Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

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> On Dec 18, 2021, at 10:42 AM, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> 
> On Fri, Dec 17, 2021 at 07:38:39PM -0800, Linus Torvalds wrote:
>> On Fri, Dec 17, 2021 at 7:30 PM Nadav Amit <namit@xxxxxxxxxx> wrote:
>>> 
>>> In such a case, I do think it makes sense to fail uffd-wp (when
>>> page_count() > 1), and in a prototype I am working on I do something
>>> like that.
>> 
>> Ack. If uddf-wp finds a page that is pinned, just skip it as not
>> write-protectable.
>> 
>> Because some of the pinners might be writing to it, of course - just
>> not through the page tables.
> 
> That doesn't address the qemu use case though. The RDMA pin is the
> 'coherent r/o pin' we discussed before, which requires that the pages
> remain un-write-protected and the HW DMA is read only.
> 
> The VFIO pin will enable dirty page tracking in the system IOMMU so it
> gets the same effect from qemu's perspective as the CPU WP is doing.
> 
> In these operations every single page of the guest will be pinned, so
> skip it just means userfault fd wp doesn't work at all.
> 
> Qemu needs some solution to be able to dirty track the CPU memory for
> migration..

My bad. I misunderstood the scenario.

Yes, I guess that you pin the pages early for RDMA registration, which
is also something you may do for IO-uring buffers. This would render
userfaultfd unusable.

I do not see how it can be solved without custom, potentially
complicated logic, which the page_count() approach wants to avoid.

The only thing I can think of is requiring the pinned regions to be
first madvise’d with MADV_DONTFORK and not COW’ing in such case.
But this would break existing code though.





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