From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198]) by kanga.kvack.org (Postfix) with ESMTP id 0CEBA6B0033 for ; Tue, 28 Nov 2017 20:21:06 -0500 (EST) Received: by mail-wr0-f198.google.com with SMTP id c3so1019014wrd.0 for ; Tue, 28 Nov 2017 17:21:06 -0800 (PST) Received: from smtp.nue.novell.com (smtp.nue.novell.com. [195.135.221.5]) by mx.google.com with ESMTPS id 59si635981edy.340.2017.11.28.17.21.04 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 28 Nov 2017 17:21:04 -0800 (PST) Date: Wed, 29 Nov 2017 09:20:40 +0800 From: joeyli Subject: Re: [PATCH v2 2/5] mm: memory_hotplug: Remove assumption on memory state before hotremove Message-ID: <20171129012040.GC1469@linux-l9pv.suse> References: <4e21a27570f665793debf167c8567c6752116d0a.1511433386.git.ar@linux.vnet.ibm.com> <20171124144917.GB1966@samekh> <20171124154317.copbe3u6y2q4mura@dhcp22.suse.cz> <20171124155458.GC1966@samekh> <20171124164042.3crcoz2lwgwv725l@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171124164042.3crcoz2lwgwv725l@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Andrea Reale , "Rafael J. Wysocki" , "linux-arm-kernel@lists.infradead.org" , Linux Kernel Mailing List , Linux Memory Management List , m.bielski@virtualopensystems.com, arunks@qti.qualcomm.com, Mark Rutland , scott.branden@broadcom.com, Will Deacon , qiuxishi@huawei.com, Catalin Marinas , Rafael Wysocki , ACPI Devel Maling List On Fri, Nov 24, 2017 at 07:17:41PM +0100, Michal Hocko wrote: > On Fri 24-11-17 15:54:59, Andrea Reale wrote: > > On Fri 24 Nov 2017, 16:43, Michal Hocko wrote: > > > On Fri 24-11-17 14:49:17, Andrea Reale wrote: > > > > Hi Rafael, > > > > > > > > On Fri 24 Nov 2017, 15:39, Rafael J. Wysocki wrote: > > > > > On Fri, Nov 24, 2017 at 11:22 AM, Andrea Reale wrote: > > > > > > Resending the patch adding linux-acpi in CC, as suggested by Rafael. > > > > > > Everyone else: apologies for the noise. > > > > > > > > > > > > Commit 242831eb15a0 ("Memory hotplug / ACPI: Simplify memory removal") > > > > > > introduced an assumption whereas when control > > > > > > reaches remove_memory the corresponding memory has been already > > > > > > offlined. In that case, the acpi_memhotplug was making sure that > > > > > > the assumption held. > > > > > > This assumption, however, is not necessarily true if offlining > > > > > > and removal are not done by the same "controller" (for example, > > > > > > when first offlining via sysfs). > > > > > > > > > > > > Removing this assumption for the generic remove_memory code > > > > > > and moving it in the specific acpi_memhotplug code. This is > > > > > > a dependency for the software-aided arm64 offlining and removal > > > > > > process. > > > > > > > > > > > > Signed-off-by: Andrea Reale > > > > > > Signed-off-by: Maciej Bielski > > > > > > --- > > > > > > drivers/acpi/acpi_memhotplug.c | 2 +- > > > > > > include/linux/memory_hotplug.h | 9 ++++++--- > > > > > > mm/memory_hotplug.c | 13 +++++++++---- > > > > > > 3 files changed, 16 insertions(+), 8 deletions(-) > > > > > > > > > > > > diff --git a/drivers/acpi/acpi_memhotplug.c b/drivers/acpi/acpi_memhotplug.c > > > > > > index 6b0d3ef..b0126a0 100644 > > > > > > --- a/drivers/acpi/acpi_memhotplug.c > > > > > > +++ b/drivers/acpi/acpi_memhotplug.c > > > > > > @@ -282,7 +282,7 @@ static void acpi_memory_remove_memory(struct acpi_memory_device *mem_device) > > > > > > nid = memory_add_physaddr_to_nid(info->start_addr); > > > > > > > > > > > > acpi_unbind_memory_blocks(info); > > > > > > - remove_memory(nid, info->start_addr, info->length); > > > > > > + BUG_ON(remove_memory(nid, info->start_addr, info->length)); > > > > > > > > > > Why does this have to be BUG_ON()? Is it really necessary to kill the > > > > > system here? > > > > > > > > Actually, I hoped you would help me understand that: that BUG() call was introduced > > > > by yourself in Commit 242831eb15a0 ("Memory hotplug / ACPI: Simplify memory removal") > > > > in memory_hoptlug.c:remove_memory()). > > > > > > > > Just reading at that commit my understanding was that you were assuming > > > > that acpi_memory_remove_memory() have already done the job of offlining > > > > the target memory, so there would be a bug if that wasn't the case. > > > > > > > > In my case, that assumption did not hold and I found that it might not > > > > hold for other platforms that do not use ACPI. In fact, the purpose of > > > > this patch is to move this assumption out of the generic hotplug code > > > > and move it to ACPI code where it originated. > > > > > > remove_memory failure is basically impossible to handle AFAIR. The > > > original code to BUG in remove_memory is ugly as hell and we do not want > > > to spread that out of that function. Instead we really want to get rid > > > of it. > > > > Today, BUG() is called even in the simple case where remove fails > > because the section we are removing is not offline. > > You cannot hotremove memory which is still online. This is what caller > should enforce. This is too late to handle the failure. At least for > ACPI. > The logic in acpi_scan_hot_remove() calls memory_subsys_offline(). If there doesn't have any error returns by memory_subsys_offline, then ACPI assumes all devices are offlined by subsystem (memory subsystem in this case). Then system moves to remove stage, ACPI calls acpi_memory_device_remove(). Here > > I cannot see any need to > > BUG() in such a case: an error code seems more than sufficient to me. > > I do not rememeber details but AFAIR ACPI is in a deferred (kworker) > context here and cannot simply communicate error code down the road. > I agree that we should be able to simply return an error but what is the > actual error condition that might happen here? > Currently acpi_bus_trim() didn't handle any return error. If subsystem returns error, then ACPI can only interrupt hot-remove process. > > This is why this patch removes the BUG() call when the "offline" check > > fails from the generic code. > > As I've said we should simply get rid of BUG rather than move it around. > As I remember that the original BUG() helped us to find out a bug about the offline state doesn't sync between memblock device with memory state. Something likes: mem->dev.offline != (mem->state == MEM_OFFLINE) So, the BUG() is useful to capture bug about state sync between device object and subsystem object. Thanks Joey Lee -- 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