Messages in this thread | | | Subject | Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse | From | John Hubbard <> | Date | Fri, 15 Jan 2021 19:40:46 -0800 |
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On 1/15/21 11:46 AM, David Hildenbrand wrote: >>> 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 :) >
I proposed this exact idea a few days ago [1]. It's remarkable that we both picked nearly identical values for the layout! :)
But as the responses show, security problems prevent pursuing that approach.
[1] https://lore.kernel.org/r/45806a5a-65c2-67ce-fc92-dc8c2144d766@nvidia.com
thanks, -- John Hubbard NVIDIA
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