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

[Vlastimil Babka <vbabka@suse.cz>]

On 1/26/22 19:34, Pasha Tatashin wrote:
> The problems with page->_refcount are hard to debug, because usually
> when they are detected, the damage has occurred a long time ago. Yet,
> the problems with invalid page refcount may be catastrophic and lead to
> memory corruptions.
> 
> Reduce the scope of when the _refcount problems manifest themselves by
> adding checks for underflows and overflows into functions that modify
> _refcount.
> 
> Use atomic_fetch_* functions to get the old values of the _refcount,
> and use it to check for overflow/underflow.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
>  include/linux/page_ref.h | 59 +++++++++++++++++++++++++++++-----------
>  1 file changed, 43 insertions(+), 16 deletions(-)
> 
> 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?