On Wed, 8 Nov 2017, Yisheng Xie wrote:

Another case is current process is *not* the same as target process, and
when current process try to migrate pages of target process from old_nodes
to new_nodes, the new_nodes should be a subset of target process cpuset.

The caller of migrate_pages should be able to migrate the target process
pages anywhere the caller can allocate memory. If that is outside the
target processes cpuset then that is fine. Pagecache pages that are not
allocated by the target process already are not subject to the target
processes restriction. So this is not that unusual.