From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754255AbZELEha (ORCPT ); Tue, 12 May 2009 00:37:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751518AbZELEhV (ORCPT ); Tue, 12 May 2009 00:37:21 -0400 Received: from TYO202.gate.nec.co.jp ([202.32.8.206]:59173 "EHLO tyo202.gate.nec.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751270AbZELEhU (ORCPT ); Tue, 12 May 2009 00:37:20 -0400 Date: Tue, 12 May 2009 13:32:38 +0900 From: Daisuke Nishimura To: KAMEZAWA Hiroyuki Cc: nishimura@mxp.nes.nec.co.jp, "linux-mm@kvack.org" , "balbir@linux.vnet.ibm.com" , "akpm@linux-foundation.org" , mingo@elte.hu, "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 2/3] fix swap cache account leak at swapin-readahead Message-Id: <20090512133238.0fb41722.nishimura@mxp.nes.nec.co.jp> In-Reply-To: <20090512104603.ac4ca1f4.kamezawa.hiroyu@jp.fujitsu.com> References: <20090512104401.28edc0a8.kamezawa.hiroyu@jp.fujitsu.com> <20090512104603.ac4ca1f4.kamezawa.hiroyu@jp.fujitsu.com> Organization: NEC Soft, Ltd. X-Mailer: Sylpheed 2.6.0 (GTK+ 2.10.14; i686-pc-mingw32) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 12 May 2009 10:46:03 +0900, KAMEZAWA Hiroyuki wrote: > From: KAMEZAWA Hiroyuki > > In general, Linux's swp_entry handling is done by combination of lazy techniques > and global LRU. It works well but when we use mem+swap controller, some more > strict control is appropriate. Otherwise, swp_entry used by a cgroup will be > never freed until global LRU works. In a system where memcg is well-configured, > global LRU doesn't work frequently. > > Example) Assume swapin-readahead. > CPU0 CPU1 > zap_pte() read_swap_cache_async() > swap_duplicate(). > swap_entry_free() = 1 > find_get_page()=> NULL. > add_to_swap_cache(). > issue swap I/O. > > There are many patterns of this kind of race (but no problems). > > free_swap_and_cache() is called for freeing swp_entry. But it is a best-effort > function. If the swp_entry/page seems busy, swp_entry is not freed. > This is not a problem because global-LRU will find SwapCache at page reclaim. > > If memcg is used, on the other hand, global LRU may not work. Then, above > unused SwapCache will not be freed. > (unmapped SwapCache occupy swp_entry but never be freed if not on memcg's LRU) > > So, even if there are no tasks in a cgroup, swp_entry usage still remains. > In bad case, OOM by mem+swap controller is triggered by this "leak" of > swp_entry as Nishimura reported. > > Considering this issue, swapin-readahead itself is not very good for memcg. > It read swap cache which will not be used. (and _unused_ swapcache will > not be accounted.) Even if we account swap cache at add_to_swap_cache(), > we need to account page to several _unrelated_ memcg. This is bad. > > This patch tries to fix racy case of free_swap_and_cache() and page status. > > After this patch applied, following test works well. > > # echo 1-2M > ../memory.limit_in_bytes > # run tasks under memcg. > # kill all tasks and make memory.tasks empty > # check memory.memsw.usage_in_bytes == memory.usage_in_bytes and > there is no _used_ swp_entry. > > What this patch does is > - avoid swapin-readahead when memcg is activated. > > Changelog: v6 -> v7 > - just handle races in readahead. > - races in writeback is handled in the next patch. > > Changelog: v5 -> v6 > - works only when memcg is activated. > - check after I/O works only after writeback. > - avoid swapin-readahead when memcg is activated. > - fixed page refcnt issue. > Changelog: v4->v5 > - completely new design. > > Reported-by: Daisuke Nishimura > Signed-off-by: KAMEZAWA Hiroyuki Looks good to me. Reviewed-by: Daisuke Nishimura From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with ESMTP id 6FA3F6B005D for ; Tue, 12 May 2009 00:36:04 -0400 (EDT) Date: Tue, 12 May 2009 13:32:38 +0900 From: Daisuke Nishimura Subject: Re: [PATCH 2/3] fix swap cache account leak at swapin-readahead Message-Id: <20090512133238.0fb41722.nishimura@mxp.nes.nec.co.jp> In-Reply-To: <20090512104603.ac4ca1f4.kamezawa.hiroyu@jp.fujitsu.com> References: <20090512104401.28edc0a8.kamezawa.hiroyu@jp.fujitsu.com> <20090512104603.ac4ca1f4.kamezawa.hiroyu@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: KAMEZAWA Hiroyuki Cc: nishimura@mxp.nes.nec.co.jp, "linux-mm@kvack.org" , "balbir@linux.vnet.ibm.com" , "akpm@linux-foundation.org" , mingo@elte.hu, "linux-kernel@vger.kernel.org" List-ID: On Tue, 12 May 2009 10:46:03 +0900, KAMEZAWA Hiroyuki wrote: > From: KAMEZAWA Hiroyuki > > In general, Linux's swp_entry handling is done by combination of lazy techniques > and global LRU. It works well but when we use mem+swap controller, some more > strict control is appropriate. Otherwise, swp_entry used by a cgroup will be > never freed until global LRU works. In a system where memcg is well-configured, > global LRU doesn't work frequently. > > Example) Assume swapin-readahead. > CPU0 CPU1 > zap_pte() read_swap_cache_async() > swap_duplicate(). > swap_entry_free() = 1 > find_get_page()=> NULL. > add_to_swap_cache(). > issue swap I/O. > > There are many patterns of this kind of race (but no problems). > > free_swap_and_cache() is called for freeing swp_entry. But it is a best-effort > function. If the swp_entry/page seems busy, swp_entry is not freed. > This is not a problem because global-LRU will find SwapCache at page reclaim. > > If memcg is used, on the other hand, global LRU may not work. Then, above > unused SwapCache will not be freed. > (unmapped SwapCache occupy swp_entry but never be freed if not on memcg's LRU) > > So, even if there are no tasks in a cgroup, swp_entry usage still remains. > In bad case, OOM by mem+swap controller is triggered by this "leak" of > swp_entry as Nishimura reported. > > Considering this issue, swapin-readahead itself is not very good for memcg. > It read swap cache which will not be used. (and _unused_ swapcache will > not be accounted.) Even if we account swap cache at add_to_swap_cache(), > we need to account page to several _unrelated_ memcg. This is bad. > > This patch tries to fix racy case of free_swap_and_cache() and page status. > > After this patch applied, following test works well. > > # echo 1-2M > ../memory.limit_in_bytes > # run tasks under memcg. > # kill all tasks and make memory.tasks empty > # check memory.memsw.usage_in_bytes == memory.usage_in_bytes and > there is no _used_ swp_entry. > > What this patch does is > - avoid swapin-readahead when memcg is activated. > > Changelog: v6 -> v7 > - just handle races in readahead. > - races in writeback is handled in the next patch. > > Changelog: v5 -> v6 > - works only when memcg is activated. > - check after I/O works only after writeback. > - avoid swapin-readahead when memcg is activated. > - fixed page refcnt issue. > Changelog: v4->v5 > - completely new design. > > Reported-by: Daisuke Nishimura > Signed-off-by: KAMEZAWA Hiroyuki Looks good to me. Reviewed-by: Daisuke Nishimura -- 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/ . Don't email: email@kvack.org