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Tue, 14 Apr 2020 10:49:32 -0400 X-MC-Unique: hA029pEbPR2_lkQqz8HIyg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id DD68E8017F5; Tue, 14 Apr 2020 14:49:30 +0000 (UTC) Received: from [10.36.113.201] (ovpn-113-201.ams2.redhat.com [10.36.113.201]) by smtp.corp.redhat.com (Postfix) with ESMTP id D9CF918A85; Tue, 14 Apr 2020 14:49:27 +0000 (UTC) Subject: Re: [PATCH 1/3] kexec: Prevent removal of memory in use by a loaded kexec image To: Baoquan He References: <20200411093009.GH25745@shell.armlinux.org.uk> <20200412053507.GA4247@MiWiFi-R3L-srv> <20200412080836.GM25745@shell.armlinux.org.uk> <87wo6klbw0.fsf@x220.int.ebiederm.org> <20200413023701.GA20265@MiWiFi-R3L-srv> <871rorjzmc.fsf@x220.int.ebiederm.org> <20200414064031.GB4247@MiWiFi-R3L-srv> <86e96214-7053-340b-5c1a-ff97fb94d8e0@redhat.com> <20200414092201.GD4247@MiWiFi-R3L-srv> <20200414143912.GE4247@MiWiFi-R3L-srv> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200414143912.GE4247@MiWiFi-R3L-srv> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200414_074937_213752_4E02B853 X-CRM114-Status: GOOD ( 30.80 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: piliu@redhat.com, Anshuman Khandual , Catalin Marinas , Bhupesh Sharma , linuxppc-dev@lists.ozlabs.org, kexec@lists.infradead.org, Russell King - ARM Linux admin , linux-mm@kvack.org, James Morse , "Eric W. Biederman" , Andrew Morton , Will Deacon , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 14.04.20 16:39, Baoquan He wrote: > On 04/14/20 at 11:37am, David Hildenbrand wrote: >> On 14.04.20 11:22, Baoquan He wrote: >>> On 04/14/20 at 10:00am, David Hildenbrand wrote: >>>> On 14.04.20 08:40, Baoquan He wrote: >>>>> On 04/13/20 at 08:15am, Eric W. Biederman wrote: >>>>>> Baoquan He writes: >>>>>> >>>>>>> On 04/12/20 at 02:52pm, Eric W. Biederman wrote: >>>>>>>> >>>>>>>> The only benefit of kexec_file_load is that it is simple enough from a >>>>>>>> kernel perspective that signatures can be checked. >>>>>>> >>>>>>> We don't have this restriction any more with below commit: >>>>>>> >>>>>>> commit 99d5cadfde2b ("kexec_file: split KEXEC_VERIFY_SIG into KEXEC_SIG >>>>>>> and KEXEC_SIG_FORCE") >>>>>>> >>>>>>> With KEXEC_SIG_FORCE not set, we can use kexec_load_file to cover both >>>>>>> secure boot or legacy system for kexec/kdump. Being simple enough is >>>>>>> enough to astract and convince us to use it instead. And kexec_file_load >>>>>>> has been in use for several years on systems with secure boot, since >>>>>>> added in 2014, on x86_64. >>>>>> >>>>>> No. Actaully kexec_file_load is the less capable interface, and less >>>>>> flexible interface. Which is why it is appropriate for signature >>>>>> verification. >>>>> >>>>> Well, everyone has a stance and the corresponding view. You could have >>>>> wider view from long time maintenance and in upstrem position, and think >>>>> kexec_file_load is horrible. But I can only see from our work as a front >>>>> line engineer to maintain/develop kexec/kdump in RHEL, and think >>>>> kexec_file_load is easier to maintain. >>>>> >>>>> Surely except of multiple kernel image format support. No matter it is >>>>> kexec_load and kexec_file_load, e.g in x86_64, we only support bzImage. >>>>> This is produced from kerel building by default. We have no way to >>>>> support it in our distros and add it into kexec_file_load. >>>>> >>>>> [RFC PATCH] x86/boot: make ELF kernel multiboot-able >>>>> https://lkml.org/lkml/2017/2/15/654 >>>>> >>>>>> >>>>>>>> kexec_load in every other respect is the more capable and functional >>>>>>>> interface. It makes no sense to get rid of it. >>>>>>>> >>>>>>>> It does make sense to reload with a loaded kernel on memory hotplug. >>>>>>>> That is simple and easy. If we are going to handle something in the >>>>>>>> kernel it should simple an automated unloading of the kernel on memory >>>>>>>> hotplug. >>>>>>>> >>>>>>>> >>>>>>>> I think it would be irresponsible to deprecate kexec_load on any >>>>>>>> platform. >>>>>>>> >>>>>>>> I also suspect that kexec_file_load could be taught to copy the dtb >>>>>>>> on arm32 if someone wants to deal with signatures. >>>>>>>> >>>>>>>> We definitely can not even think of deprecating kexec_load until >>>>>>>> architecture that supports it also supports kexec_file_load and everyone >>>>>>>> is happy with that interface. That is Linus's no regression rule. >>>>>>> >>>>>>> I should pick a milder word to express our tendency and tell our plan >>>>>>> then 'obsolete'. Even though I added 'gradually', seems it doesn't help >>>>>>> much. I didn't mean to say 'deprecate' at all when replied. >>>>>>> >>>>>>> The situation and trend I understand about kexec_load and kexec_file_load >>>>>>> are: >>>>>>> >>>>>>> 1) Supporting kexec_file_load is suggested to add in ARCHes which don't >>>>>>> have yet, just as x86_64, arm64 and s390 have done; >>>>>>> >>>>>>> 2) kexec_file_load is suggested to use, and take precedence over >>>>>>> kexec_load in the future, if both are supported in one ARCH. >>>>>> >>>>>> The deep problem is that kexec_file_load is distinctly less expressive >>>>>> than kexec_load. >>>>>> >>>>>>> 3) Kexec_load is kept being used by ARCHes w/o kexc_file_load support, >>>>>>> and by ARCHes for back compatibility w/ kexec_file_load support. >>>>>>> >>>>>>> For 1) and 2), I think the reason is obvious as Eric said, >>>>>>> kexec_file_load is simple enough. And currently, whenever we got a bug >>>>>>> report, we may need fix them twice, for kexec_load and kexec_file_load. >>>>>>> If kexec_file_load is made by default, e.g on x86_64, we will change it >>>>>>> in kernel space only, for kexec_file_load. This is what I meant about >>>>>>> 'obsolete gradually'. I think for arm64, s390, they will do these too. >>>>>>> Unless there's some critical/blocker bug in kexec_load, to corrupt the >>>>>>> old kexec_load interface in old product. >>>>>> >>>>>> Maybe. The code that kexec_file_load sucked into the kernel is quite >>>>>> stable and rarely needs changes except during a port of kexec to >>>>>> another architecture. >>>>>> >>>>>> Last I looked the real maintenance effor of kexec and kexec on panic was >>>>>> in the drivers. So I don't think we can use maintenance to do anything. >>>>> >>>>> Not sure if I got it. But if check Lianbo's patches, a lot of effort has >>>>> been taken to make SEV work well on kexec_file_load. And we have >>>>> switched to use kexec_file_load in the newly published Fedora release >>>>> on x86_64 by default. Before this, Lianbo has investigated and done many >>>>> experiments to make sure the switching is safe. We finally made this >>>>> decision. Next we will do the switch in Enterprise distros. Once these >>>>> are proved safe, we will suggest customers to use kexec_file_load for >>>>> kexec rebooting too. In the future, we will only care about >>>>> kexec_file_load if everying is going well. But as I have explained >>>>> repeatedly, only caring about kexec_file_load means we will leave >>>>> kexec_load as is, we will not add new feature or improvement patches >>>>> for it. >>>>> >>>>> commit 6a20bd54473e11011bf2b47efb52d0759d412854 >>>>> Author: Lianbo Jiang >>>>> Date: Thu Jan 16 13:47:35 2020 +0800 >>>>> >>>>> kdump-lib: switch to the kexec_file_load() syscall on x86_64 by default >>>>> >>>>>> >>>>>>> For 3), people can still use kexec_load and develop/fix for it, if no >>>>>>> kexec_file_load supported. But 32-bit arm should be a different one, >>>>>>> more like i386, we will leave it as is, and fix anything which could >>>>>>> break it. But people really expects to improve or add feature to it? E.g >>>>>>> in this patchset, the mem hotplug issue James raised, I assume James is >>>>>>> focusing on arm64, x86_64, but not 32-bit arm. As DavidH commented in >>>>>>> another reply, people even don't agree to continue supporting memory >>>>>>> hotplug on 32-bit system. We ever took effort to fix a memory hotplug >>>>>>> bug on i386 with a patch, but people would rather set it as BROKEN. >>>>>> >>>>>> For memory hotplug just reload. Userspace already gets good events. >>>>> >>>>> Kexec_file_load is easy to maintain. This is an example. >>>>> >>>>> Lock the hotplug area where kexed-ed kernel is targeted in this patchset, >>>>> it's obviously not right. We can't disable memory hotplug just because >>>>> kexec-ed kernel is loaded ahead of time. >>>>> >>>>> Reloading is also not a good fix. Kexec-ed kernel is targeted at a >>>>> movable area, reloading can avoid kexec rebooting corruption if that >>>>> area is hot removed. But if that area is not removed, locating kernel >>>>> into the hotpluggable area will change the area into ummovable zone. >>>>> Unless we decide to not support memory hotplug in kexec-ed kernel, I >>>>> guess it's very hard. Now in our distros kexec rebooting has been >>>>> supported, the big cloud providers are deploying linux in guest, bugs on >>>>> kexec reboot failure has been reported. They need the memory hotplug to >>>>> increase/decrease memory. >>>>> >>>>> The root cause is kexec-ed kernel is targeted at hotpluggable memory >>>>> region. Just avoiding the movable area can fix it. In kexec_file_load(), >>>>> just checking or picking those unmovable region to put kernel/initrd in >>>>> function locate_mem_hole_callback() can fix it. The page or pageblock's >>>>> zone is movable or not, it's easy to know. This fix doesn't need to >>>>> bother other component. >>>> >>>> I don't fully agree. E.g., just because memory is onlined to ZONE_NORMAL >>>> does not imply that it cannot get offlined and removed e.g., this is >>>> heavily used on ppc64, with 16MB sections. >>> >>> Really? I just know there are two kinds of mem hoplug in ppc, but don't >>> know the details. So in this case, is there any flag or a way to know >>> those memory block are hotpluggable? I am curious how those kernel data >>> is avoided to be put in this area. Or ppc just freely uses it for kernel >>> data or user space data, then try to migrate when hot remove? >> >> See >> arch/powerpc/platforms/pseries/hotplug-memory.c:dlpar_memory_remove_by_count() >> >> Under DLAPR, it can remove memory in LMB granularity, which is usually >> 16MB (== single section on ppc64). DLPAR will directly online all >> hotplugged memory (LMBs) from the kernel using device_online(), which >> will go to ZONE_NORMAL. >> >> When trying to remove memory, it simply scans for offlineable 16MB >> memory blocks (==section == LMB), offlines and removes them. No need for >> the movable zone and all the involved issues. > > Yes, this is a different one, thanks for pointing it out. It sounds like > balloon driver in virt platform, doesn't it? With DLPAR there is a hypervisor involved (which manages the actual HW DIMMs), so yes. > > Avoiding to put kexec kernel into movable zone can't solve this DLPAR > case as you said. > >> >> Now, the interesting question is, can we have LMBs added during boot >> (not via add_memory()), that will later be removed via remove_memory(). >> IIRC, we had BUGs related to that, so I think yes. If a section contains >> no unmovable allocations (after boot), it can get removed. > > I do want to ask this question. If we can add LMB into system RAM, then > reload kexec can solve it. > > Another better way is adding a common function to filter out the > movable zone when search position for kexec kernel, use a arch specific > funciton to filter out DLPAR memory blocks for ppc only. Over there, > we can simply use for_each_drmem_lmb() to do that. I was thinking about something similar. Maybe something like a notifier that can be used to test if selected memory can be used for kexec images. It would apply to - arm64 and filter out all hotadded memory (IIRC, only boot memory can be used). - powerpc to filter out all LMBs that can be removed (assuming not all memory corresponds to LMBs that can be removed, otherwise we're in trouble ... :) ) - virtio-mem to filter out all memory it added. - hyper-v to filter out partially backed memory blocks (esp. the last memory block it added and only partially backed it by memory). This would make it work for kexec_file_load(), however, I do wonder how we would want to approach that from userspace kexec-tools when handling it from kexec_load(). -- Thanks, David / dhildenb _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel