Re: [RFC] rmap: fix "race" between do_wp_page and shrink_active_list

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, May 11, 2015 at 10:51:17AM +0300, Vladimir Davydov wrote:
> Hi,
> 
> I've been arguing with Minchan for a while about whether store-tearing
> is possible while setting page->mapping in __page_set_anon_rmap and
> friends, see
> 
>   http://thread.gmane.org/gmane.linux.kernel.mm/131949/focus=132132
> 
> This patch is intended to draw attention to this discussion. It fixes a
> race that could happen if store-tearing were possible. The race is as
> follows.
> 
> In do_wp_page() we can call page_move_anon_rmap(), which sets
> page->mapping as follows:
> 
>         anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
>         page->mapping = (struct address_space *) anon_vma;
> 
> The page in question may be on an LRU list, because nowhere in
> do_wp_page() we remove it from the list, neither do we take any LRU
> related locks. Although the page is locked, shrink_active_list() can
> still call page_referenced() on it concurrently, because the latter does
> not require an anonymous page to be locked.
> 
> If store tearing described in the thread were possible, we could face
> the following race resulting in kernel panic:
> 
>   CPU0                          CPU1
>   ----                          ----
>   do_wp_page                    shrink_active_list
>    lock_page                     page_referenced
>                                   PageAnon->yes, so skip trylock_page
>    page_move_anon_rmap
>     page->mapping = anon_vma
>                                   rmap_walk
>                                    PageAnon->no
>                                    rmap_walk_file
>                                     BUG
>     page->mapping += PAGE_MAPPING_ANON
> 
> This patch fixes this race by explicitly forbidding the compiler to
> split page->mapping store in __page_set_anon_rmap() and friends and load
> in PageAnon() with the aid of WRITE/READ_ONCE.
> 
> Personally, I don't believe that this can ever happen on any sane
> compiler, because such an "optimization" would only result in two stores
> vs one (note, anon_vma is not a constant), but since I can be mistaken I
> would like to hear from synchronization experts what they think about
> it.

An example "insane" compiler might notice that the value set cannot be
safely observed without multiple CPUs accessing that variable at the
same time.  A paper entitled "No Sane Compiler Would Optimize Atomics"
has some examples:

	http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html

If this paper doesn't scare you, then you didn't read it carefully enough.
And yes, I did give the author a very hard time about the need to suppress
some of these optimizations in order to correctly compile old code, and
will continue to do so.  However, a READ_ONCE() would be a most excellent
and very cheap way to future-proof this code, and is highly recommended.

							Thanx, Paul

> Thanks,
> Vladimir
> ---
>  include/linux/page-flags.h |    3 ++-
>  mm/rmap.c                  |    6 +++---
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 5e7c4f50a644..a529e0a35fe9 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -320,7 +320,8 @@ PAGEFLAG(Idle, idle)
> 
>  static inline int PageAnon(struct page *page)
>  {
> -	return ((unsigned long)page->mapping & PAGE_MAPPING_ANON) != 0;
> +	return ((unsigned long)READ_ONCE(page->mapping) &
> +		PAGE_MAPPING_ANON) != 0;
>  }
> 
>  #ifdef CONFIG_KSM
> diff --git a/mm/rmap.c b/mm/rmap.c
> index eca7416f55d7..aa60c63704e6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -958,7 +958,7 @@ void page_move_anon_rmap(struct page *page,
>  	VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), page);
> 
>  	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
> -	page->mapping = (struct address_space *) anon_vma;
> +	WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
>  }
> 
>  /**
> @@ -987,7 +987,7 @@ static void __page_set_anon_rmap(struct page *page,
>  		anon_vma = anon_vma->root;
> 
>  	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
> -	page->mapping = (struct address_space *) anon_vma;
> +	WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
>  	page->index = linear_page_index(vma, address);
>  }
> 
> @@ -1579,7 +1579,7 @@ static void __hugepage_set_anon_rmap(struct page *page,
>  		anon_vma = anon_vma->root;
> 
>  	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
> -	page->mapping = (struct address_space *) anon_vma;
> +	WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
>  	page->index = linear_page_index(vma, address);
>  }
> 
> -- 
> 1.7.10.4
> 

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