linux-kernel - Re: PFs on pages pinned with get_user

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Message-Id: <1233312321.4495.169.camel@laptop>
Date:	Fri, 30 Jan 2009 11:45:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frank Mehnert <Frank.Mehnert@....COM>
Cc:	linux-kernel@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: PFs on pages pinned with get_user_pages()

On Fri, 2009-01-30 at 11:34 +0100, Frank Mehnert wrote:

> > Thing is, lock_page() and down_read() require to be able to schedule(),
> > so there's no way around that.
> >
> > So even if there was another way to disable scheduling, you'd still have
> > the same problem.
> 
> Yes, makes sense.
> 
> Back to my initial question: The problem arises for us because we depend
> on permanent mappings of memory which were
> 
>  - allocated with alloc_pages() or alloc_page()
>  - mapped into ring 3 with remap_pfn_range() and
>  - pinned with get_user_pages()
> 
> There are potential pagefaults when touching into these ring-3-mappings
> from ring 0. So I assume we could prevent such pagefaults if we access
> that memory from ring-0-mappings, right? Unfortunately, the space for
> ring-0-mappings (< 1GB) is smaller than userland (~ 3GB), at least on
> 32-bit systems.

if you only need to access one or two pages, you could kmap_atomic() the
actual pages from ring-0.

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