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



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