Subject: Re: [patch 3/9] Guest page hinting: volatile page cache.
From: Martin Schwidefsky <schwidefsky@de.ibm.com>
Reply-To: schwidefsky@de.ibm.com
To: Dave Hansen <haveblue@us.ibm.com>
Cc: Andy Whitcroft <apw@shadowen.org>, linux-kernel@vger.kernel.org,
       virtualization@lists.osdl.org, akpm@osdl.org, nickpiggin@yahoo.com.au,
       frankeh@watson.ibm.com
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Organization: IBM Corporation
Date: Fri, 01 Sep 2006 18:25:43 +0200
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On Fri, 2006-09-01 at 09:18 -0700, Dave Hansen wrote:
> > 1) The page-is-discarded (PG_discarded) bit is set for pages that have
> > been recognized as removed by the host. The page needs to be removed
> > from the page cache while there are still page references floating
> > around. To prevent multiple removals from the page cache the discarded
> > bit is needed.
> 
> OK, so the page has data in it, and is in the page cache.  The
> hypervisor kills the page, gives the notification to the kernel that the
> page has gone away, and the kernel marks PG_discarded.  There still
> might be active references to the page.

No, the hypervisor does not give the notification immediatly. A discard
fault is delivered to the guest if it tries to access a page that has
been removed by the host. That is the fundamental difference between a
memory ballooner and the guest page hinting.

> So, is the problem trying to communicate with the reference holders that
> the page is no longer valid?  How is this fundamentally different from
> page truncating?

Truncating is similar but the reaction is different. A truncated page is
gone and will not be recreated. A discarded page can be reloaded.

> > 2) The page-state-change (PG_state_change) bit is required to prevent
> > that an make_stable "overtakes" a make_volatile. In order to make a page
> > volatile a number of conditions are check. After this is done the state
> > change will be done. The critical section is the code that performs the
> > checks up to the instruction that does the state change. No make_stable
> > may be done in between. The granularity is per page, to use a global
> > lock like a spinlock would severly limit the scalability for large smp
> > systems.
> 
> How about doing it in the NUMA node?  Or the mem_section?  Or, even a
> bit in the mem_map[] for the area guarding the 'struct page' itself?
> Even a hashed table of locks based on the page address.  You just need
> something that allows _some_ level of concurrency.  You certainly never
> have a number of CPUs which is anywhere close to the number of 'struct
> page's in the system.

NUMA node is not granular enough, mem_section is probably doable. I do
not understand the part about the bit in the mem_map[] area, a bit in
the page->flags is exactly that, isn't it?

-- 
blue skies,
  Martin.

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH

"Reality continues to ruin my life." - Calvin.


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