Ceph

master PR: https://github.com/ceph/ceph/pull/3595

Note that the hammer backport will not be a straight-forward cherry pick. Sam wrote:

"@smithfarm @xie xingguo This backports way too much. I think @Haomai Wang 's commit fixed it basically by accident. We do not want to backport that whole feature merely to fix the two read() callers. Please create a new commit which only restores the correct behavior for the affected callers."

In other words, the backport should not look like this: https://github.com/ceph/ceph/pull/9341