From mboxrd@z Thu Jan 1 00:00:00 1970 From: ric.masonn@gmail.com (Ric Mason) Date: Tue, 19 Feb 2013 16:32:38 +0800 Subject: Should a swapped out page be deleted from swap cache? In-Reply-To: References: Message-ID: <512338A6.1030602@gmail.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 02/19/2013 02:06 AM, Hugh Dickins wrote: > 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. Hi Hugh, There is a call of try_to_free_swap in function swap_writepage, if swap_writepage is call from shrink_page_list path, PageSwapCache(page) == trure, PageWriteback(page) maybe false, page_swapcount(page) == 0, then will delete the page from swap cache and free swap slot, where I miss? > > 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 at kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email at kvack.org