linux-kernel - Re: [PATCH] mm: avoid dirtying shared mappings on mlock

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Message-Id: <200710120414.11026.nickpiggin@yahoo.com.au>
Date:	Fri, 12 Oct 2007 04:14:10 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Suleiman Souhlal <ssouhlal@...ebsd.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Suleiman Souhlal <suleiman@...gle.com>,
	linux-mm <linux-mm@...ck.org>, hugh <hugh@...itas.com>
Subject: Re: [PATCH] mm: avoid dirtying shared mappings on mlock

On Friday 12 October 2007 20:37, Peter Zijlstra wrote:
> On Fri, 2007-10-12 at 02:57 +1000, Nick Piggin wrote:
> > On Friday 12 October 2007 19:03, Peter Zijlstra wrote:
> > > Subject: mm: avoid dirtying shared mappings on mlock
> > >
> > > Suleiman noticed that shared mappings get dirtied when mlocked.
> > > Avoid this by teaching make_pages_present about this case.
> > >
> > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > > Acked-by: Suleiman Souhlal <suleiman@...gle.com>
> >
> > Umm, I don't see the other piece of this thread, so I don't
> > know what the actual problem was.
> >
> > But I would really rather not do this. If you do this, then you
> > now can get random SIGBUSes when you write into the memory if it
> > can't allocate blocks or ... (some other filesystem specific
> > condition).
>
> I'm not getting this, make_pages_present() only has to ensure all the
> pages are read from disk and in memory. How is this different from a
> read-scan?

I guess because we've mlocked a region that has PROT_WRITE access...
but actually, I suppose mlock doesn't technically require that we
can write to the memory, only that the page isn't swapped out.

Still, it is nice to be able to have a reasonable guarantee of
writability.


> The pages will still be read-only due to dirty tracking, so the first
> write will still do page_mkwrite().

Which can SIGBUS, no?
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