This is first time to look at mmap module, so sorry if I'm saying
totally wrong thing. (I noticed this when I saw issue2733) I think the
behavior of mmap_resize_method is unclear when mapping object is created
with offset > 0.

From view of other functions, it seems that self->offset is offset from
beginning of the file, and self->size is the size of mapped area from
self->offset. If so,

#if SIZEOF_SIZE_T > 4
    newSizeHigh = (DWORD)((self->offset + new_size) >> 32);
    newSizeLow = (DWORD)((self->offset + new_size) & 0xFFFFFFFF);
#else
    newSizeHigh = 0;
    newSizeLow = (DWORD)new_size; /* shouldn't add self->offset? this is
not doing same thing as SIZEOF_SIZE_T > 4 */
#endif

And on unix part,
    newmap = mremap(self->data, self->size, new_size, MREMAP_MAYMOVE);
self->offset is totally ignored. I think when self->offset > 0,
something wrong happens.

And comment above function definition saids,

 / Is this really necessary?  This could easily be done
 / from python by just closing and re-opening with the
 / new size?

I think this function is not tested well. There is no test about
resize+offset in test_mmap.py.