其中结构“struct page"是存储在linux内核上吗?

我了解到linux内核管理内存，分配/释放内存的单位是4KB，也就是页面大小.而且我知道这个页面是由 struct page 处理的.我在这里得到了一个实际的代码.

I learned that linux kernel manages the memory and the unit for allocate/deallocate the memory is 4KB, which is the page size. And I know that this pages are handled by struct page. I got a actual code here.

struct page {
    unsigned long flags;        /* Atomic flags, some possibly
                     * updated asynchronously */
    /*
     * Five words (20/40 bytes) are available in this union.
     * WARNING: bit 0 of the first word is used for PageTail(). That
     * means the other users of this union MUST NOT use the bit to
     * avoid collision and false-positive PageTail().
     */
    union {
        struct {    /* Page cache and anonymous pages */
            /**
             * @lru: Pageout list, eg. active_list protected by
             * pgdat->lru_lock.  Sometimes used as a generic list
             * by the page owner.
             */
            struct list_head lru;
            /* See page-flags.h for PAGE_MAPPING_FLAGS */
            struct address_space *mapping;
            pgoff_t index;      /* Our offset within mapping. */
            /**
             * @private: Mapping-private opaque data.
             * Usually used for buffer_heads if PagePrivate.
             * Used for swp_entry_t if PageSwapCache.
             * Indicates order in the buddy system if PageBuddy.
             */
            unsigned long private;
        };
        struct {    /* page_pool used by netstack */
            /**
             * @dma_addr: might require a 64-bit value even on
             * 32-bit architectures.
             */
            dma_addr_t dma_addr;
        };
        struct {    /* slab, slob and slub */
            union {
                struct list_head slab_list;
                struct {    /* Partial pages */
                    struct page *next;
#ifdef CONFIG_64BIT
                    int pages;  /* Nr of pages left */
                    int pobjects;   /* Approximate count */
#else
                    short int pages;
                    short int pobjects;
#endif
                };
            };
            struct kmem_cache *slab_cache; /* not slob */
            /* Double-word boundary */
            void *freelist;     /* first free object */
            union {
                void *s_mem;    /* slab: first object */
                unsigned long counters;     /* SLUB */
                struct {            /* SLUB */
                    unsigned inuse:16;
                    unsigned objects:15;
                    unsigned frozen:1;
                };
            };
        };
        struct {    /* Tail pages of compound page */
            unsigned long compound_head;    /* Bit zero is set */

            /* First tail page only */
            unsigned char compound_dtor;
            unsigned char compound_order;
            atomic_t compound_mapcount;
        };
        struct {    /* Second tail page of compound page */
            unsigned long _compound_pad_1;  /* compound_head */
            atomic_t hpage_pinned_refcount;
            /* For both global and memcg */
            struct list_head deferred_list;
        };
        struct {    /* Page table pages */
            unsigned long _pt_pad_1;    /* compound_head */
            pgtable_t pmd_huge_pte; /* protected by page->ptl */
            unsigned long _pt_pad_2;    /* mapping */
            union {
                struct mm_struct *pt_mm; /* x86 pgds only */
                atomic_t pt_frag_refcount; /* powerpc */
            };
#if ALLOC_SPLIT_PTLOCKS
            spinlock_t *ptl;
#else
            spinlock_t ptl;
#endif
        };
        struct {    /* ZONE_DEVICE pages */
            /** @pgmap: Points to the hosting device page map. */
            struct dev_pagemap *pgmap;
            void *zone_device_data;
            /*
             * ZONE_DEVICE private pages are counted as being
             * mapped so the next 3 words hold the mapping, index,
             * and private fields from the source anonymous or
             * page cache page while the page is migrated to device
             * private memory.
             * ZONE_DEVICE MEMORY_DEVICE_FS_DAX pages also
             * use the mapping, index, and private fields when
             * pmem backed DAX files are mapped.
             */
        };

        /** @rcu_head: You can use this to free a page by RCU. */
        struct rcu_head rcu_head;
    };

    union {     /* This union is 4 bytes in size. */
        /*
         * If the page can be mapped to userspace, encodes the number
         * of times this page is referenced by a page table.
         */
        atomic_t _mapcount;

        /*
         * If the page is neither PageSlab nor mappable to userspace,
         * the value stored here may help determine what this page
         * is used for.  See page-flags.h for a list of page types
         * which are currently stored here.
         */
        unsigned int page_type;

        unsigned int active;        /* SLAB */
        int units;          /* SLOB */
    };

    /* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
    atomic_t _refcount;

#ifdef CONFIG_MEMCG
    struct mem_cgroup *mem_cgroup;
#endif

    /*
     * On machines where all RAM is mapped into kernel address space,
     * we can simply calculate the virtual address. On machines with
     * highmem some memory is mapped into kernel virtual memory
     * dynamically, so we need a place to store that address.
     * Note that this field could be 16 bits on x86 ... ;)
     *
     * Architectures with slow multiplication can define
     * WANT_PAGE_VIRTUAL in asm/page.h
     */
#if defined(WANT_PAGE_VIRTUAL)
    void *virtual;          /* Kernel virtual address (NULL if
                       not kmapped, ie. highmem) */
#endif /* WANT_PAGE_VIRTUAL */

#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
    int _last_cpupid;
#endif
} _struct_page_alignment;

而且我不知道 linux 内核在哪里存储这个巨大的(?)结构.在 linux 内核中处理了很多页面，这意味着我们有很多这种 struct page 结构.它可以存储在内存中的什么位置?

And I have no idea where the linux kernel stores this huge(?) structure. There are a lot of pages handled in the linux kernel and that means that we have a lot of this struct page structures. Where can it be stored on the memory?

我也不知道上面的工会是干什么的.

Also I have no idea what the union up there is up for.

提前致谢.