userfaultfd: add UFFDIO_CONTINUE ioctl

This ioctl is how userspace ought to resolve "minor" userfaults.  The
idea is, userspace is notified that a minor fault has occurred.  It
might change the contents of the page using its second non-UFFD mapping,
or not.  Then, it calls UFFDIO_CONTINUE to tell the kernel "I have
ensured the page contents are correct, carry on setting up the mapping".

Note that it doesn't make much sense to use UFFDIO_{COPY,ZEROPAGE} for
MINOR registered VMAs.  ZEROPAGE maps the VMA to the zero page; but in
the minor fault case, we already have some pre-existing underlying page.
Likewise, UFFDIO_COPY isn't useful if we have a second non-UFFD mapping.
We'd just use memcpy() or similar instead.

It turns out hugetlb_mcopy_atomic_pte() already does very close to what
we want, if an existing page is provided via `struct page **pagep`.  We
already special-case the behavior a bit for the UFFDIO_ZEROPAGE case, so
just extend that design: add an enum for the three modes of operation,
and make the small adjustments needed for the MCOPY_ATOMIC_CONTINUE
case.  (Basically, look up the existing page, and avoid adding the
existing page to the page cache or calling set_page_huge_active() on
it.)

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Axel Rasmussen <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Cc: Adam Ruprecht <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Cannon Matthews <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Chinwen Chang <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: "Dr . David Alan Gilbert" <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: "Michal Koutn" <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Oliver Upton <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Shawn Anastasio <[email protected]>
Cc: Steven Price <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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