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 <[email protected]> > Cc: "Paul E. McKenney" <[email protected]> > Cc: "Kirill A. Shutemov" <[email protected]> > Cc: Rik van Riel <[email protected]> > Cc: Hugh Dickins <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]> -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
