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SubjectRe: [PATCH v5 3/4] mm: introduce page memcg flags
On Fri 02-10-20 10:25:58, Roman Gushchin 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>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> include/linux/memcontrol.h | 32 ++++++++++++++++++++------------
> 1 file changed, 20 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index c7ac0a5b8989..99a4841d658b 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 = (1UL << 0),
> + /* the next bit after the last actual flag */
> + __NR_MEMCG_DATA_FLAGS = (1UL << 1),
> +};
> +
> +#define MEMCG_DATA_FLAGS_MASK (__NR_MEMCG_DATA_FLAGS - 1)
> +
> /*
> * 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 & MEMCG_DATA_OBJCGS)
> return NULL;
>
> return (struct mem_cgroup *)memcg_data;
> @@ -429,7 +432,11 @@ static inline struct mem_cgroup *page_memcg_check(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 & MEMCG_DATA_OBJCGS), page);
> +
> + return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> }
>
> /*
> @@ -444,10 +451,10 @@ 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 & MEMCG_DATA_OBJCGS))
> + return NULL;
>
> - return NULL;
> + return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> }
>
> /*
> @@ -460,7 +467,8 @@ 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);
> + return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs |
> + MEMCG_DATA_OBJCGS);
> }
> #else
> static inline struct obj_cgroup **page_objcgs(struct page *page)
> --
> 2.26.2

--
Michal Hocko
SUSE Labs

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