On 1/7/21 2:00 PM, Linus Torvalds wrote:
On Thu, Jan 7, 2021 at 1:53 PM John Hubbard <jhubbard@xxxxxxxxxx> 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
