Re: [PATCH v2] rmap: fix theoretical race between do_wp_page and shrink_active_list

[Minchan Kim]

Hi, Rik

I'd like to bring up the issue in this thread although I already gave
my Acked-by.

Below issue causes by no PG_locked page in page_referenced while
page_move_anon_rmap depends on PG_locked to prevent race with rmap code.

So, although this patch fixes below one example, we still have a problem
in rmap.

If page_referenced holds PG_locked for all of pages unconditionally,
we don't need this patch and might remove READ_ONCE introduced by
80e148 and more than.

What do you think about?

On Tue, May 12, 2015 at 01:18:39PM +0300, Vladimir Davydov wrote:
> As noted by Paul the compiler is free to store a temporary result in a
> variable on stack, heap or global unless it is explicitly marked as
> volatile, see:
> 
>   http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html#sample-optimizations
> 
> This can result in a race between do_wp_page() and shrink_active_list()
> 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:
> 
>   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_move_anon_rmap() with the aid of
> WRITE_ONCE.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
> Changes in v2:
>  - do not add READ_ONCE to PageAnon and WRITE_ONCE to
>    __page_set_anon_rmap and __hugepage_set_anon_rmap (Kirill)
> 
>  mm/rmap.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 24dd3f9fee27..8b18fd4227d1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -950,7 +950,7 @@ 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);
>  }
>  
>  /**
> -- 
> 1.7.10.4
> 
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-- 
Kind regards,
Minchan Kim
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