Should a swapped out page be deleted from swap cache?

[Hugh Dickins]

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.

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.