linux-kernel - Re: [patch 4/6] Guest page hinting: writable page table entries.

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Date:	Wed, 1 Apr 2009 16:36:58 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, frankeh@...son.ibm.com,
	akpm@...l.org, nickpiggin@...oo.com.au, hugh@...itas.com
Subject: Re: [patch 4/6] Guest page hinting: writable page table entries.

On Wed, 01 Apr 2009 09:25:34 -0400
Rik van Riel <riel@...hat.com> wrote:

> Martin Schwidefsky wrote:
> 
> This code has me stumped.  Does it mean that if a page already
> has the PageWritable bit set (and count_ok stays 0), we will
> always mark the page as volatile?
> 
> How does that work out on !s390?

No, we will not always mark the page as volatile. If PG_writable is
already set count_ok will stay 0 and a call to page_make_volatile is
done. This differs from page_set_volatile as it repeats all the
required checks, then calls page_set_volatile with a PageWritable(page)
as second argument. What state the page will get depends on the
architecture definition of page_set_volatile. For s390 this will do a
state transition to potentially volatile as the PG_writable bit is set.
On architecture that cannot check the dirty bit on a physical page basis
you need to make the page stable.

> >  /**
> > + * __page_check_writable() - check page state for new writable pte
> > + *
> > + * @page: the page the new writable pte refers to
> > + * @pte: the new writable pte
> > + */
> > +void __page_check_writable(struct page *page, pte_t pte, unsigned int offset)
> > +{
> > +	int count_ok = 0;
> > +
> > +	preempt_disable();
> > +	while (page_test_set_state_change(page))
> > +		cpu_relax();
> > +
> > +	if (!TestSetPageWritable(page)) {
> > +		count_ok = check_counts(page, offset);
> > +		if (check_bits(page) && count_ok)
> > +			page_set_volatile(page, 1);
> > +		else
> > +			/*
> > +			 * If two processes create a write mapping at the
> > +			 * same time check_counts will return false or if
> > +			 * the page is currently isolated from the LRU
> > +			 * check_bits will return false but the page might
> > +			 * be in volatile state.
> > +			 * We have to take care about the dirty bit so the
> > +			 * only option left is to make the page stable but
> > +			 * we can try to make it volatile a bit later.
> > +			 */
> > +			page_set_stable_if_present(page);
> > +	}
> > +	page_clear_state_change(page);
> > +	if (!count_ok)
> > +		page_make_volatile(page, 1);
> > +	preempt_enable();
> > +}
> > +EXPORT_SYMBOL(__page_check_writable);
> 
> 

-- 
blue skies,
   Martin.

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

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