Date: Mon, 8 Mar 2004 00:36:39 -0600
From: Andy Isaacson <adi@hexapodia.org>
To: Pavel Machek <pavel@ucw.cz>, linux-kernel@vger.kernel.org,
       Andrew Morton <akpm@osdl.org>
Subject: Re: Some highmem pages still in use after shrink_all_memory()?
Message-ID: <20040308063639.GA20793@hexapodia.org>
References: <20040307144921.GA189@elf.ucw.cz> <20040307164052.0c8a212b.akpm@osdl.org>
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On Sun, Mar 07, 2004 at 04:40:52PM -0800, Andrew Morton wrote:
> Pavel Machek <pavel@ucw.cz> wrote:
> > For swsusp, I need to free as much memory as possible. Well, and it
> > would be great if no highmem pages remained, so that I would not have
> > to deal with that. Is that possible?
> 
> No, it isn't.  There are pagetable pages and mlocked user pages which we
> cannot do anything with.
> 
> We could perhaps swap out the mlocked pages anyway if a suspend is in
> progress, but the highmem pagetable pages are not presently reclaimed
> by the VM.

Note that there are some applications for which it is a *bug* if an
mlocked page gets written out to magnetic media.  (gpg, for example.)
I imagine that they'd rather lose the mapping and get a page fault on
the next reference (which they can then fix up with a new mmap and
mlock) than have precious key material written to disk.

... unless, of course, the swap device is securely encrypted a la
OpenBSD's 'sysctl vm.swapencrypt.enable'.

http://www.openbsd.org/papers/swapencrypt.ps

However, I don't see how to implement a cryptographically secure swsusp.

(The importance of this behavior is obviously dependent on your threat
model.  Perhaps the Sufficiently Paranoid gpg users will simply need to
avoid using swsusp.)

-andy
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