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SubjectRe: [v2.6.26] what's brewing in x86.git for v2.6.26
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.


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