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

Paul E. McKenney
2015-05-11T14:29:43Z

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
>