Re: [PATCH 04 of 30] clear compound mapping

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Jan 21, 2010 at 09:43:30AM -0800, Dave Hansen wrote:
> On Thu, 2010-01-21 at 07:20 +0100, Andrea Arcangeli wrote:
> > Clear compound mapping for anonymous compound pages like it already happens for
> > regular anonymous pages.
> 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -584,6 +584,8 @@ static void __free_pages_ok(struct page 
> > 
> >  	kmemcheck_free_shadow(page, order);
> > 
> > +	if (PageAnon(page))
> > +		page->mapping = NULL;
> >  	for (i = 0 ; i < (1 << order) ; ++i)
> >  		bad += free_pages_check(page + i);
> >  	if (bad)
> 
> This one may at least need a bit of an enhanced patch description.  I
> didn't immediately remember that __free_pages_ok() is only actually
> called for compound pages.

In short the problem is that the mapping is only cleared if page is
freed through free_hot_cold_page. So we've to clear it also if we
don't pass through free_hot_cold_page.

> Would it make more sense to pull the page->mapping=NULL out of
> free_hot_cold_page(), and just put a single one in __free_pages()?
>
> I guess we'd also need one in free_compound_page() since it calls
> __free_pages_ok() directly.  But, if this patch were putting modifying
> free_compound_page() it would at least be super obvious what was going
> on.

I could as well set_compound_page_dtor and have my own callback that
calls free_compound_page. Or I can move it to __free_one_page and
remove the one from free_hot_cold_page. What do you prefer?

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