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 > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>