2.4.19 ( try_to_swap_out) does not set page->mapping to NULL

[Bourne]

Hi all, A small piece of code in mm/vmscan.c kept me confused. Please 
could some one explain this ?

File: mm/vmscan.c , try_to_swap_out()

....SNIP....
drop_pte: mm->rss--
                UnlockPage(page);
                ---SNIP--
                page_cache_release(page);
                return freeable;
                ----SNIP----
if(page->mapping)
    goto drop_pte;

What i can get is if page->mapping is NOT NULL , then do a 
page_cache_release(). This boils down to __free_pages_ok(). Here the 
code snippet is if(page->mapping) BUG();

So if try_to_swap_out wants to drop a page which can be brought in 
always ( i.e , not dirty ), then what is the harm in setting it to NULL ?

TIA

Bourne