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SubjectRe: [PATCH v2] rmap: fix theoretical race between do_wp_page and shrink_active_list
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@parallels.com>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

--
Kirill A. Shutemov


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