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

[Pasha Tatashin]

On Tue, Oct 26, 2021 at 5:34 PM Pasha Tatashin
<pasha.tatashin@xxxxxxxxxx> wrote:
>
> 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.

I think we can do that by using:
atomic_fetch_*() and check for overflow/underflow after operation. I
will send the updated series soon.

Pasha