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 B9A376B005D for ; Thu, 10 Jan 2013 01:10:58 -0500 (EST) Message-ID: <50EE57F7.1000304@cn.fujitsu.com> Date: Thu, 10 Jan 2013 13:56:07 +0800 From: Tang Chen MIME-Version: 1.0 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> Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=ISO-8859-1; format=flowed Sender: owner-linux-mm@kvack.org List-ID: 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 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. :) > -- 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