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SubjectRe: [PATCH 0/1] mm: restore full accuracy in COW page reuse
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@infradead.org> 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

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