Re: [PATCH RFC v2 4/4] mm/mempolicy: add nodes_empty check in SYSC_migrate_pages

[Christopher Lameter]

On Tue, 7 Nov 2017, Vlastimil Babka wrote:

> > Migrate pages moves the pages of a single process there is no TARGET
> > process.
>
> migrate_pages(2) takes a pid argument
>
> "migrate_pages()  attempts  to  move all pages of the process pid that
> are in memory nodes old_nodes to the memory nodes in new_nodes. "

Ok missed that. Most use cases here are on the current process.

Fundamentally a process can have shared pages outside of the cpuset that
a process is restricted to. Thus I would think that migration to any of
the allowed nodes of the current process that is calling migrate pages
is ok. The caller wants this and the caller has a right to allocate on
these nodes. It would be strange if migrate_pages would allow allocation
outside of the current cpuset.

> > Thus thehe *target* nodes need to be a subset of the current cpu set.

And therefore the above still holds.