Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

David Hildenbrand
2024-06-20T09:00:51Z

>> Yes, and I think we might have to revive that discussion, unfortunately.
>> I started thinking about this, but did not reach a conclusion. Sharing
>> my thoughts.
>>
>> The minimum we might need to make use of guest_memfd (v1 or v2 ;) ) not
>> just for private memory should be:
>>
>> (1) Have private + shared parts backed by guest_memfd. Either the same,
>> or a fd pair.
>> (2) Allow to mmap only the "shared" parts.
>> (3) Allow in-place conversion between "shared" and "private" parts.
>
> These three were covered (modulo bugs) in the guest_memfd() RFC I'd
> sent a while back:
>
> https://lore.kernel.org/all/20240222161047.402609-1-tabba@google.com/

I remember there was a catch to it (either around mmap or pinning
detection -- or around support for huge pages in the future; maybe these
count as BUGs :) ).

I should probably go back and revisit the whole thing, I was only CCed
on some part of it back then.

>
>> (4) Allow migration of the "shared" parts.
>
> We would really like that too, if they allow us :)
>
>> A) Convert shared -> private?
>> * Must not be GUP-pinned
>> * Must not be mapped
>> * Must not reside on ZONE_MOVABLE/MIGRATE_CMA
>> * (must rule out any other problematic folio references that could
>> read/write memory, might be feasible for guest_memfd)
>>
>> B) Convert private -> shared?
>> * Nothing to consider
>>
>> C) Map something?
>> * Must not be private
>
> A,B and C were covered (again, modulo bugs) in the RFC.
>
>> For ordinary (small) pages, that might be feasible.
>> (ZONE_MOVABLE/MIGRATE_CMA might be feasible, but maybe we could just not
>> support them initially)
>>
>> The real fun begins once we want to support huge pages/large folios and
>> can end up having a mixture of "private" and "shared" per huge page. But
>> really, that's what we want in the end I think.
>
> I agree.
>
>> Unless we can teach the VM to not convert arbitrary physical memory
>> ranges on a 4k basis to a mixture of private/shared ... but I've been
>> told we don't want that. Hm.
>>
>>
>> There are two big problems with that that I can see:
>>
>> 1) References/GUP-pins are per folio
>>
>> What if some shared part of the folio is pinned but another shared part
>> that we want to convert to private is not? Core-mm will not provide the
>> answer to that: the folio maybe pinned, that's it. *Disallowing* at
>> least long-term GUP-pins might be an option.
>
> Right.
>
>> To get stuff into an IOMMU, maybe a per-fd interface could work, and
>> guest_memfd would track itself which parts are currently "handed out",
>> and with which "semantics" (shared vs. private).
>>
>> [IOMMU + private parts might require that either way? Because, if we
>> dissallow mmap, how should that ever work with an IOMMU otherwise].
>
> Not sure if IOMMU + private makes that much sense really, but I think
> I might not really understand what you mean by this.

A device might be able to access private memory. In the TDX world, this
would mean that a device "speaks" encrypted memory.

At the same time, a device might be able to access shared memory. Maybe
devices can do both?

What do do when converting between private and shared? I think it
depends on various factors (e.g., device capabilities).

[...]

>> I recall quite some details with memory renting or so on pKVM ... and I
>> have to refresh my memory on that.
>
> I really would like to get to a place where we could investigate and
> sort out all of these issues. It would be good to know though, what,
> in principle (and not due to any technical limitations), we might be
> allowed to do and expand guest_memfd() to do, and what out of
> principle is off the table.

As Jason said, maybe we need a revised model that can handle
[...] private+shared properly.

--
Cheers,

David / dhildenb