two questions (issues?) about phobos/std/mmfile.d

[mw]

I'm checking file:

https://github.com/dlang/phobos/blob/master/std/mmfile.d

and found two  questions (issues?) I don't understand, can 
someone explain?

1) This ctor:

https://github.com/dlang/phobos/blob/master/std/mmfile.d#L79

version (linux) this(File file, Mode mode = Mode.read, ulong size 
= 0,
             void* address = null, size_t window = 0) scope

is not doc-ed: https://dlang.org/phobos/std_mmfile.html

and there is a version (Posix) implementation inside the doc-ed 
ctor:

https://github.com/dlang/phobos/blob/master/std/mmfile.d#L275

There is some code duplication of these two implementation, so is 
the version (linux) needed at all? or it's some dead legacy code 
that should be deleted together with this:

https://github.com/dlang/phobos/blob/master/std/mmfile.d#L87


2) in ensureMapped() why the requested mmap length are 3x window 
(2x is enough, since the start offset of the mmap must be 
multiples of pagesize):

https://github.com/dlang/phobos/blob/master/std/mmfile.d#L556
```
                     map(window*(block-1),3*window);
// should this be?:
                     map(window*(block),  2*window);
```

and
https://github.com/dlang/phobos/blob/master/std/mmfile.d#L582

```
                     
map(window*(iblock-1),cast(size_t)(window*(jblock-iblock+3)));
// should this be?:
                     map(window*(iblock),  
cast(size_t)(window*(jblock-iblock+2)));

```

So the mapped buffer size is consistent of 2x window?


Thanks.