Messages in this thread | | | Subject | Re: [PATCH v6 0/6] mm: introduce memfd_secret system call to create "secret" memory areas | From | David Hildenbrand <> | Date | Mon, 2 Nov 2020 18:51:09 +0100 |
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>> 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
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