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

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Message-Id: <1205399511.26537.3.camel@localhost>
Date:	Thu, 13 Mar 2008 10:11:51 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	virtualization@...ts.osdl.org, akpm@...l.org,
	nickpiggin@...oo.com.au, frankeh@...son.ibm.com, hugh@...itas.com
Subject: Re: [patch 4/6] Guest page hinting: writable page table entries.

On Thu, 2008-03-13 at 10:35 +1100, Rusty Russell wrote:
> On Thursday 13 March 2008 00:21:36 Martin Schwidefsky wrote:
> > Index: linux-2.6/fs/exec.c
> > ===================================================================
> > --- linux-2.6.orig/fs/exec.c
> > +++ linux-2.6/fs/exec.c
> > @@ -51,6 +51,7 @@
> >  #include <linux/tsacct_kern.h>
> >  #include <linux/cn_proc.h>
> >  #include <linux/audit.h>
> > +#include <linux/page-states.h>
> >
> >  #include <asm/uaccess.h>
> >  #include <asm/mmu_context.h>
> 
> I haven't compile-tested, but this seems unnecessary; it's the only change to 
> this file.

True. I removed the include.

> > +/**
> > + * __page_reset_writable() - clear the PageWritable bit
> > + *
> > + * @page: the page
> > + */
> > +void __page_reset_writable(struct page *page)
> > +{
> > +	preempt_disable();
> > +	if (!page_test_set_state_change(page)) {
> > +		ClearPageWritable(page);
> > +		page_clear_state_change(page);
> > +	}
> > +	preempt_enable();
> > +}
> > +EXPORT_SYMBOL(__page_reset_writable);
> 
> If I understand correctly, you don't bother resetting the writable bit if you 
> don't get the state_change lock.  Is this best effort, or is there some 
> correctness issue here?

It is an error on the safe side. If the page writable bit is set then
the page state has to indicate to the host that the page dirty bit needs
to be checked. 

-- 
blue skies,
  Martin.

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


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