Re: [PATCH v2] mm: don't expose page to fast gup prematurely

[Kirill A. Shutemov]

On Tue, Sep 24, 2019 at 04:05:50PM -0600, Yu Zhao wrote:
> On Tue, Sep 24, 2019 at 02:23:16PM +0300, Kirill A. Shutemov wrote:
> > On Sat, Sep 14, 2019 at 01:05:18AM -0600, Yu Zhao wrote:
> > > We don't want to expose page to fast gup running on a remote CPU
> > > before all local non-atomic ops on page flags are visible first.
> > > 
> > > For anon page that isn't in swap cache, we need to make sure all
> > > prior non-atomic ops, especially __SetPageSwapBacked() in
> > > page_add_new_anon_rmap(), are order before set_pte_at() to prevent
> > > the following race:
> > > 
> > > 	CPU 1				CPU1
> > > set_pte_at()			get_user_pages_fast()
> > > page_add_new_anon_rmap()		gup_pte_range()
> > > 	__SetPageSwapBacked()			SetPageReferenced()
> > 
> > Is there a particular codepath that has what you listed for CPU?
> > After quick look, I only saw that we page_add_new_anon_rmap() called
> > before set_pte_at().
> 
> I think so. One in do_swap_page() and another in unuse_pte(). Both
> are on KSM paths. Am I referencing a stale copy of the source?

I *think* it is a bug. Setting a pte before adding the page to rmap may
lead to rmap (like try_to_unmap() or something) to miss the VMA.

Do I miss something?

-- 
 Kirill A. Shutemov