page->_count in shrink_cache() and shrink_list() ??

Hi,I am a kernel newbie and have a question about page_count in shrink_list() ( 2.6.11 source code ).
When kswapd began to shrink_cache(),it will first collect the pages
in zone->inactive_list which are not about to free into a temp list:page_list .
565 if (get_page_testone(page)) {
569 __put_page(page);
570 SetPageLRU(page);
571 list_add(&page->lru, &zone->inactive_list);
572 continue;
573 }
574 list_add(&page->lru, &page_list);
...
Then shrink_list() will check and try to free some fit pages. 589 nr_freed = shrink_list(&page_list, sc);

in shrink_list(),I dont know why kernel will judge the expression if(page_count(page)!=2) before doing something with this page. After a page is allocated ,its page_count() is 1 and again kernel add 1 in shrink_cache (line 565).So I think if the page is in page cache or swap cache ,its page_count() is at least 3 and line 485 will not satisfied.
480 /*
481 * The non-racy check for busy page. It is critical to check 482 * PageDirty _after_ making sure that the page is freeable and 483 * not in use by anybody. (pagecache + us == 2)
484 */
485 if (page_count(page) != 2 || PageDirty(page)) {
486 spin_unlock_irq(&mapping->tree_lock);
487 goto keep_locked;
488 }
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