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SubjectRe: Should a swapped out page be deleted from swap cache?
From
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.

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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