Re: 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@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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