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    Subject[PATCH v6 03/34] mm: introduce page memcg flags
    Date
    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.

    Link: https://lkml.kernel.org/r/20201027001657.3398190-4-guro@fb.com
    Signed-off-by: Roman Gushchin <guro@fb.com>
    Reviewed-by: Shakeel Butt <shakeelb@google.com>
    Acked-by: Johannes Weiner <hannes@cmpxchg.org>
    Acked-by: Michal Hocko <mhocko@suse.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    ---
    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
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