Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934723AbdCaAAv (ORCPT ); Thu, 30 Mar 2017 20:00:51 -0400 Received: from smtp.nue.novell.com ([195.135.221.5]:42821 "EHLO smtp.nue.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933350AbdCaAAt (ORCPT ); Thu, 30 Mar 2017 20:00:49 -0400 Date: Fri, 31 Mar 2017 08:00:30 +0800 From: joeyli To: "Rafael J. Wysocki" Cc: Michal Hocko , Jiri Kosina , linux-mm@kvack.org, LKML , linux-api@vger.kernel.org Subject: Re: memory hotplug and force_remove Message-ID: <20170331000030.GO28365@linux-l9pv.suse> References: <20170320192938.GA11363@dhcp22.suse.cz> <20170330162031.GE4326@dhcp22.suse.cz> <20170330165729.GN28365@linux-l9pv.suse> <8340752.8oM5M3L67Q@aspire.rjw.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8340752.8oM5M3L67Q@aspire.rjw.lan> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3994 Lines: 96 On Thu, Mar 30, 2017 at 10:15:04PM +0200, Rafael J. Wysocki wrote: > On Friday, March 31, 2017 12:57:29 AM joeyli wrote: > > On Thu, Mar 30, 2017 at 06:20:34PM +0200, Michal Hocko wrote: > > > On Thu 30-03-17 10:47:52, Jiri Kosina wrote: > > > > On Tue, 28 Mar 2017, Rafael J. Wysocki wrote: > > > > > > > > > > > > we have been chasing the following BUG() triggering during the memory > > > > > > > > hotremove (remove_memory): > > > > > > > > ret = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL, > > > > > > > > check_memblock_offlined_cb); > > > > > > > > if (ret) > > > > > > > > BUG(); > > > > > > > > > > > > > > > > and it took a while to learn that the issue is caused by > > > > > > > > /sys/firmware/acpi/hotplug/force_remove being enabled. I was really > > > > > > > > surprised to see such an option because at least for the memory hotplug > > > > > > > > it cannot work at all. Memory hotplug fails when the memory is still > > > > > > > > in use. Even if we do not BUG() here enforcing the hotplug operation > > > > > > > > will lead to problematic behavior later like crash or a silent memory > > > > > > > > corruption if the memory gets onlined back and reused by somebody else. > > > > > > > > > > > > > > > > I am wondering what was the motivation for introducing this behavior and > > > > > > > > whether there is a way to disallow it for memory hotplug. Or maybe drop > > > > > > > > it completely. What would break in such a case? > > > > > > > > > > > > > > Honestly, I don't remember from the top of my head and I haven't looked at > > > > > > > that code for several months. > > > > > > > > > > > > > > I need some time to recall that. > > > > > > > > > > > > Did you have any chance to look into this? > > > > > > > > > > Well, yes. > > > > > > > > > > It looks like that was added for some people who depended on the old behavior > > > > > at that time. > > > > > > > > > > I guess we can try to drop it and see what happpens. :-) > > > > > > > > I'd agree with that; at the same time, udev rule should be submitted to > > > > systemd folks though. I don't think there is anything existing in this > > > > area yet (neither do distros ship their own udev rules for this AFAIK). > > > > > > Another option would keepint the force_remove knob but make the code be > > > error handling aware. In other words rather than ignoring offline error > > > simply propagate it up the chain and do not consider the offline. Would > > > that be acceptable? > > > > Then the only difference between normal mode is that the force_remove mode > > doesn't send out uevent for not-offline-yet container. > > Which would be rather confusing. > > The whole point of the thing was the "remove no matter what" behavior and > there's not much point in keeping it around without that. > OK~ Understood. Then back the "remove no matter waht" behavior, the point is force_remove knob causes that acpi_scan_try_to_offline() ignored the offline error of parent/children devices in the second pass: drivers/acpi/scan.c static int acpi_scan_try_to_offline(struct acpi_device *device) { ... /* first pass to call bus offline for parent */ acpi_bus_offline(handle, 0, (void *)false, (void **)&errdev); /* if failed, then second pass */ if (errdev) { errdev = NULL; /* children devices, second pass */ acpi_walk_namespace(ACPI_TYPE_ANY, handle, ACPI_UINT32_MAX, NULL, acpi_bus_offline, (void *)true, (void **)&errdev); /* ignored children's offline error here */ if (!errdev || acpi_force_hot_remove) /* ignored parent's offline error */ acpi_bus_offline(handle, 0, (void *)true, (void **)&errdev); /* will not set devices back to online */ if (errdev && !acpi_force_hot_remove) { ... } return 0; } Then acpi_scan_try_to_offline() returns 0 and go to _remove_ stage, then memory subsystem raises BUG() because the device offline state doesn't sync with memory block state. Thanks a lot! Joey Lee