From: Arnd Bergmann <arnd@arndb.de>
To: Marek Szyprowski <m.szyprowski@samsung.com>
Subject: Re: [RFC/PATCH 0/5] Contiguous Memory Allocator and get_user_pages()
Date: Tue, 5 Mar 2013 08:50:26 +0000
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Cc: linux-mm@kvack.org, linaro-mm-sig@lists.linaro.org,
        linux-kernel@vger.kernel.org,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mel@csn.ul.ie>,
        Michal Nazarewicz <mina86@mina86.com>,
        Minchan Kim <minchan@kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
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On Tuesday 05 March 2013, Marek Szyprowski wrote:
> To solving this issue requires preventing locking of the pages, which
> are placed in CMA regions, for a long time. Our idea is to migrate
> anonymous page content before locking the page in get_user_pages(). This
> cannot be done automatically, as get_user_pages() interface is used very
> often for various operations, which usually last for a short period of
> time (like for example exec syscall). We have added a new flag
> indicating that the given get_user_space() call will grab pages for a
> long time, thus it is suitable to use the migration workaround in such
> cases.

Can you explain the tradeoff here? I would have expected that the default
should be to migrate pages out, and annotate the instances that we know
are performance critical and short-lived. That would at least appear
more reliable to me.

	Arnd
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