Messages in this thread | | | From | Pasha Tatashin <> | Date | Tue, 26 Oct 2021 17:34:18 -0400 | Subject | Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount |
| |
On Tue, Oct 26, 2021 at 3:50 PM Matthew Wilcox <willy@infradead.org> 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
| |