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

[David Hildenbrand]

>> 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.

> 
> But again, there is nothing in the current form of secretmem that
> prevents allocation from ZONE_MOVABLE.

Oh, there is something: That the pages are not movable.

-- 
Thanks,

David / dhildenb