Re: [PATCH v3 1/9] mm: add overflow and underflow checks for page->_refcount

[Pasha Tatashin]

> > diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> > index 2e677e6ad09f..fe4864f7f69c 100644
> > --- a/include/linux/page_ref.h
> > +++ b/include/linux/page_ref.h
> > @@ -117,7 +117,10 @@ static inline void init_page_count(struct page *page)
> >
> >  static inline void page_ref_add(struct page *page, int nr)
> >  {
> > -     atomic_add(nr, &page->_refcount);
> > +     int old_val = atomic_fetch_add(nr, &page->_refcount);
> > +     int new_val = old_val + nr;
> > +
> > +     VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);
>
> This seems somewhat weird, as it will trigger not just on overflow, but also
> if nr is negative. Which I think is valid usage, even though the function
> has 'add' in name, because 'nr' is signed?

I have not found any places in the mainline kernel where nr is
negative in page_ref_add(). I think, by adding this assert we ensure
that when 'add' shows up in backtraces it can be assured that the ref
count has increased, and if page_ref_sub() showed up it means it
decreased. It is strange to have both functions, and yet allow them to
do the opposite. We can also change the type to unsigned.

Pasha