Re: [patch 3/9] Guest page hinting: volatile page cache.

[Dave Hansen]

On Fri, 2006-09-01 at 18:25 +0200, Martin Schwidefsky wrote:
> 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 the page is no longer valid?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.

Can you give me the sequence of events that occur so that we need to
set, then check PG_discarded?  I'm not getting it.

1. there is good data in a page
...
50.  ... and PG_discarded gets set
...
99.  We check PG_discarded and ...

> > > 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?

No, I'm being tricky.  There are struct pages for all memory, including
kernel memory.  mem_map[] is in kernel memory.  So, the memory for the
mem_map[] has struct pages, which themselves are in the mem_map[].

void lock_page_for_state_change(struct page *page)
{
	struct pages_backing_page = virt_to_page(page);
	lock_page(pages_backing_page)
}

We did this for a bit with sparsemem, I think.  That is, until Andy came
up with something even more clever.

-- Dave

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