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Tue, 14 Jan 2020 12:30:49 +0000 (UTC) Received: from [10.36.118.60] (unknown [10.36.118.60]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0CD5EA4B60; Tue, 14 Jan 2020 12:30:43 +0000 (UTC) Subject: Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating the hot remove range To: Anshuman Khandual , David Hildenbrand Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com, cai@lca.pw, logang@deltatee.com, cpandya@codeaurora.org, arunks@codeaurora.org, dan.j.williams@intel.com, mgorman@techsingularity.net, osalvador@suse.de, ard.biesheuvel@arm.com, steve.capper@arm.com, broonie@kernel.org, valentin.schneider@arm.com, robin.murphy@arm.com, steven.price@arm.com, suzuki.poulose@arm.com, ira.weiny@intel.com References: <6f0efddc-f124-58ca-28b6-4632469cf992@arm.com> <3C3BE5FA-0CFC-4C90-8657-63EF5B680B0B@redhat.com> <6b8fb779-31e8-1b63-85a8-9f6c93a04494@arm.com> <19194427-1295-3596-2c2c-463c4adf4f35@redhat.com> <78f04711-2ca6-280c-d0c2-ab9eea866e59@arm.com> <2c4b04d6-6d86-e87a-9b09-b931133a0d9c@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 MIME-Version: 1.0 In-Reply-To: <2c4b04d6-6d86-e87a-9b09-b931133a0d9c@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14.01.20 12:09, Anshuman Khandual wrote: >=20 >=20 > On 01/14/2020 07:43 AM, Anshuman Khandual wrote: >> >> >> On 01/13/2020 04:07 PM, David Hildenbrand wrote: >>> On 13.01.20 10:50, Anshuman Khandual wrote: >>>> >>>> >>>> On 01/13/2020 02:44 PM, David Hildenbrand wrote: >>>>> >>>>> >>>>>> Am 13.01.2020 um 10:10 schrieb Anshuman Khandual : >>>>>> >>>>>> =EF=BB=BF >>>>>> >>>>>>> On 01/10/2020 02:12 PM, David Hildenbrand wrote: >>>>>>>> On 10.01.20 04:09, Anshuman Khandual wrote: >>>>>>>> Currently there are two interfaces to initiate memory range hot = removal i.e >>>>>>>> remove_memory() and __remove_memory() which then calls try_remov= e_memory(). >>>>>>>> Platform gets called with arch_remove_memory() to tear down requ= ired kernel >>>>>>>> page tables and other arch specific procedures. But there are pl= atforms >>>>>>>> like arm64 which might want to prevent removal of certain specif= ic memory >>>>>>>> ranges irrespective of their present usage or movability propert= ies. >>>>>>> >>>>>>> Why? Is this only relevant for boot memory? I hope so, otherwise = the >>>>>>> arch code needs fixing IMHO. >>>>>> >>>>>> Right, it is relevant only for the boot memory on arm64 platform. = But this >>>>>> new arch callback makes it flexible to reject any given memory ran= ge. >>>>>> >>>>>>> >>>>>>> If it's only boot memory, we should disallow offlining instead vi= a a >>>>>>> memory notifier - much cleaner. >>>>>> >>>>>> Dont have much detail understanding of MMU notifier mechanism but = from some >>>>>> initial reading, it seems like we need to have a mm_struct for a n= otifier >>>>>> to monitor various events on the page table. Just wondering how a = physical >>>>>> memory range like boot memory can be monitored because it can be u= sed both >>>>>> for for kernel (init_mm) or user space process at same time. Is th= ere some >>>>>> mechanism we could do this ? >>>>>> >>>>>>> >>>>>>>> >>>>>>>> Current arch call back arch_remove_memory() is too late in the p= rocess to >>>>>>>> abort memory hot removal as memory block devices and firmware me= mory map >>>>>>>> entries would have already been removed. Platforms should be abl= e to abort >>>>>>>> the process before taking the mem_hotplug_lock with mem_hotplug_= begin(). >>>>>>>> This essentially requires a new arch callback for memory range v= alidation. >>>>>>> >>>>>>> I somewhat dislike this very much. Memory removal should never fa= il if >>>>>>> used sanely. See e.g., __remove_memory(), it will BUG() whenever >>>>>>> something like that would strike. >>>>>>> >>>>>>>> >>>>>>>> This differentiates memory range validation between memory hot a= dd and hot >>>>>>>> remove paths before carving out a new helper check_hotremove_mem= ory_range() >>>>>>>> which incorporates a new arch callback. This call back provides = platforms >>>>>>>> an opportunity to refuse memory removal at the very onset. In fu= ture the >>>>>>>> same principle can be extended for memory hot add path if requir= ed. >>>>>>>> >>>>>>>> Platforms can choose to override this callback in order to rejec= t specific >>>>>>>> memory ranges from removal or can just fallback to a default imp= lementation >>>>>>>> which allows removal of all memory ranges. >>>>>>> >>>>>>> I suspect we want really want to disallow offlining instead. E.g.= , I >>>>>> >>>>>> If boot memory pages can be prevented from being offlined for sure= , then it >>>>>> would indirectly definitely prevent hot remove process as well. >>>>>> >>>>>>> remember s390x does that with certain areas needed for dumping/ke= xec. >>>>>> >>>>>> Could not find any references to mmu_notifier in arch/s390 or any = other arch >>>>>> for that matter apart from KVM (which has an user space component)= , could you >>>>>> please give some pointers ? >>>>> >>>>> Memory (hotplug) notifier, not MMU notifier :) >>>> >>>> They are so similarly named :) >>>> >>>>> >>>>> Not on my notebook right now, grep for MEM_GOING_OFFLINE, that shou= ld be it. >>>>> >>>> >>>> Got it, thanks ! But we will still need boot memory enumeration via = MEMBLOCK_BOOT >>>> to reject affected offline requests in the callback. >>> >>> Do you really need that? >>> >>> We have SECTION_IS_EARLY. You could iterate all involved sections (fo= r >>> which you are getting notified) and check if any one of these is mark= ed >>> SECTION_IS_EARLY. then, it was added during boot and not via add_memo= ry(). >> >> Seems to be a better approach than adding a new memblock flag. >=20 > These additional changes do the trick and prevent boot memory removal. > Hope this is in line with your earlier suggestion. >=20 > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 00f3e1836558..3b59e6a29dea 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -17,6 +17,7 @@ > +++ b/arch/arm64/mm/mmu.c > @@ -17,6 +17,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -1365,4 +1366,37 @@ void arch_remove_memory(int nid, u64 start, u64 = size, > __remove_pages(start_pfn, nr_pages, altmap); > __remove_pgd_mapping(swapper_pg_dir, __phys_to_virt(start), siz= e); > } > + > +static int boot_mem_remove_notifier(struct notifier_block *nb, > + unsigned long action, void *data) > +{ > + unsigned long start_pfn, end_pfn, pfn, section_nr; > + struct mem_section *ms; > + struct memory_notify *arg =3D data; > + > + start_pfn =3D=20 > + end_pfn =3D start_pfn + arg->nr_pages; You can initialize some of these directly struct memory_notify *arg =3D data; const unsigned long end_pfn =3D arg->start_pfn; + arg->nr_pages; unsigned long pfn =3D arg->start_pfn; and avoid start_pfn. > + > + if (action !=3D MEM_GOING_OFFLINE) > + return NOTIFY_OK; > + > + for (pfn =3D start_pfn; pfn < end_pfn; pfn +=3D PAGES_PER_SECTI= ON) { > + section_nr =3D ; > + ms =3D __nr_to_section(section_nr); Also, I think you can avoid section_nr. ms =3D __nr_to_section(pfn_to_section_nr(pfn)); > + > + if (early_section(ms)) > + return NOTIFY_BAD; > + } > + return NOTIFY_OK; > +} > + > +static struct notifier_block boot_mem_remove_nb =3D { > + .notifier_call =3D boot_mem_remove_notifier, > +}; > + > +static int __init boot_mem_remove_init(void) > +{ > + return register_memory_notifier(&boot_mem_remove_nb); > +} > +device_initcall(boot_mem_remove_init); > #endif Exactly what I was suggesting :) If we ever need to offline+re-online boot memory (e.g., to a different zone), we can think of something else. --=20 Thanks, David / dhildenb