Re: [RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages)

[Jason Gunthorpe <jgg@nvidia.com>]

On Sat, Jul 29, 2023 at 12:46:59AM +0000, Kasireddy, Vivek wrote:

> > Later the importer decides it needs the memory again so it again asks
> > for the dmabuf to be present, which does hmm_range_fault and gets
> > whatever is appropriate at the time.
> Unless I am missing something, I think just doing the above still won't solve
> the problem. Consider this sequence:
>      write_to_memfd(addr1, size, 'a');
>      buf = create_udmabuf_list(devfd, memfd, size);
>      addr2 = mmap_fd(buf, NUM_PAGES * NUM_ENTRIES * getpagesize());
>      read(addr2);
>      write_to_memfd(addr1, size, 'b');
>      punch_hole(memfd, MEMFD_SIZE / 2);
> -> Since we can process the invalidate at this point, as per your suggestion,
>      we can trigger dmabuf move to let the importers know that the dmabuf's
>      backing memory has changed (or moved).
> 
>      read(addr2);
> -> Because there is a hole, we can handle the read by either providing the
>      old pages or zero pages (if using hmm_range_fault()) to the
> importers.

You never provide the old pages. After trunctate the only correct
value to read is zero.

>      Maybe it is against convention, but I think it makes sense to provide old
>      pages (that were mapped before the hole punch) because the importers
>      have not read the data in these pages ('b' above) yet.

Nope.

>      And, another reason to provide old pages is because the data in
>      these pages is shown in a window on the Host's screen so it
>      doesn't make sense to show zero page data.

So why did you trucate it if you want to keep the data?


> -> write_to_memfd(addr1, size, 'c');
>      As the hole gets refilled (with new pages) after the above write, AFAIU, we
>      have to tell the importers again that since the backing memory has changed,
>      (new pages) they need to recreate their mappings. But herein lies the problem:
>      from inside the udmabuf driver, we cannot know when this write occurs, so we
>      would not be able to notify the importers of the dmabuf move.

You get another invalidate because the memfd removes the zero pages
that hmm_range_fault installed in the PTEs before replacing them with
actual writable pages. Then you do the move, and another
hmm_range_fault, and basically the whole thing over again. Except this
time instead of returning zero pages it returns actual writable page.

Jason