mm/rmap: stop store reordering issue on page->mapping

Hugh Dickins and Minchan Kim observed a long time issue which discussed
here, but actully the mentioned fix in

  https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop/

was missed.

The store reordering may cause problem in the scenario:

	CPU 0						CPU1
   do_anonymous_page
	page_add_new_anon_rmap()
	  page->mapping = anon_vma + PAGE_MAPPING_ANON
	lru_cache_add_inactive_or_unevictable()
	  spin_lock(lruvec->lock)
	  SetPageLRU()
	  spin_unlock(lruvec->lock)
						/* idletacking judged it as LRU
						 * page so pass the page in
						 * page_idle_clear_pte_refs
						 */
						page_idle_clear_pte_refs
						  rmap_walk
						    if PageAnon(page)

Johannes give detailed examples how the store reordering could cause
trouble: "The concern is the SetPageLRU may get reorder before
'page->mapping' setting, That would make CPU 1 will observe at
page->mapping after observing PageLRU set on the page.

1. anon_vma + PAGE_MAPPING_ANON

   That's the in-order scenario and is fine.

2. NULL

   That's possible if the page->mapping store gets reordered to occur
   after SetPageLRU. That's fine too because we check for it.

3. anon_vma without the PAGE_MAPPING_ANON bit

   That would be a problem and could lead to all kinds of undesirable
   behavior including crashes and data corruption.

   Is it possible? AFAICT the compiler is allowed to tear the store to
   page->mapping and I don't see anything that would prevent it.

That said, I also don't see how the reader testing PageLRU under the
lru_lock would prevent that in the first place.  AFAICT we need that
WRITE_ONCE() around the page->mapping assignment."

[[email protected]: updated for comments change from Johannes]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alex Shi <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: "Chen, Rong A" <[email protected]>
Cc: Daniel Jordan <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mika Penttilä <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Yang Shi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 7 insertions, 1 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 6657000b18d4..ab16c96efdfb 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1054,8 +1054,14 @@ static void __page_set_anon_rmap(struct page *page,
 	if (!exclusive)
 		anon_vma = anon_vma->root;
 
+	/*
+	 * page_idle does a lockless/optimistic rmap scan on page->mapping.
+	 * Make sure the compiler doesn't split the stores of anon_vma and
+	 * the PAGE_MAPPING_ANON type identifier, otherwise the rmap code
+	 * could mistake the mapping for a struct address_space and crash.
+	 */
 	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);
 }