In the Linux kernel, the following vulnerability has been resolved:

migrate: correct lock ordering for hugetlb file folios

Syzbot has found a deadlock (analyzed by Lance Yang):

1) Task (5749): Holds folio_lock, then tries to acquire i_mmap_rwsem(read lock).
2) Task (5754): Holds i_mmap_rwsem(write lock), then tries to acquire
folio_lock.

migrate_pages()
  -> migrate_hugetlbs()
    -> unmap_and_move_huge_page()     <- Takes folio_lock!
      -> remove_migration_ptes()
        -> __rmap_walk_file()
          -> i_mmap_lock_read()       <- Waits for i_mmap_rwsem(read lock)!

hugetlbfs_fallocate()
  -> hugetlbfs_punch_hole()           <- Takes i_mmap_rwsem(write lock)!
    -> hugetlbfs_zero_partial_page()
     -> filemap_lock_hugetlb_folio()
      -> filemap_lock_folio()
        -> __filemap_get_folio        <- Waits for folio_lock!

The migration path is the one taking locks in the wrong order according to
the documentation at the top of mm/rmap.c.  So expand the scope of the
existing i_mmap_lock to cover the calls to remove_migration_ptes() too.

This is (mostly) how it used to be after commit c0d0381ade79.  That was
removed by 336bf30eb765 for both file & anon hugetlb pages when it should
only have been removed for anon hugetlb pages.