With an encrypted filesystem, you can't do that. Or rather: you can do it
if the filesystem is read-only, but you definitely CANNOT do it on
writing. For writing you have to marshall the output buffer somewhere
else (and quite frankly, it tends to become a lot easier if you can do
that for reading too).
And that in turn causes problems. You get all kinds of interesting
deadlock schenarios when write-out requires more memory in order to
succeed. So you need to get careful. Reading ends up being the much easier
case (doesn't have the same deadlock issues _and_ you could do it in-place
anyway).