Re: [PATCH v6 0/6] mm: introduce memfd_secret system call to create "secret" memory areas

[David Hildenbrand <david@redhat.com>]

On 03.11.20 10:52, Mike Rapoport wrote:
> On Mon, Nov 02, 2020 at 06:51:09PM +0100, David Hildenbrand wrote:
>>>> Assume you have a system with quite some ZONE_MOVABLE memory (esp. in
>>>> virtualized environments), eating up a significant amount of !ZONE_MOVABLE
>>>> memory dynamically at runtime can lead to non-obvious issues. It looks like
>>>> you have plenty of free memory, but the kernel might still OOM when trying
>>>> to do kernel allocations e.g., for pagetables. With CMA we at least know
>>>> what we're dealing with - it behaves like ZONE_MOVABLE except for the owner
>>>> that can place unmovable pages there. We can use it to compute statically
>>>> the amount of ZONE_MOVABLE memory we can have in the system without doing
>>>> harm to the system.
>>>
>>> Why would you say that secretmem allocates from !ZONE_MOVABLE?
>>> If we put boot time reservations aside, the memory allocation for
>>> secretmem follows the same rules as the memory allocations for any file
>>> descriptor. That means we allocate memory with GFP_HIGHUSER_MOVABLE.
>>
>> Oh, okay - I missed that! I had the impression that pages are unmovable and
>> allocating from ZONE_MOVABLE would be a violation of that?
>>
>>> After the allocation the memory indeed becomes unmovable but it's not
>>> like we are eating memory from other zones here.
>>
>> ... and here you have your problem. That's a no-no. We only allow it in very
>> special cases where it can't be avoided - e.g., vfio having to pin guest
>> memory when passing through memory to VMs.
>>
>> Hotplug memory, online it to ZONE_MOVABLE. Allocate secretmem. Try to unplug
>> the memory again -> endless loop in offline_pages().
>>
>> Or have a CMA area that gets used with GFP_HIGHUSER_MOVABLE. Allocate
>> secretmem. The owner of the area tries to allocate memory - always fails.
>> Purpose of CMA destroyed.
>>
>>>
>>>> Ideally, we would want to support page migration/compaction and allow for
>>>> allocation from ZONE_MOVABLE as well. Would involve temporarily mapping,
>>>> copying, unmapping. Sounds feasible, but not sure which roadblocks we would
>>>> find on the way.
>>>
>>> We can support migration/compaction with temporary mapping. The first
>>> roadblock I've hit there was that migration allocates 4K destination
>>> page and if we use it in secret map we are back to scrambling the direct
>>> map into 4K pieces. It still sounds feasible but not as trivial :)
>>
>> That sounds like the proper way for me to do it then.
>   
> Although migration of secretmem pages sounds feasible now, there maybe
> other issues I didn't see because I'm not very familiar with
> migration/compaction code.

Migration of PMDs might also be feasible -  and it would be even 
cleaner. But I agree that that might require more work and starting with 
something simpler (!movable) is the right way to move forward.

-- 
Thanks,

David / dhildenb
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