Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount
From: Matthew Wilcox <willy@infradead.org>
Date: 2021-10-26 19:50:35
Also in:
linux-m68k, lkml
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.