linux-kernel - Re: [PATCH] Thaw userspace and kernel space separately.

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Message-Id: <20061023095124.7be583ce.akpm@osdl.org>
Date:	Mon, 23 Oct 2006 09:51:24 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Nigel Cunningham <ncunningham@...uxmail.org>
Cc:	rjw@...k.pl, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Thaw userspace and kernel space separately.

> On Mon, 23 Oct 2006 22:00:11 +1000 Nigel Cunningham <ncunningham@...uxmail.org> wrote:
> Hi.
> 
> On Mon, 2006-10-23 at 12:26 +0200, Rafael J. Wysocki wrote:
> > On Monday, 23 October 2006 01:48, Nigel Cunningham wrote:
> > > Modify process thawing so that we can thaw kernel space without thawing
> > > userspace, and thaw kernelspace first. This will be useful in later
> > > patches, where I intend to get swsusp thawing kernel threads only before
> > > seeking to free memory.
> > 
> > Please explain why you think it will be necessary/useful.
> > 
> > I remember a discussion about it some time ago that didn't indicate
> > we would need/want to do this.
> 
> This is needed to make suspending faster and more reliable when the
> system is in a low memory situation. Imagine that you have a number of
> processes trying to allocate memory at the time you're trying to
> suspend. They want so much memory that when you come to prepare the
> image, you find that you need to free pages. But your swapfile is on
> ext3, and you've just frozen all processes, so any attempt to free
> memory could result in a deadlock while the vm tries to swap out pages
> using the frozen kjournald. So you need to thaw processes to free the
> memory. But thawing processes will start the processes allocating memory
> again, so you'll be fighting an uphill battle.
> 
> If you can only thaw the kernel threads, you can free memory without
> restarting userspace or deadlocking against a frozen kjournald.
> 

kjournald will not participate in writing to swapfiles.

The situation where we would need this feature is where the loop driver is
involved in the path-to-disk.  But I doubt if that's a thing we'd want to
support.

otoh there may be other kernel threads which are a saner thing to have in
the swapout path and which we do want to support.  md_thread, perhaps?
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