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

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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;
  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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