On Fri, Dec 17, 2021 at 07:38:39PM -0800, Linus Torvalds wrote:

On Fri, Dec 17, 2021 at 7:30 PM Nadav Amit [off-list ref] wrote:

quoted

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..

So that sounds like the right thing to do. I _think_ we discussed this
the last time this came up. I have some dim memory of that. Jason,
ring a bell?

We talked about clear_refs alot, but it was never really clear the use
case, I think. Plus that discussion never finialized to anything.

David's latest summary seems accurate, if I paraphrase at a high
level, Linus's approach always does enough COWs but might do extra and
David's approach tries to do exactly the right number of COWs.

It looks like to have the same functionality with Linus's approach we
need to have a way for userspace to opt out of COW and work in an
entirely deterministic non-COW world. WP&GUP can never work together
otherwise which leaves qemu stranded.

Or, we follow David's approach and make COW be precise and accept the
complexity..

Jason