Re: [RFC][PATCH 3/3] account swapcache

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 11 Apr 2008 21:20:55 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> IMO, not charging swap caches as memory occasionally causes a problem
> that swap caches are not freed even when a process that owns
> those pages try to free them(e.g. task exit).
> 
> For example:
> 
>   Some pages are being reclaimed via memcg memory reclaim.
> 
>   Assume that shrink_page_list() has already moved those pages
>   to swap cache, unmapped them from ptes, removed from mz->lru,
>   and is working on other pages on page_list.
>   Those swap cache pages are unlocked and
>   page_count of them are 2(swap cache, isolate_page).
> 
>   At the same time on other CPU, if the process that owns those
>   pages are trying to free them, free_swap_and_cache() cannot
>   free those pages unless vm_swap_full, because find_get_pages()
>   increases page_count.
> 
> I think this rare case itself also exists on global memory reclaim,
> but global memory reclaim does not assume that those pases have
> been freed, so, if it need to free more memory, those pases
> will be freed later because they remain on global inactive list.
> 
yes.

> The problem here is that those swap cache pages are uncharged
> from memcg, so memcg can never reclaim those pages that belonged
> to the group.
> 
why "never" uncharged ? 

Assume "page" is SwapCache and unmapped and clean. 
==
 shrink_page_list()
	-> PageSwapCache() == true
	-> PageWriteback() == false
	-> PageDirty()     == false
	-> PagePrivate()   == true or false
	-> remove_mapping()
		-> page_count() == 2
		-> PageDirty()  == false
		-> PageSwapCache() == true
			-> __delete_from_swap_cache()
			-> true
	-> page will be freed
==

page shirinking can free SwapCache regardless of vm_swap_full() result.
Of course, my patch handles __delete_from_swap_cache().
 
Thanks,
-Kame



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