Re: page_launder() bug

[Mikulas Patocka]

>  > My point is that its _ok_ for us to check if the page is a dead swap cache
>  > page _without_ the lock since writepage() will recheck again with the page
>  > _locked_. Quoting you two messages back: 
>  > 
>  > "But it is important to re-calculate the deadness after getting the lock.
>  > Before, it was just an informed guess. After the lock, it is knowledge."
>  > 
>  > See ? 
> 
> In fact my patch isn't changing writepage behavior wrt. that page, it
> is changing behavior with respect to laundering policy for that page.
> 
> Here, let's talk code a little bit so there are no misunderstandings,
> I really want to put this to rest:
> 
> +		int dead_swap_page;
> +
>  		page = list_entry(page_lru, struct page, lru);
>  
> +		dead_swap_page =
> +			(PageSwapCache(page) &&
> +			 page_count(page) == (1 + !!page->buffers));
> +
> 
> Calculate dead_swap_page outside of lock.
> 
>  		/* Page is or was in use?  Move it to the active list. */
> -		if (PageTestandClearReferenced(page) || page->age > 0 ||
> -				(!page->buffers && page_count(page) > 1) ||
> -				page_ramdisk(page)) {
			     ^^^^^^^^^^^^^^^^^
> +		if (!dead_swap_page &&
> +		    (PageTestandClearReferenced(page) || page->age > 0 ||
> +		     (!page->buffers && page_count(page) > 1) ||
> +		     page_ramdisk(page))) {
		^^^^^^^^^^^^^^^^^^^^^^
>  			del_page_from_inactive_dirty_list(page);
>  			add_page_to_active_list(page);
>  			continue;

#define page_ramdisk(page) \
        (page->buffers && (MAJOR(page->buffers->b_dev) == RAMDISK_MAJOR))

Are you sure that no one will release buffers under your hands?

Mikulas