Subject: Re: [RFC] mlock/stack guard interaction fixup
From: Peter Zijlstra <peterz@infradead.org>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Ian Jackson <ijackson@chiark.greenend.org.uk>,
        Greg KH <gregkh@suse.de>, Ian Campbell <ijc@hellion.org.uk>,
        linux-kernel@vger.kernel.org, stable@kernel.org,
        stable-review@kernel.org, akpm@linux-foundation.org,
        alan@lxorguk.ukuu.org.uk
In-Reply-To: <4C72D1E1.1040709@goop.org>
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Date: Tue, 24 Aug 2010 09:08:14 +0200
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On Mon, 2010-08-23 at 12:54 -0700, Jeremy Fitzhardinge wrote:
> On 08/23/2010 12:26 PM, Peter Zijlstra wrote:
> > On Mon, 2010-08-23 at 12:23 -0700, Jeremy Fitzhardinge wrote:
> >> On 08/23/2010 12:07 PM, Peter Zijlstra wrote:
> >>> mlock() simply avoids major faults, nothing more.
> >>>
> >>> I think both page migration and page-out for shared pages where some
> >>> maps are !mlocked can cause unmaps and thus minor faults.
> >>>
> >>> mlock and dirty do not interact, they will still be cleaned/written out
> >>> as normal.
> >> So mlock is useless for preventing secret stuff from being written to disk.
> > Well, if you put your sekrit in a file map, sure.
> >
> > Use a mmap(MAP_ANONYMOUS|MAP_LOCK) and madvise(MADV_DONTFORK) for your
> > sekrits.
> 
> Won't dirty anonymous pages also get written to swap?

Not if all the maps are mlocked (private like above would only have a
single map), there'd be no point would there.
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