On Tue, 4 Feb 2014, Andrew Morton wrote:
> > Commit bf6bddf1924e ("mm: introduce compaction and migration for ballooned
> > pages") introduces page_count(page) into memory compaction which
> > dereferences page->first_page if PageTail(page).
> >
> > Introduce a store memory barrier to ensure page->first_page is properly
> > initialized so that code that does page_count(page) on pages off the lru
> > always have a valid p->first_page.
>
> Could we have a code comment please? Even checkpatch knows this rule!
>
Ok.
> > Reported-by: Holger Kiehl <holger.ki...@dwd.de>
>
> What did Holger report?
>
A once-in-five-years NULL pointer dereference on the aforementioned
page_count(page).
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