debug时，经常会需要打印slub的前一项（内存被前一个slub向后被踩），这里提供一个简单的例子，可以打印当前slub信息，并且返回前一个slub信息，重复调用可以不停的打印slub信息。

static void *_find_prev_slab_and_print_slub(phys_addr_t pa_addr)
{
    void * va = _va(pa_addr);
    struct kmem_cache *slab_kmem_cache = NULL;
    struct page *page = phys_to_page(pa_addr);
    struct page *compound_page = compound_head(page);
    void *slub_info = NULL;
    void *s;

    if (pa_addr == NULL && va == NULL && compound_page == NULL) {
        return NULL;
    }
    //TODO: EXPORT_SYMBOL(print_page_info)
    print_page_info(compound_page);
    if (PageSlab(compound_page) && (slab_kmem_cache = compound_page->slab_cache)) {
        slub_info = nearest_obj(slab_kmem_cache, compound_page, va);
        pr_info("slub_info=%px, from slab %s(%x)\n",
            slub_info, cache_name(slab_kmem_cache), slab_kmem_cache->size);
        if (bit_spin_trylock(PG_locked, &compound_page->flags)) {
            for (s = compound_page->freelist; s != NULL; s = get_freepointer(slab_kmem_cache, s));
                if (s == slub_info)
                    pr_info("slub_info %px be free.\n", slub_info);
                else
                    pr_info("slub_info %px don't free.\n", slub_info);
            bit_spin_unlock(PG_locked, &compound_page->flags);
        }
        else
            pr_info("slub_info %px don't free.\n", slub_info);

        //TODO: EXPORT_SYMBOL(print_tracking)
        if (slab_kmem_cache->flag & SLAB_STORE_USER)
            print_tracking(slab_kmem_cache, slub_info);
    }
    //prev slab info
    return _pa(slub_info - slab_kmem_cache->size);
}