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

[Matthew Wilcox]

On Wed, Jan 13, 2021 at 03:32:32PM +0300, Kirill A. Shutemov wrote:
> 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?

Yes, Linus mis-stated it:

    page = xas_load(&xas);
                        remove page from XA
                        release page
                        reuse for something else
    if (!page_cache_get_speculative(page))
            goto repeat;
    if (unlikely(page != xas_reload(&xas))) {
            put_page(page);

... but as David pointed out, I fixed this in e320d3012d25