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SubjectRe: Should a swapped out page be deleted from swap cache?
On 02/19/2013 10:04 AM, Li Haifeng wrote:
> 2013/2/19 Hugh Dickins <hughd@google.com>
>> 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.

>
> 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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