Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating the hot remove range

From: David Hildenbrand <david@redhat.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
	David Hildenbrand <dhildenb@redhat.com>
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
Subject: Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating the hot remove range
Date: Mon, 13 Jan 2020 11:37:48 +0100	[thread overview]
Message-ID: <19194427-1295-3596-2c2c-463c4adf4f35@redhat.com> (raw)
In-Reply-To: <6b8fb779-31e8-1b63-85a8-9f6c93a04494@arm.com>

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 <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.

Do you really need that?

We have SECTION_IS_EARLY. You could iterate all involved sections (for
which you are getting notified) and check if any one of these is marked
SECTION_IS_EARLY. then, it was added during boot and not via add_memory().

-- 
Thanks,

David / dhildenb