Re: [PATCH v3 3/4] mm: introduce page memcg flags

[Shakeel Butt <shakeelb@google.com>]

On Tue, Sep 29, 2020 at 4:59 PM Roman Gushchin <guro@fb.com> wrote:
>
> The lowest bit in page->memcg_data is used to distinguish between
> struct memory_cgroup pointer and a pointer to a objcgs array.
> All checks and modifications of this bit are open-coded.
>
> Let's formalize it using page memcg flags, defined in enum
> page_memcg_data_flags.
>
> Additional flags might be added later.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>

One nit below:

Reviewed-by: Shakeel Butt <shakeelb@google.com>

> ---
>  include/linux/memcontrol.h | 35 ++++++++++++++++++++++++-----------
>  1 file changed, 24 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index d4d15c04bbaf..35f846c6b89b 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -343,6 +343,15 @@ struct mem_cgroup {
>
>  extern struct mem_cgroup *root_mem_cgroup;
>
> +enum page_memcg_data_flags {
> +       /* page->memcg_data is a pointer to an objcgs vector */
> +       MEMCG_DATA_OBJCGS,
> +       /* the next bit after the last actual flag */
> +       __NR_MEMCG_DATA_FLAGS,
> +};
> +
> +#define MEMCG_DATA_FLAGS_MASK ((0x1UL << __NR_MEMCG_DATA_FLAGS) - 1)
> +

Nit:

#define PG_MEMCG_OBJCGS (0x1UL << MEMCG_DATA_OBJCGS)

and use this macro below.

>  /*
>   * page_memcg - get the memory cgroup associated with a page
>   * @page: a pointer to the page struct
> @@ -404,13 +413,7 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page)
>          */
>         unsigned long memcg_data = READ_ONCE(page->memcg_data);
>
> -       /*
> -        * 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 doesn't belong
> -        * to any specific memory cgroup.
> -        */
> -       if (memcg_data & 0x1UL)
> +       if (memcg_data & (0x1UL << MEMCG_DATA_OBJCGS))
>                 return NULL;
>
>         return (struct mem_cgroup *)memcg_data;
> @@ -459,7 +462,12 @@ static inline void clear_page_memcg(struct page *page)
>   */
>  static inline struct obj_cgroup **page_objcgs(struct page *page)
>  {
> -       return (struct obj_cgroup **)(READ_ONCE(page->memcg_data) & ~0x1UL);
> +       unsigned long memcg_data = READ_ONCE(page->memcg_data);
> +
> +       VM_BUG_ON_PAGE(memcg_data &&
> +                      !(memcg_data & (0x1UL << MEMCG_DATA_OBJCGS)), page);
> +
> +       return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
>  }
>
>  /*
> @@ -474,8 +482,9 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
>  {
>         unsigned long memcg_data = READ_ONCE(page->memcg_data);
>
> -       if (memcg_data && (memcg_data & 0x1UL))
> -               return (struct obj_cgroup **)(memcg_data & ~0x1UL);
> +       if (memcg_data && (memcg_data & (0x1UL << MEMCG_DATA_OBJCGS)))
> +               return (struct obj_cgroup **)
> +                       (memcg_data & ~MEMCG_DATA_FLAGS_MASK);
>
>         return NULL;
>  }
> @@ -490,7 +499,11 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
>  static inline bool set_page_objcgs(struct page *page,
>                                         struct obj_cgroup **objcgs)
>  {
> -       return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs | 0x1UL);
> +       unsigned long memcg_data = (unsigned long)objcgs;
> +
> +       memcg_data |= (0x1UL << MEMCG_DATA_OBJCGS);
> +
> +       return !cmpxchg(&page->memcg_data, 0, memcg_data);
>  }
>
>  /*
> --
> 2.26.2
>