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Date: Thu, 17 Apr 2008 03:50:15 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Johannes Weiner <hannes@saeurebad.de>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [v2.6.26] what's brewing in x86.git for v2.6.26
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On Thu, 17 Apr 2008 12:32:03 +0200 Johannes Weiner <hannes@saeurebad.de> wrote:

> Hi,
> 
> Andrew Morton <akpm@linux-foundation.org> writes:
> 
> > - extensive damage to the page-flags patches
> >
> >   Did you check that all architectures and configurations still have
> >   sufficient page flags for us to be able to consume another one for
> >   kmemcheck?  The MM developers have put much, much effort into avoiding
> >   running out of flags over numerous years and afaik none of them even know
> >   that this debug feature is using one of the few remaining ones.
> >
> >   What do we do when we run out?
> 
> Would it be feasible to add another unsigned long to struct page?  I
> mean, extending such a common structure always sucks, but for
> emergency...
> 
> #define PageFoobar(page)	test_bit(PG_foobar, &(page)->flags2)
> 
> Of course the essential core flags should always be in ->flags but
> perhaps we could have a symbol CONFIG_NEED_EXTRA_PAGE_FLAGS that gets
> selected by kmemcheck (and other candidates that are unlikely to be
> enabled most of the time) and then #ifndef ->flags2 out.
> 

Yes, but I think that only applies to PG_tracked.

We may be able to reclaim PG_buddy by putting various fields in the
pageframe to idiotic otherwise-cant-happen states.  Like

static inline bool PageBuddy(struct page *page)
{
	return page->mapping == (long)&page->private;
}

or something.  But these things are so overloaded it gets tricky.