Revert "mm/memory.c: share the i_mmap_rwsem"

This reverts commit c8475d1.

There are several[1][2] of bug reports which points to this commit as potential
cause[3].

Let's revert it until we figure out what's going on.

[1] https://lkml.org/lkml/2014/11/14/342
[2] https://lkml.org/lkml/2014/12/22/213
[3] https://lkml.org/lkml/2014/12/9/741

Signed-off-by: Kirill A. Shutemov <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Acked-by: Davidlohr Bueso <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Zijlstra (Intel) <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Srikar Dronamraju <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: kiryl

mm/memory.c

@@ -2378,12 +2378,12 @@ void unmap_mapping_range(struct address_space *mapping,
 		details.last_index = ULONG_MAX;
 
 
-	i_mmap_lock_read(mapping);
+	i_mmap_lock_write(mapping);
 	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap)))
 		unmap_mapping_range_tree(&mapping->i_mmap, &details);
 	if (unlikely(!list_empty(&mapping->i_mmap_nonlinear)))
 		unmap_mapping_range_list(&mapping->i_mmap_nonlinear, &details);
-	i_mmap_unlock_read(mapping);
+	i_mmap_unlock_write(mapping);
 }
 EXPORT_SYMBOL(unmap_mapping_range);