After investigating with @lesteve it turns out the tests were not failing because X.astype('float64') does make a copy for float64 (e.g. here), so the cython functions was not getting the read-only mmap.

Once that is fixed in #11966, this PR will not work unfortunately due to #10624 (comment) :/

Or more explicitly it would be necessary to fix cython/cython#1772 to make this possible

FWIW astype does take a copy keyword argument. So this copying behavior could be disabled. Though it will still copy if the type does not match.

FWIW astype does take a copy keyword argument. So this copying behavior could be disabled. Though it will still copy if the type does not match.

Yes, but than we get the overhead of a copy in the hope of gaining something when transitioning to memoryviews.

Generally the goal of this PR was to check whether we could easily use memoryviews. We cant (except for workarounds you mentioned) until the corresponding Cython issue is fixed. Closing this for now.