Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount

[Pasha Tatashin]

On Tue, Oct 26, 2021 at 3:50 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Tue, Oct 26, 2021 at 05:38:15PM +0000, Pasha Tatashin wrote:
> >  static inline void page_ref_add(struct page *page, int nr)
> >  {
> > -     atomic_add(nr, &page->_refcount);
> > +     int ret;
> > +
> > +     VM_BUG_ON(nr <= 0);
> > +     ret = atomic_add_return(nr, &page->_refcount);
> > +     VM_BUG_ON_PAGE(ret <= 0, page);
>
> This isn't right.  _refcount is allowed to overflow into the negatives.
> See page_ref_zero_or_close_to_overflow() and the conversations that led
> to it being added.

#define page_ref_zero_or_close_to_overflow(page) \
1204   ((unsigned int) page_ref_count(page) + 127u <= 127u)


Uh, right, I saw the macro but did not realize there was an (unsigned int) cast.

OK, I think we can move this macro inside:
include/linux/page_ref.h

modify it to something like this:
#define page_ref_zero_or_close_to_overflow(page) \
   ((unsigned int) page_ref_count(page) + v + 127u <= v + 127u)

The sub/dec can also be fixed to ensure that we do not underflow but
still working with the fact that we use all 32bits of _refcount.

Pasha