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

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

[ren zhy]

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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