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

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Message-Id: <201303051959.35471.arnd@arndb.de>
Date:	Tue, 5 Mar 2013 19:59:35 +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:
> On 3/5/2013 9:50 AM, Arnd Bergmann wrote:
> > On Tuesday 05 March 2013, Marek Szyprowski wrote:
> 
> The problem is that the opposite approach is imho easier.

I can understand that, yes ;-)

> get_user_pages()
> is used in quite a lot of places (I was quite surprised when I've added some
> debug to it and saw the logs) and it seems to be easier to identify places
> where references are kept for significant amount of time. Usually such 
> places
> are in the device drivers. In our case only videobuf2 and some closed-source
> driver were causing the real migration problems, so I decided to leave the
> default approach unchanged.
> 
> If we use this workaround for every get_user_pages() call we will sooner or
> later end with most of the anonymous pages migrated to non-movable 
> pageblocks
> what make the whole CMA approach a bit pointless.

But you said that most users are in device drivers, and I would expect drivers
not to touch that many pages.

We already have two interfaces: the generic get_user_pages and the "fast" version
"get_user_pages_fast" that has a number of restrictions. We could add another
such restriction to get_user_pages_fast(), which is that it must not hold
the page reference count for an extended time because it will not migrate
pages out.

I would assume that most of the in-kernel users of get_user_pages() that
are called a lot either already use get_user_pages_fast, or can be easily
converted to it.

	Arnd
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