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

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Message-id: <200901301134.07242.frank.mehnert@sun.com>
Date:	Fri, 30 Jan 2009 11:34:03 +0100
From:	Frank Mehnert <Frank.Mehnert@....COM>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <peterz@...radead.org>, Avi Kivity <avi@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: PFs on pages pinned with get_user_pages()

On Thursday 29 January 2009, Peter Zijlstra wrote:
> On Thu, 2009-01-29 at 17:03 +0100, Frank Mehnert wrote:
> > On Thursday 29 January 2009, Peter Zijlstra wrote:
> > > > > That aside, is there any reason you have to avoid scheduling?
> > > > > Otherwise I would just allow so and be done with it.
> > > >
> > > > The reason is that our code expects that to ensure syncing of the CPU
> > > > state with the saved state. I fear it is quite difficult to change
> > > > that...
> > >
> > > Ah, is that what KVM uses the preempt notifiers for? Could you too?
> >
> > Right, that could be an option.
> >
> > We will try to change our code which is a big effort as we try
> > to keep the code as unique as possible between the different
> > hosts we support (Linux, Solaris, Windows, Mac OS X).
> >
> > Just to be sure: There is no other option than disabling interrupts
> > or calling disable_preemption() to prevent scheduling?
>
> 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.

Kind regards,

Frank
-- 
Dr.-Ing. Frank Mehnert    Sun Microsystems    http://www.sun.com/

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