Messages in this thread | | | Subject | Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending | From | John Hubbard <> | Date | Thu, 7 Jan 2021 14:14:46 -0800 |
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On 1/7/21 2:00 PM, Linus Torvalds wrote: > On Thu, Jan 7, 2021 at 1:53 PM John Hubbard <jhubbard@nvidia.com> wrote: >> >>> >>> Now, I do agree that from a QoI standpoint, it would be really lovely >>> if we actually enforced it. I'm not entirely sure we can, but maybe it >>> would be reasonable to use that >>> >>> mm->has_pinned && page_maybe_dma_pinned(page) >>> >>> at least as the beginning of a heuristic. >>> >>> In fact, I do think that "page_maybe_dma_pinned()" could possibly be >>> made stronger than it is. Because at *THAT* point, we might say "we >> >> What exactly did you have in mind, to make it stronger? I think the >> answer is in this email but I don't quite see it yet... > > Literally just adding a " && page_mapcount(page) == 1" in there > (probably best done inside page_maybe_dma_pinned() itself)
Well, that means that pages that are used for pinned DMA like this, can not be shared with other processes. Is that an acceptable limitation for the RDMA users? It seems a bit constraining, at first glance anyway.
> >> Direct IO pins, on the other hand, are more transient. We can probably live >> without tagging Direct IO pages as FOLL_PIN. I think. > > Yes. I think direct-IO writes should be able to just do a transient > GUP, and if it causes a COW fault that isn't coherent, that's the > correct semantics, I think (ie the direct-IO will see the original > data, the COW faulter will get it's own private copy to make changes > to). > > I think pinning should be primarily limited to things that _require_ > coherency (ie you pin because you're going to do some active two-way > communication using that page) > > Does that match your thinking? >
Yes, perfectly. I'm going to update Documentation/core-api/pin_user_pages.rst accordingly, once the dust settles on these discussions.
thanks, -- John Hubbard NVIDIA
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