linux-kernel - Re: [PATCH] mm: numa: Do not trap faults on shared data section pages.

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Date:   Wed, 17 Jan 2018 12:23:04 -0600 (CST)
From:   Christopher Lameter <cl@...ux.com>
To:     Mel Gorman <mgorman@...e.de>
cc:     Henry Willard <henry.willard@...cle.com>,
        akpm@...ux-foundation.org, kstewart@...uxfoundation.org,
        zi.yan@...rutgers.edu, pombredanne@...b.com, aarcange@...hat.com,
        gregkh@...uxfoundation.org, aneesh.kumar@...ux.vnet.ibm.com,
        kirill.shutemov@...ux.intel.com, jglisse@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: numa: Do not trap faults on shared data section
 pages.

On Tue, 16 Jan 2018, Mel Gorman wrote:

> My main source of discomfort is the fact that this is permanent as two
> processes perfectly isolated but with a suitably shared COW mapping
> will never migrate the data. A potential improvement to get the reported
> bandwidth up in the test program would be to skip the rest of the VMA if
> page_mapcount != 1 in a COW mapping as it would be reasonable to assume
> the remaining pages in the VMA are also affected and the scan is wasteful.
> There are counter-examples to this but I suspect that the full VMA being
> shared is the common case. Whether you do that or not;

Same concern here. Typically CAP_SYS_NICE will bypass the check that the
page is only mapped to a single process and the check looks exactly like
the ones for manual migration. Using CAP_SYS_NICE would be surprising
here since autonuma is not triggered by the currently running process.

Can we configure this somehow via sysfs?