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

From: Will Huck
Date: Mon Feb 18 2013 - 21:38:22 EST

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.

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;
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
980 */
981 __clear_page_locked(page);
982 free_it:
983 nr_reclaimed++;
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.

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.

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));
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
656 return 0;
658 /*
659 * Once hibernation has begun to create its image of
660 * there's a danger that one of the calls to
661 * - most probably a call from __try_to_reclaim_swap()
662 * hibernation is allocating its own swap pages for the
663 * but conceivably even a call from memory reclaim - will
664 * the swap from a page which has already been recorded in
665 * image as a clean swapcache page, and then reuse its swap
666 * another page of the image. On waking from hibernation,
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;
676 delete_from_swap_cache(page);
677 SetPageDirty(page);
678 return 1;
679 }

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