slab pages

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jan 27, 2020 at 09:34:41AM -0800, Roman Gushchin wrote:
> Allocate and release memory to store obj_cgroup pointers for each
> non-root slab page. Reuse page->mem_cgroup pointer to store a pointer
> to the allocated space.
> 
> To distinguish between obj_cgroups and memcg pointers in case
> when it's not obvious which one is used (as in page_cgroup_ino()),
> let's always set the lowest bit in the obj_cgroup case.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
>  include/linux/mm.h       | 25 ++++++++++++++++++--
>  include/linux/mm_types.h |  5 +++-
>  mm/memcontrol.c          |  5 ++--
>  mm/slab.c                |  3 ++-
>  mm/slab.h                | 51 +++++++++++++++++++++++++++++++++++++++-
>  mm/slub.c                |  2 +-
>  6 files changed, 83 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 080f8ac8bfb7..65224becc4ca 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1264,12 +1264,33 @@ static inline void set_page_links(struct page *page, enum zone_type zone,
>  #ifdef CONFIG_MEMCG
>  static inline struct mem_cgroup *page_memcg(struct page *page)
>  {
> -	return page->mem_cgroup;
> +	struct mem_cgroup *memcg = page->mem_cgroup;
> +
> +	/*
> +	 * The lowest bit set means that memcg isn't a valid memcg pointer,
> +	 * but a obj_cgroups pointer. In this case the page is shared and
> +	 * isn't charged to any specific memory cgroup. Return NULL.
> +	 */
> +	if ((unsigned long) memcg & 0x1UL)
> +		memcg = NULL;
> +
> +	return memcg;

That should really WARN instead of silently returning NULL. Which
callsite optimistically asks a page's cgroup when it has no idea
whether that page is actually a userpage or not?

>  static inline struct mem_cgroup *page_memcg_rcu(struct page *page)
>  {
> +	struct mem_cgroup *memcg = READ_ONCE(page->mem_cgroup);
> +
>  	WARN_ON_ONCE(!rcu_read_lock_held());
> -	return READ_ONCE(page->mem_cgroup);
> +
> +	/*
> +	 * The lowest bit set means that memcg isn't a valid memcg pointer,
> +	 * but a obj_cgroups pointer. In this case the page is shared and
> +	 * isn't charged to any specific memory cgroup. Return NULL.
> +	 */
> +	if ((unsigned long) memcg & 0x1UL)
> +		memcg = NULL;
> +
> +	return memcg;

Same here.

>  }
>  #else
>  static inline struct mem_cgroup *page_memcg(struct page *page)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 270aa8fd2800..5102f00f3336 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -198,7 +198,10 @@ struct page {
>  	atomic_t _refcount;
>  
>  #ifdef CONFIG_MEMCG
> -	struct mem_cgroup *mem_cgroup;
> +	union {
> +		struct mem_cgroup *mem_cgroup;
> +		struct obj_cgroup **obj_cgroups;
> +	};

Since you need the casts in both cases anyway, it's safer (and
simpler) to do

	unsigned long mem_cgroup;

to prevent accidental direct derefs in future code.

Otherwise, this patch looks good to me!