>> 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
