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Subject: Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse
To: David Hildenbrand <david@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>
CC: Andrea Arcangeli <aarcange@redhat.com>, Andrew Morton
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From: John Hubbard <jhubbard@nvidia.com>
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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