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

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2015-05-11 15:51 GMT+08:00 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>:
> 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.
>
> 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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i am confused about your analysis ,
for the race stack:

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

the page should must change from PageAnon() to !PageAnon() when crash happened.
but page_move_anon_rmap() is doing change a page from !PageAnon()
(swapcache page)
to PageAnon() , how does this race condition crash happened ?

BRs,
Yalin

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