Re: Should a swapped out page be deleted from swap cache?

From: Li Haifeng
Date: Tue Feb 19 2013 - 01:53:48 EST

2013/2/19 Will Huck <will.huckk@xxxxxxxxx>:
> On 02/19/2013 10:04 AM, Li Haifeng wrote:
>>
>> 2013/2/19 Hugh Dickins <hughd@xxxxxxxxxx>
>>>
>>> On Mon, 18 Feb 2013, Li Haifeng wrote:
>>>
>>>> For explain my question, the two points should be displayed as below.
>>>>
>>>> 1. If an anonymous page is swapped out, this page will be deleted
>>>> from swap cache and be put back into buddy system.
>>>
>>> Yes, unless the page is referenced again before it comes to be
>>> deleted from swap cache.
>>>
>>>> 2. When a page is swapped out, the sharing count of swap slot must not
>>>> be zero. That is, page_swapcount(page) will not return zero.
>>>
>>> I would not say "must not": we just prefer not to waste time on swapping
>>> a page out if its use count has already gone to 0. And its use count
>>> might go down to 0 an instant after swap_writepage() makes that check.
>>>
>> Thanks for your reply and patience.
>>
>> If a anonymous page is swapped out and comes to be reclaimable,
>> shrink_page_list() will call __remove_mapping() to delete the page
>> swapped out from swap cache. Corresponding code lists as below.
>
>
> I'm not sure if
> if (PageAnon(page) && !PageSwapCache(page)) {
> .................
> }
> will add the page to swap cache again.
>

Adding the page to swap cache is the first stage of memory reclaiming.

When an anonymous page will be reclaimed, it should be swapped out. If
it's not in the swap cache, it will insert into swap cache first and
set the bit of PG_swapcache on page->flags. Then, it will be swapped
out by try_to_unmap(). After it's swapped out, and no processes swap
in this page, the page will be deleted from swap cache and be put into
buddy system.

Please correct me if my understanding is wrong.

Regards,
Haifeng Li.

>>
>> 765 static unsigned long shrink_page_list(struct list_head *page_list,
>> 766 struct mem_cgroup_zone *mz,
>> 767 struct scan_control *sc,
>> 768 int priority,
>> 769 unsigned long *ret_nr_dirty,
>> 770 unsigned long
>> *ret_nr_writeback)
>> 771 {
>> ...
>> 971 if (!mapping || !__remove_mapping(mapping, page))
>> 972 goto keep_locked;
>> 973
>> 974 /*
>> 975 * At this point, we have no other references and
>> there is
>> 976 * no way to pick any more up (removed from LRU,
>> removed
>> 977 * from pagecache). Can use non-atomic bitops now
>> (and
>> 978 * we obviously don't have to worry about waking
>> up a process
>> 979 * waiting on the page lock, because there are no
>> references.
>> 980 */
>> 981 __clear_page_locked(page);
>> 982 free_it:
>> 983 nr_reclaimed++;
>> 984
>> 985 /*
>> 986 * Is there need to periodically free_page_list? It
>> would
>> 987 * appear not as the counts should be low
>> 988 */
>> 989 list_add(&page->lru, &free_pages);
>> 990 continue;
>>
>> Please correct me if my understanding is wrong.
>>
>> Thanks.
>>>>
>>>> Are both of them above right?
>>>>
>>>> According the two points above, I was confused to the line 655 below.
>>>> When a page is swapped out, the return value of page_swapcount(page)
>>>> will not be zero. So, the page couldn't be deleted from swap cache.
>>>
>>> Yes, we cannot free the swap as long as its data might be needed again.
>>>
>>> But a swap cache page may linger in memory for an indefinite time,
>>> in between being queued for write out, and actually being freed from
>>> the end of the lru by memory pressure.
>>>
>>> At various points where we hold the page lock on a swap cache page,
>>> it's worth checking whether it is still actually needed, or could
>>> now be freed from swap cache, and the corresponding swap slot freed:
>>> that's what try_to_free_swap() does.
>>
>> I do agree. Thanks again.
>>>
>>> Hugh
>>>
>>>> 644 * If swap is getting full, or if there are no more mappings of
>>>> this page,
>>>> 645 * then try_to_free_swap is called to free its swap space.
>>>> 646 */
>>>> 647 int try_to_free_swap(struct page *page)
>>>> 648 {
>>>> 649 VM_BUG_ON(!PageLocked(page));
>>>> 650
>>>> 651 if (!PageSwapCache(page))
>>>> 652 return 0;
>>>> 653 if (PageWriteback(page))
>>>> 654 return 0;
>>>> 655 if (page_swapcount(page))//Has referenced by other swap
>>>> out
>>>> page.
>>>> 656 return 0;
>>>> 657
>>>> 658 /*
>>>> 659 * Once hibernation has begun to create its image of
>>>> memory,
>>>> 660 * there's a danger that one of the calls to
>>>> try_to_free_swap()
>>>> 661 * - most probably a call from __try_to_reclaim_swap()
>>>> while
>>>> 662 * hibernation is allocating its own swap pages for the
>>>> image,
>>>> 663 * but conceivably even a call from memory reclaim - will
>>>> free
>>>> 664 * the swap from a page which has already been recorded in
>>>> the
>>>> 665 * image as a clean swapcache page, and then reuse its
>>>> swap
>>>> for
>>>> 666 * another page of the image. On waking from hibernation,
>>>> the
>>>> 667 * original page might be freed under memory pressure,
>>>> then
>>>> 668 * later read back in from swap, now with the wrong data.
>>>> 669 *
>>>> 670 * Hibration suspends storage while it is writing the
>>>> image
>>>> 671 * to disk so check that here.
>>>> 672 */
>>>> 673 if (pm_suspended_storage())
>>>> 674 return 0;
>>>> 675
>>>> 676 delete_from_swap_cache(page);
>>>> 677 SetPageDirty(page);
>>>> 678 return 1;
>>>> 679 }
>>>>
>>>> Thanks.
>>
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