From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx183.postini.com [74.125.245.183]) by kanga.kvack.org (Postfix) with SMTP id B89F96B005A for ; Mon, 9 Jan 2012 15:42:51 -0500 (EST) Received: by iacb35 with SMTP id b35so8881787iac.14 for ; Mon, 09 Jan 2012 12:42:51 -0800 (PST) Message-ID: <4F0B5146.6090200@gmail.com> Date: Mon, 09 Jan 2012 15:42:46 -0500 From: KOSAKI Motohiro MIME-Version: 1.0 Subject: Re: [PATCH 2/2] SHM_UNLOCK: fix Unevictable pages stranded after swap References: In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins Cc: kosaki.motohiro@gmail.com, Andrew Morton , Minchan Kim , Rik van Riel , Shaohua Li , Eric Dumazet , Johannes Weiner , Michel Lespinasse , linux-kernel@vger.kernel.org, linux-mm@kvack.org 2012/1/6 Hugh Dickins : > Commit cc39c6a9bbde "mm: account skipped entries to avoid looping in > find_get_pages" correctly fixed an infinite loop; but left a problem > that find_get_pages() on shmem would return 0 (appearing to callers > to mean end of tree) when it meets a run of nr_pages swap entries. > > The only uses of find_get_pages() on shmem are via pagevec_lookup(), > called from invalidate_mapping_pages(), and from shmctl SHM_UNLOCK's > scan_mapping_unevictable_pages(). The first is already commented, > and not worth worrying about; but the second can leave pages on the > Unevictable list after an unusual sequence of swapping and locking. > > Fix that by using shmem_find_get_pages_and_swap() (then ignoring > the swap) instead of pagevec_lookup(). > > But I don't want to contaminate vmscan.c with shmem internals, nor > shmem.c with LRU locking. So move scan_mapping_unevictable_pages() > into shmem.c, renaming it shmem_unlock_mapping(); and rename > check_move_unevictable_page() to check_move_unevictable_pages(), > looping down an array of pages, oftentimes under the same lock. > > Leave out the "rotate unevictable list" block: that's a leftover > from when this was used for /proc/sys/vm/scan_unevictable_pages, > whose flawed handling involved looking at pages at tail of LRU. > > Was there significance to the sequence first ClearPageUnevictable, > then test page_evictable, then SetPageUnevictable here? I think > not, we're under LRU lock, and have no barriers between those. If I understand correctly, this is not exactly correct. Because of, PG_mlocked operation is not protected by LRU lock. So, I think we have three choice. 1) check_move_unevictable_pages() aimed retry logic and put pages back into correct lru. 2) check_move_unevictable_pages() unconditionally move the pages into evictable lru, and vmacan put them back into correct lru later. 3) To protect PG_mlock operation by lru lock. other parts looks fine to me. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org