linux-kernel - Re: [PATCH RFC v2 4/4] mm/mempolicy: add nodes_empty check in SYSC_migrate

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Date:   Tue, 7 Nov 2017 09:55:47 -0600 (CST)
From:   Christopher Lameter <cl@...ux.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     Yisheng Xie <xieyisheng1@...wei.com>, akpm@...ux-foundation.org,
        mhocko@...e.com, mingo@...nel.org, rientjes@...gle.com,
        n-horiguchi@...jp.nec.com, salls@...ucsb.edu, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, tanxiaojun@...wei.com,
        linux-api@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH RFC v2 4/4] mm/mempolicy: add nodes_empty check in
 SYSC_migrate_pages

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.