linux-kernel - Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

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Date:	Fri, 26 Mar 2010 10:39:10 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped 
	anonymous pages

On Fri, Mar 26, 2010 at 9:58 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Fri, 26 Mar 2010 00:29:01 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
>
>> Hi, Kame.
> <snip>
>
>> Which case do we have PageAnon && (page_mapcount == 0) && PageSwapCache ?
>> With looking over code which add_to_swap_cache, I found somewhere.
>>
>> 1) shrink_page_list
>> I think this case doesn't matter by isolate_lru_xxx.
>>
>> 2) shmem_swapin
>> It seems to be !PageAnon
>>
>> 3) shmem_writepage
>> It seems to be !PageAnon.
>>
>> 4) do_swap_page
>> page_add_anon_rmap increases _mapcount before setting page->mapping to anon_vma.
>> So It doesn't matter.
>
>>
>>
>> I think following codes in unmap_and_move seems to handle 3) case.
>>
>> ---
>>          * Corner case handling:
>>          * 1. When a new swap-cache page is read into, it is added to the LRU
>>          * and treated as swapcache but it has no rmap yet.
>>         ...
>>         if (!page->mapping) {
>>                 if (!PageAnon(page) && page_has_private(page)) {
>>                 ....
>>                 }
>>                 goto skip_unmap;
>>         }
>>
>> ---
>>
>> Do we really check PageSwapCache in there?
>> Do I miss any case?
>>
>
> When a page is fully unmapped, page->mapping is not cleared.
>
> from rmap.c
> ==
>  734 void page_remove_rmap(struct page *page)
>  735 {
>        ....
>  758         /*
>  759          * It would be tidy to reset the PageAnon mapping here,
>  760          * but that might overwrite a racing page_add_anon_rmap
>  761          * which increments mapcount after us but sets mapping
>  762          * before us: so leave the reset to free_hot_cold_page,
>  763          * and remember that it's only reliable while mapped.
>  764          * Leaving it set also helps swapoff to reinstate ptes
>  765          * faster for those pages still in swapcache.
>  766          */
>  767 }
> ==
>
> What happens at memory reclaim is...
>
>        the first vmscan
>        1. isolate a page from LRU.
>        2. add_to_swap_cache it.
>        3. try_to_unmap it
>        4. pageout it (PG_reclaim && PG_writeback)
>        5. move page to the tail of LRU.
>        .....<after some time>
>        6. I/O ends and PG_writeback is cleared.
>
> Here, in above cycle, the page is not freed. Still in LRU list.
>        next vmscan
>        7. isolate a page from LRU.
>        8. finds a unmapped clean SwapCache
>        9. drop it.
>
> So, to _free_ unmapped SwapCache, sequence 7-9 should happen.
> If enough memory is freed by the first itelation of vmscan before I/O end,
> next vmscan doesn't happen. Then, we have "unmmaped clean Swapcache which has
> anon_vma pointer on page->mapping" on LRU.

Thanks for open my eye. Kame. :)



-- 
Kind regards,
Minchan Kim
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