Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse

[Kirill A. Shutemov]

On Tue, Jan 12, 2021 at 07:31:07PM -0800, Linus Torvalds wrote:
> On Tue, Jan 12, 2021 at 6:16 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > The thing about the speculative page cache references is that they can
> > temporarily bump a refcount on a page which _used_ to be in the page
> > cache and has now been reallocated as some other kind of page.
> 
> Oh, and thinking about this made me think we might actually have a
> serious bug here, and it has nothing what-so-ever to do with COW, GUP,
> or even the page count itself.
> 
> It's unlikely enough that I think it's mostly theoretical, but tell me
> I'm wrong.
> 
> PLEASE tell me I'm wrong:
> 
> CPU1 does page_cache_get_speculative under RCU lock
> 
> CPU2 frees and re-uses the page
> 
>     CPU1                CPU2
>     ----                ----
> 
>     page = xas_load(&xas);
>     if (!page_cache_get_speculative(page))
>             goto repeat;
>     .. succeeds ..
> 
>                         remove page from XA
>                         release page
>                         reuse for something else

How can it be reused if CPU1 hold reference to it?

> 
>     .. and then re-check ..
>     if (unlikely(page != xas_reload(&xas))) {
>             put_page(page);
>             goto repeat;
>     }
> 

-- 
 Kirill A. Shutemov