hugetlbfs: revert use i_mmap_rwsem for more pmd sharing synchronization

Commit c0d0381ade79 ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
synchronization") added code to take i_mmap_rwsem in read mode for the
duration of fault processing.  However, this has been shown to cause
performance/scaling issues.  Revert the code and go back to only taking
the semaphore in huge_pmd_share during the fault path.

Keep the code that takes i_mmap_rwsem in write mode before calling
try_to_unmap as this is required if huge_pmd_unshare is called.

NOTE: Reverting this code does expose the following race condition.

Faulting thread                                 Unsharing thread
...                                                  ...
ptep = huge_pte_offset()
      or
ptep = huge_pte_alloc()
...
                                                i_mmap_lock_write
                                                lock page table
ptep invalid   <------------------------        huge_pmd_unshare()
Could be in a previously                        unlock_page_table
sharing process or worse                        i_mmap_unlock_write
...
ptl = huge_pte_lock(ptep)
get/update pte
set_pte_at(pte, ptep)

It is unknown if the above race was ever experienced by a user.  It was
discovered via code inspection when initially addressed.

In subsequent patches, a new synchronization mechanism will be added to
coordinate pmd sharing and eliminate this race.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Kravetz <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: James Houghton <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Prakash Sangappa <[email protected]>
Cc: Sven Schnelle <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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