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Date:   Wed, 17 Jan 2018 12:23:04 -0600 (CST)
From:   Christopher Lameter <cl@linux.com>
To:     Mel Gorman <mgorman@suse.de>
cc:     Henry Willard <henry.willard@oracle.com>,
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        kirill.shutemov@linux.intel.com, jglisse@redhat.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: numa: Do not trap faults on shared data section
 pages.
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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?