Question on i_mmap_mutex locking rule

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Hi,

I started looking at unmap_hugepage_range. unmap_hugepage_range takes
i_mmap_mutex. But then the same lock is also taken higher up in the
stack. ie via the below chain

unmap_mapping_range (i_mmap_mutex)
 -> unmap_mapping_range_tree
    -> unmap_mapping_range_vma
       -> zap_page_range_single
          -> unmap_single_vma
             -> unmap_hugepage_range (i_mmap_mutex)

But there are other code path that doesn't take i_mmap_mutex. For ex:
-> madvise_dontneed
   -> zap_page_range
      -> unmap_vmas
         -> unmap_single_vma
            -> unmap_page_range
      

similarly unmap_region also don't take i_mmap_mutex. Don't we need to
ensure that we take i_mmap_mutex when we unmap a file backed range ?

-aneesh

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