Should a swapped out page be deleted from swap cache?

[Will Huck]

On 02/19/2013 10:04 AM, Li Haifeng wrote:
> 2013/2/19 Hugh Dickins <hughd at 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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