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 > > -- > 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> -- Kind regards, Minchan Kim -- 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>