linux-kernel - Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

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Date:	Tue, 20 Mar 2007 07:00:17 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Blaisorblade <blaisorblade@...oo.it>
Cc:	Bill Irwin <bill.irwin@...cle.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management <linux-mm@...ck.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

On Mon, Mar 19, 2007 at 09:44:28PM +0100, Blaisorblade wrote:
> On Sunday 18 March 2007 03:50, Nick Piggin wrote:
> > > >
> > > > Yes, I believe that is the case, however I wonder if that is going to
> > > > be a problem for you to distinguish between write faults for clean
> > > > writable ptes, and write faults for readonly ptes?
> > >
> > > I wouldn't be able to distinguish them, but am I going to get write
> > > faults for clean ptes when vma_wants_writenotify() is false (as seems to
> > > be for tmpfs)? I guess not.
> > >
> > > For tmpfs pages, clean writable PTEs are mapped as writable so they won't
> > > give any problem, since vma_wants_writenotify() is false for tmpfs.
> > > Correct?
> >
> > Yes, that should be the case. So would this mean that nonlinear protections
> > don't work on regular files?
> 
> They still work in most cases (including for UML), but if the initial mmap() 
> specified PROT_WRITE, that is ignored, for pages which are not remapped via 
> remap_file_pages(). UML uses PROT_NONE for the initial mmap, so that's no 
> problem.

But how are you going to distinguish a write fault on a readonly pte for
dirty page accounting vs a read-only nonlinear protection?

You can't store any more data in a present pte AFAIK, so you'd have to
have some out of band data. At which point, you may as well just forget
about vma_wants_writenotify vmas, considering that everybody is using
shmem/ramfs.
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