Re: [PATCH v3 3/4] mm: don't expose non-hugetlb page to fast gup prematurely

[Peter Zijlstra]

On Tue, Sep 24, 2019 at 05:24:58PM -0600, Yu Zhao wrote:
> We don't want to expose a non-hugetlb page to the fast gup running
> on a remote CPU before all local non-atomic ops on the page flags
> are visible first.
> 
> For an 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 ordered 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()
> 
> This demonstrates a non-fatal scenario. Though haven't been directly
> observed, the fatal ones can exist, e.g., PG_lock set by fast gup
> caller and then overwritten by __SetPageSwapBacked().
> 
> For an anon page that is already in swap cache or a file page, we
> don't need smp_wmb() before set_pte_at() because adding to swap or
> file cach serves as a valid write barrier. Using non-atomic ops
> thereafter is a bug, obviously.
> 
> smp_wmb() is added following 11 of total 12 page_add_new_anon_rmap()
> call sites, with the only exception being
> do_huge_pmd_wp_page_fallback() because of an existing smp_wmb().
> 

I'm thinking this patch make stuff rather fragile.. Should we instead
stick the barrier in set_p*d_at() instead? Or rather, make that store a
store-release?