Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751502AbbLUMJn (ORCPT ); Mon, 21 Dec 2015 07:09:43 -0500 Received: from mail-ob0-f182.google.com ([209.85.214.182]:34783 "EHLO mail-ob0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751081AbbLUMJk (ORCPT ); Mon, 21 Dec 2015 07:09:40 -0500 MIME-Version: 1.0 In-Reply-To: <5677B1B9.2080707@cn.fujitsu.com> References: <20151221031501.GA32524@js1304-P5Q-DELUXE> <5677A378.6010703@cn.fujitsu.com> <20151221071747.GA4396@js1304-P5Q-DELUXE> <5677B1B9.2080707@cn.fujitsu.com> Date: Mon, 21 Dec 2015 21:09:39 +0900 Message-ID: Subject: Re: [RFC] theoretical race between memory hotplug and pfn iterator From: Joonsoo Kim To: Zhu Guihua Cc: Joonsoo Kim , Linux Memory Management List , Andrew Morton , Gu Zheng , Tang Chen , Naoya Horiguchi , Toshi Kani , Mel Gorman , Vlastimil Babka , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2855 Lines: 74 2015-12-21 17:00 GMT+09:00 Zhu Guihua : > > On 12/21/2015 03:17 PM, Joonsoo Kim wrote: >> >> On Mon, Dec 21, 2015 at 03:00:08PM +0800, Zhu Guihua wrote: >>> >>> On 12/21/2015 11:15 AM, Joonsoo Kim wrote: >>>> >>>> Hello, memory-hotplug folks. >>>> >>>> I found theoretical problems between memory hotplug and pfn iterator. >>>> For example, pfn iterator works something like below. >>>> >>>> for (pfn = zone_start_pfn; pfn < zone_end_pfn; pfn++) { >>>> if (!pfn_valid(pfn)) >>>> continue; >>>> >>>> page = pfn_to_page(pfn); >>>> /* Do whatever we want */ >>>> } >>>> >>>> Sequence of hotplug is something like below. >>>> >>>> 1) add memmap (after then, pfn_valid will return valid) >>>> 2) memmap_init_zone() >>>> >>>> So, if pfn iterator runs between 1) and 2), it could access >>>> uninitialized page information. >>>> >>>> This problem could be solved by re-ordering initialization steps. >>>> >>>> Hot-remove also has a problem. If memory is hot-removed after >>>> pfn_valid() succeed in pfn iterator, access to page would cause NULL >>>> deference because hot-remove frees corresponding memmap. There is no >>>> guard against free in any pfn iterators. >>>> >>>> This problem can be solved by inserting get_online_mems() in all pfn >>>> iterators but this looks error-prone for future usage. Another idea is >>>> that delaying free corresponding memmap until synchronization point such >>>> as system suspend. It will guarantee that there is no running pfn >>>> iterator. Do any have a better idea? >>>> >>>> Btw, I tried to memory-hotremove with QEMU 2.5.5 but it didn't work. I >>>> followed sequences in doc/memory-hotplug. Do you have any comment on >>>> this? >>> >>> I tried memory hot remove with qemu 2.5.5 and RHEL 7, it works well. >>> Maybe you can provide more details, such as guest version, err log. >> >> I'm testing with qemu 2.5.5 and linux-next-20151209 with reverting >> following two patches. >> >> "mm/memblock.c: use memblock_insert_region() for the empty array" >> >> "mm-memblock-use-memblock_insert_region-for-the-empty-array-checkpatch-fixes" >> >> When I type "device_del dimm1" in qemu monitor, there is no err log in >> kernel and it looks like command has no effect. I inserted log to >> acpi_memory_device_remove() but there is no message, too. Is there >> another way to check that device_del event is actually transmitted to >> kernel? > > > You can use udev to monitor memory device remove event. (udevadm monitor) > I have tried it but there is no message when I type hot-remove command. Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/