linux-kernel - Re: [RFC/PATCH 0/5] Contiguous Memory Allocator and get_user

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Message-Id: <201303050850.26615.arnd@arndb.de>
Date:	Tue, 5 Mar 2013 08:50:26 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	linux-mm@...ck.org, linaro-mm-sig@...ts.linaro.org,
	linux-kernel@...r.kernel.org,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Michal Nazarewicz <mina86@...a86.com>,
	Minchan Kim <minchan@...nel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [RFC/PATCH 0/5] Contiguous Memory Allocator and get_user_pages()

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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