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Subject: Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating
 the hot remove range
To:     David Hildenbrand <dhildenb@redhat.com>
Cc:     David Hildenbrand <david@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
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        osalvador@suse.de, ard.biesheuvel@arm.com, steve.capper@arm.com,
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        robin.murphy@arm.com, steven.price@arm.com, suzuki.poulose@arm.com,
        ira.weiny@intel.com
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 <3C3BE5FA-0CFC-4C90-8657-63EF5B680B0B@redhat.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <6b8fb779-31e8-1b63-85a8-9f6c93a04494@arm.com>
Date:   Mon, 13 Jan 2020 15:20:09 +0530
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On 01/13/2020 02:44 PM, David Hildenbrand wrote:
> 
> 
>> Am 13.01.2020 um 10:10 schrieb Anshuman Khandual <anshuman.khandual@arm.com>:
>>
>> ﻿
>>
>>> 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_remove_memory().
>>>> Platform gets called with arch_remove_memory() to tear down required kernel
>>>> page tables and other arch specific procedures. But there are platforms
>>>> like arm64 which might want to prevent removal of certain specific memory
>>>> ranges irrespective of their present usage or movability properties.
>>>
>>> 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 range.
>>
>>>
>>> If it's only boot memory, we should disallow offlining instead via 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 notifier
>> 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 used both
>> for for kernel (init_mm) or user space process at same time. Is there some
>> mechanism we could do this ?
>>
>>>
>>>>
>>>> Current arch call back arch_remove_memory() is too late in the process to
>>>> abort memory hot removal as memory block devices and firmware memory map
>>>> entries would have already been removed. Platforms should be able to abort
>>>> the process before taking the mem_hotplug_lock with mem_hotplug_begin().
>>>> This essentially requires a new arch callback for memory range validation.
>>>
>>> I somewhat dislike this very much. Memory removal should never fail 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 add and hot
>>>> remove paths before carving out a new helper check_hotremove_memory_range()
>>>> which incorporates a new arch callback. This call back provides platforms
>>>> an opportunity to refuse memory removal at the very onset. In future the
>>>> same principle can be extended for memory hot add path if required.
>>>>
>>>> Platforms can choose to override this callback in order to reject specific
>>>> memory ranges from removal or can just fallback to a default implementation
>>>> 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/kexec.
>>
>> 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 should be it.
> 

Got it, thanks ! But we will still need boot memory enumeration via MEMBLOCK_BOOT
to reject affected offline requests in the callback.