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

From: Ric Mason
Date: Tue Feb 19 2013 - 03:33:01 EST

On 02/19/2013 02:06 AM, Hugh Dickins wrote:
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.

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.

Hi Hugh,

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?


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 page.
656 return 0;
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;
676 delete_from_swap_cache(page);
677 SetPageDirty(page);
678 return 1;
679 }

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