mm/khugepaged: don't recycle vma pgtable if uffd-wp registered

When we're trying to collapse a 2M huge shmem page, don't retract pgtable
pmd page if it's registered with uffd-wp, because that pgtable could have
pte markers installed.  Recycling of that pgtable means we'll lose the pte
markers.  That could cause data loss for an uffd-wp enabled application on
shmem.

Instead of disabling khugepaged on these files, simply skip retracting
these special VMAs, then the page cache can still be merged into a huge
thp, and other mm/vma can still map the range of file with a huge thp when
proper.

Note that checking VM_UFFD_WP needs to be done with mmap_sem held for
write, that avoids race like:

         khugepaged                             user thread
         ==========                             ===========
     check VM_UFFD_WP, not set
                                       UFFDIO_REGISTER with uffd-wp on shmem
                                       wr-protect some pages (install markers)
     take mmap_sem write lock
     erase pmd and free pmd page
      --> pte markers are dropped unnoticed!

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Nadav Amit <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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