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    SubjectRe: [PATCH v1 3/4] mm: introduce page memcg flags
    On Tue, Sep 22, 2020 at 01:36:59PM -0700, Roman Gushchin wrote:
    > @@ -422,7 +421,13 @@ static inline void clear_page_mem_cgroup(struct page *page)
    > */
    > static inline struct obj_cgroup **page_obj_cgroups(struct page *page)
    > {
    > - return (struct obj_cgroup **)(page->memcg_data & ~0x1UL);
    > + unsigned long memcg_data = page->memcg_data;
    > +
    > + VM_BUG_ON_PAGE(memcg_data && !test_bit(PG_MEMCG_OBJ_CGROUPS,
    > + &memcg_data), page);
    > + __clear_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data);
    > +
    > + return (struct obj_cgroup **)memcg_data;

    Slab allocations set up page->memcg_data locklessly, right? AFAICS,
    the page_objcg lookup functions all need READ_ONCE() loads.

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