From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932388Ab3AJGLD (ORCPT ); Thu, 10 Jan 2013 01:11:03 -0500 Received: from cn.fujitsu.com ([222.73.24.84]:3293 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751029Ab3AJGK7 (ORCPT ); Thu, 10 Jan 2013 01:10:59 -0500 X-IronPort-AV: E=Sophos;i="4.84,442,1355068800"; d="scan'208";a="6556630" Message-ID: <50EE57F7.1000304@cn.fujitsu.com> Date: Thu, 10 Jan 2013 13:56:07 +0800 From: Tang Chen User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1 MIME-Version: 1.0 To: Andrew Morton CC: rientjes@google.com, len.brown@intel.com, benh@kernel.crashing.org, paulus@samba.org, cl@linux.com, minchan.kim@gmail.com, kosaki.motohiro@jp.fujitsu.com, isimatu.yasuaki@jp.fujitsu.com, wujianguo@huawei.com, wency@cn.fujitsu.com, hpa@zytor.com, linfeng@cn.fujitsu.com, laijs@cn.fujitsu.com, mgorman@suse.de, yinghai@kernel.org, glommer@parallels.com, x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-acpi@vger.kernel.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, linux-ia64@vger.kernel.org, cmetcalf@tilera.com, sparclinux@vger.kernel.org Subject: Re: [PATCH v6 02/15] memory-hotplug: check whether all memory blocks are offlined or not when removing memory References: <1357723959-5416-1-git-send-email-tangchen@cn.fujitsu.com> <1357723959-5416-3-git-send-email-tangchen@cn.fujitsu.com> <20130109151140.76982b9e.akpm@linux-foundation.org> In-Reply-To: <20130109151140.76982b9e.akpm@linux-foundation.org> X-MIMETrack: Itemize by SMTP Server on mailserver/fnst(Release 8.5.3|September 15, 2011) at 2013/01/10 13:56:18, Serialize by Router on mailserver/fnst(Release 8.5.3|September 15, 2011) at 2013/01/10 13:56:29, Serialize complete at 2013/01/10 13:56:29 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=ISO-8859-1; format=flowed Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Andrew, On 01/10/2013 07:11 AM, Andrew Morton wrote: > On Wed, 9 Jan 2013 17:32:26 +0800 > Tang Chen wrote: > >> We remove the memory like this: >> 1. lock memory hotplug >> 2. offline a memory block >> 3. unlock memory hotplug >> 4. repeat 1-3 to offline all memory blocks >> 5. lock memory hotplug >> 6. remove memory(TODO) >> 7. unlock memory hotplug >> >> All memory blocks must be offlined before removing memory. But we don't hold >> the lock in the whole operation. So we should check whether all memory blocks >> are offlined before step6. Otherwise, kernel maybe panicked. > > Well, the obvious question is: why don't we hold lock_memory_hotplug() > for all of steps 1-4? Please send the reasons for this in a form which > I can paste into the changelog. In the changelog form: Offlining a memory block and removing a memory device can be two different operations. Users can just offline some memory blocks without removing the memory device. For this purpose, the kernel has held lock_memory_hotplug() in __offline_pages(). To reuse the code for memory hot-remove, we repeat step 1-3 to offline all the memory blocks, repeatedly lock and unlock memory hotplug, but not hold the memory hotplug lock in the whole operation. > > > Actually, I wonder if doing this would fix a race in the current > remove_memory() repeat: loop. That code does a > find_memory_block_hinted() followed by offline_memory_block(), but > afaict find_memory_block_hinted() only does a get_device(). Is the > get_device() sufficiently strong to prevent problems if another thread > concurrently offlines or otherwise alters this memory_block's state? I think we already have memory_block->state_mutex to protect the concurrently changing of memory_block's state. The find_memory_block_hinted() here is to find the memory_block corresponding to the memory section we are dealing with. Thanks. :) >