Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse

[David Hildenbrand <david@redhat.com>]

>> 7) There is no easy way to detect if a page really was pinned: we might
>> have false positives. Further, there is no way to distinguish if it was
>> pinned with FOLL_WRITE or not (R vs R/W). To perform reliable tracking
>> we most probably would need more counters, which we cannot fit into
>> struct page. (AFAIU, for huge pages it's easier).
> 
> I think this is the real issue. We can only store so much information,
> so we have to decide which things work and which things are broken. So
> far someone hasn't presented a way to record everything at least..

I do wonder how many (especially long-term) GUP readers/writers we have
to expect, and especially, support for a single base page. Do we have a
rough estimate?

With RDMA, I would assume we only need a single one (e.g., once RDMA
device; I'm pretty sure I'm wrong, sounds too easy).
With VFIO I guess we need one for each VFIO container (~ in the worst
case one for each passthrough device).
With direct I/O, vmsplice and other GUP users ?? No idea.

If we could somehow put a limit on the #GUP we support, and fail further
GUP (e.g., -EAGAIN?) once a limit is reached, we could partition the
refcount into something like (assume max #15 GUP READ and #15 GUP R/W,
which is most probably a horribly bad choice)

[ GUP READ ][ GUP R/W ] [  ordinary ]
31  ...  28 27  ...  24 23   ....   0

But due to saturate handling in "ordinary", we would lose further 2 bits
(AFAIU), leaving us "only" 22 bits for "ordinary". Now, I have no idea
how many bits we actually need in practice.

Maybe we need less for GUP READ, because most users want GUP R/W? No idea.

Just wild ideas. Most probably that has already been discussed, and most
probably people figured that it's impossible :)

-- 
Thanks,

David / dhildenb