On Fri, Mar 26, 2010 at 9:58 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Fri, 26 Mar 2010 00:29:01 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> 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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