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

[Yu Zhao]

On Wed, Sep 25, 2019 at 03:17:50PM +0300, Kirill A. Shutemov wrote:
> 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?

We have the pages locked in those two places, so for try_to_unmap()
and the rest of page_vma_mapped_walk() users, they will block on
the page lock:
	CPU 1			CPU 2
	lock_page()
	set_pte_at()
	unlock_page()
				lock_page()
				try_to_unmap()
				  page_vma_mapped_walk()
				    pte_present() without holding ptl
				unlock_page()

For others that don't use page_vma_mapped_walk(), they should either
lock pages or grab ptl before checking pte_present().

AFAIK, the fast gup is the only one doesn't fall into the either
category.