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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



2013/2/19 Will Huck <will.huckk@xxxxxxxxx>:
> 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.
>

Adding the page to swap cache is the first stage of memory reclaiming.

When an anonymous page will be reclaimed, it should be swapped out. If
it's not in the swap cache, it will insert into swap cache first and
set the bit of PG_swapcache on page->flags. Then, it will be swapped
out by try_to_unmap(). After it's swapped out, and no processes swap
in this page, the page will be deleted from swap cache and be put into
buddy system.

Please correct me if my understanding is wrong.

Regards,
Haifeng Li.

>>
>>   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.
>>
>> --
>>
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@xxxxxxxxx.  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
>
>

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>


[Index of Archives]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Bugtraq]     [Linux]     [Linux OMAP]     [Linux MIPS]     [ECOS]     [Asterisk Internet PBX]     [Linux API]