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Date: Wed, 06 Mar 2013 19:10:27 +0800
From: Ric Mason <ric.masonn@...il.com>
To: Li Haifeng <omycle@...il.com>
CC: Hugh Dickins <hughd@...gle.com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: Should a swapped out page be deleted from swap cache?
On 03/06/2013 07:04 PM, Ric Mason wrote:
> On 03/06/2013 01:34 PM, Li Haifeng wrote:
>> 2013/2/20 Ric Mason <ric.masonn@...il.com>:
>>> Hi Hugh,
>>>
>>>
>>> On 02/20/2013 02:56 AM, Hugh Dickins wrote:
>>>> On Tue, 19 Feb 2013, Ric Mason wrote:
>>>>> There is a call of try_to_free_swap in function swap_writepage, if
>>>>> swap_writepage is call from shrink_page_list path,
>>>>> PageSwapCache(page) ==
>>>>> trure, PageWriteback(page) maybe false, page_swapcount(page) == 0,
>>>>> then
>>>>> will
>>>>> delete the page from swap cache and free swap slot, where I miss?
>>>> That's correct. PageWriteback is sure to be false there.
>>>> page_swapcount
>>>> usually won't be 0 there, but sometimes it will be, and in that
>>>> case we
>>>> do want to delete from swap cache and free the swap slot.
>>>
>>> 1) If PageSwapCache(page) == true, PageWriteback(page) == false,
>>> page_swapcount(page) == 0 in swap_writepage(shrink_page_list path),
>>> then
>>> will delete the page from swap cache and free swap slot, in function
>>> swap_writepage:
>>>
>>> if (try_to_free_swap(page)) {
>>> unlock_page(page);
>>> goto out;
>>> }
>>> writeback will not execute, that's wrong. Where I miss?
>> when the page is deleted from swap cache and corresponding swap slot
>> is free, the page is set dirty. The dirty page won't be reclaimed. It
>> is not wrong.
>
> I don't think so. For dirty pages, there are two steps: 1)writeback
> 2)reclaim. Since PageSwapCache(page) == true && PageWriteback(page) ==
> false && page_swapcount(page) == 0 in swap_writeback(),
> try_to_free_swap() will return true and writeback will be skip. Then
> how can step one be executed?
s/swap_writeback()/swap_writepage()
Btw, Hi Hugh, could you explain more to us? :-)
>
>>
>> corresponding path lists as below.
>> when swap_writepage() is called by pageout() in shrink_page_list().
>> pageout() will return PAGE_SUCCESS. For PAGE_SUCCESS, when
>> PageDirty(page) is true, this reclaiming page will be keeped in the
>> inactive LRU list.
>> shrink_page_list()
>> {
>> ...
>> 904 switch (pageout(page, mapping, sc)) {
>> 905 case PAGE_KEEP:
>> 906 nr_congested++;
>> 907 goto keep_locked;
>> 908 case PAGE_ACTIVATE:
>> 909 goto activate_locked;
>> 910 case PAGE_SUCCESS:
>> 911 if (PageWriteback(page))
>> 912 goto keep_lumpy;
>> 913 if (PageDirty(page))
>> 914 goto keep;
>> ...}
>>
>>> 2) In the function pageout, page will be set PG_Reclaim flag, since
>>> this
>>> flag is set, end_swap_bio_write->end_page_writeback:
>>> if (TestClearPageReclaim(page))
>>> rotate_reclaimable_page(page);
>>> it means that page will be add to the tail of lru list, page is clean
>>> anonymous page this time and will be reclaim to buddy system soon,
>>> correct?
>> correct
>>> If is correct, what is the meaning of rotate here?
>> Rotating here is to add the page to the tail of inactive LRU list. So
>> this page will be reclaimed ASAP while reclaiming.
>>
>>>> Hugh
>>>
>
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