Re: folio_mmapped

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On Mon, Mar 04, 2024, Quentin Perret wrote:
> > As discussed in the sub-thread, that might still be required.
> > 
> > One could think about completely forbidding GUP on these mmap'ed
> > guest-memfds. But likely, there might be use cases in the future where you
> > want to use GUP on shared memory inside a guest_memfd.
> > 
> > (the iouring example I gave might currently not work because
> > FOLL_PIN|FOLL_LONGTERM|FOLL_WRITE only works on shmem+hugetlb, and
> > guest_memfd will likely not be detected as shmem; 8ac268436e6d contains some
> > details)
> 
> Perhaps it would be wise to start with GUP being forbidden if the
> current users do not need it (not sure if that is the case in Android,
> I'll check) ? We can always relax this constraint later when/if the
> use-cases arise, which is obviously much harder to do the other way
> around.

+1000.  At least on the KVM side, I would like to be as conservative as possible
when it comes to letting anything other than the guest access guest_memfd.




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