hugetlb/userfaultfd: unshare all pmds for hugetlbfs when register wp

Huge pmd sharing for hugetlbfs is racy with userfaultfd-wp because
userfaultfd-wp is always based on pgtable entries, so they cannot be
shared.

Walk the hugetlb range and unshare all such mappings if there is, right
before UFFDIO_REGISTER will succeed and return to userspace.

This will pair with want_pmd_share() in hugetlb code so that huge pmd
sharing is completely disabled for userfaultfd-wp registered range.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Adam Ruprecht <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Alexey Dobriyan <[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: Lokesh Gidra <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: "Michal Koutn" <[email protected]>
Cc: Michel Lespinasse <[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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