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Thu, 10 Sep 2020 12:38:03 +0000 (UTC) Received: from [10.36.113.88] (ovpn-113-88.ams2.redhat.com [10.36.113.88]) by smtp.corp.redhat.com (Postfix) with ESMTP id EDC1187D99; Thu, 10 Sep 2020 12:38:00 +0000 (UTC) Subject: Re: [PATCH] mm: don't rely on system state to detect hot-plug operations To: Laurent Dufour , Michal Hocko Cc: akpm@linux-foundation.org, Oscar Salvador , rafael@kernel.org, nathanl@linux.ibm.com, cheloha@linux.ibm.com, stable@vger.kernel.org, Greg Kroah-Hartman , linux-mm@kvack.org, LKML References: <5cbd92e1-c00a-4253-0119-c872bfa0f2bc@redhat.com> <20200908170835.85440-1-ldufour@linux.ibm.com> <20200909074011.GD7348@dhcp22.suse.cz> <9faac1ce-c02d-7dbc-f79a-4aaaa5a73d28@linux.ibm.com> <20200909090953.GE7348@dhcp22.suse.cz> <4cdb54be-1a92-4ba4-6fee-3b415f3468a9@linux.ibm.com> <20200909105914.GF7348@dhcp22.suse.cz> <74a62b00-235e-7deb-2814-f3b240fea25e@linux.ibm.com> <20200910072331.GB28354@dhcp22.suse.cz> <31cfdf35-618f-6f56-ef16-0d999682ad02@linux.ibm.com> <20200910111246.GE28354@dhcp22.suse.cz> <65b2680d-b02e-16c9-66a4-e38b9d3fa52b@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10.09.20 14:36, Laurent Dufour wrote: > Le 10/09/2020 à 14:00, David Hildenbrand a écrit : >> On 10.09.20 13:35, Laurent Dufour wrote: >>> Le 10/09/2020 à 13:12, Michal Hocko a écrit : >>>> On Thu 10-09-20 09:51:39, Laurent Dufour wrote: >>>>> Le 10/09/2020 à 09:23, Michal Hocko a écrit : >>>>>> On Wed 09-09-20 18:07:15, Laurent Dufour wrote: >>>>>>> Le 09/09/2020 à 12:59, Michal Hocko a écrit : >>>>>>>> On Wed 09-09-20 11:21:58, Laurent Dufour wrote: >>>>>> [...] >>>>>>>>> For the point a, using the enum allows to know in >>>>>>>>> register_mem_sect_under_node() if the link operation is due to a hotplug >>>>>>>>> operation or done at boot time. >>>>>>>> >>>>>>>> Yes, but let me repeat. We have a mess here and different paths check >>>>>>>> for the very same condition by different ways. We need to unify those. >>>>>>> >>>>>>> What are you suggesting to unify these checks (using a MP_* enum as >>>>>>> suggested by David, something else)? >>>>>> >>>>>> We do have system_state check spread at different places. I would use >>>>>> this one and wrap it behind a helper. Or have I missed any reason why >>>>>> that wouldn't work for this case? >>>>> >>>>> That would not work in that case because memory can be hot-added at the >>>>> SYSTEM_SCHEDULING system state and the regular memory is also registered at >>>>> that system state too. So system state is not enough to discriminate between >>>>> the both. >>>> >>>> If that is really the case all other places need a fix as well. >>>> Btw. could you be more specific about memory hotplug during early boot? >>>> How that happens? I am only aware of https://lkml.kernel.org/r/20200818110046.6664-1-osalvador@suse.de >>>> and that doesn't happen as early as SYSTEM_SCHEDULING. >>> >>> That points has been raised by David, quoting him here: >>> >>>> IIRC, ACPI can hotadd memory while SCHEDULING, this patch would break that. >>>> >>>> Ccing Oscar, I think he mentioned recently that this is the case with ACPI. >>> >>> Oscar told that he need to investigate further on that. >>> >>> On my side I can't get these ACPI "early" hot-plug operations to happen so I >>> can't check that. >>> >>> If this is clear that ACPI memory hotplug doesn't happen at SYSTEM_SCHEDULING, >>> the patch I proposed at first is enough to fix the issue. >>> >> >> Booting a qemu guest with 4 coldplugged DIMMs gives me: >> >> :/root# dmesg | grep link_mem >> [ 0.302247] link_mem_sections() during 1 >> [ 0.445086] link_mem_sections() during 1 >> [ 0.445766] link_mem_sections() during 1 >> [ 0.446749] link_mem_sections() during 1 >> [ 0.447746] link_mem_sections() during 1 >> >> So AFAICs everything happens during SYSTEM_SCHEDULING - boot memory and >> ACPI (cold)plug. >> >> To make forward progress with this, relying on the system_state is >> obviously not sufficient. >> >> 1. We have to fix this instance and the instance directly in >> get_nid_for_pfn() by passing in the context (I once had a patch to clean >> that up, to not have two state checks, but it got lost somewhere). >> >> 2. The "system_state < SYSTEM_RUNNING" check in >> register_memory_resource() is correct. Actual memory hotplug after boot >> is not impacted. (I remember we discussed this exact behavior back then) >> >> 3. build_all_zonelists() should work as expected, called from >> start_kernel() before sched_init(). > > I'm bit confused now. > Since hotplug operation is happening at SYSTEM_SCHEDULING like the regular > memory registration, would it be enough to add a parameter to > register_mem_sect_under_node() (reworking the memmap_context enum)? > That way the check is not based on the system state but on the calling path. > That would have been my suggestion to definitely fix it - maybe Michal/Oscar have a better suggestion know that we know what's going on. -- Thanks, David / dhildenb