mm: thp: tail page refcounting fix

Michel while working on the working set estimation code, noticed that
calling get_page_unless_zero() on a random pfn_to_page(random_pfn)
wasn't safe, if the pfn ended up being a tail page of a transparent
hugepage under splitting by __split_huge_page_refcount().

He then found the problem could also theoretically materialize with
page_cache_get_speculative() during the speculative radix tree lookups
that uses get_page_unless_zero() in SMP if the radix tree page is freed
and reallocated and get_user_pages is called on it before
page_cache_get_speculative has a chance to call get_page_unless_zero().

So the best way to fix the problem is to keep page_tail->_count zero at
all times.  This will guarantee that get_page_unless_zero() can never
succeed on any tail page.  page_tail->_mapcount is guaranteed zero and
is unused for all tail pages of a compound page, so we can simply
account the tail page references there and transfer them to
tail_page->_count in __split_huge_page_refcount() (in addition to the
head_page->_mapcount).

While debugging this s/_count/_mapcount/ change I also noticed get_page is
called by direct-io.c on pages returned by get_user_pages.  That wasn't
entirely safe because the two atomic_inc in get_page weren't atomic.  As
opposed to other get_user_page users like secondary-MMU page fault to
establish the shadow pagetables would never call any superflous get_page
after get_user_page returns.  It's safer to make get_page universally safe
for tail pages and to use get_page_foll() within follow_page (inside
get_user_pages()).  get_page_foll() is safe to do the refcounting for tail
pages without taking any locks because it is run within PT lock protected
critical sections (PT lock for pte and page_table_lock for
pmd_trans_huge).

The standard get_page() as invoked by direct-io instead will now take
the compound_lock but still only for tail pages.  The direct-io paths
are usually I/O bound and the compound_lock is per THP so very
finegrined, so there's no risk of scalability issues with it.  A simple
direct-io benchmarks with all lockdep prove locking and spinlock
debugging infrastructure enabled shows identical performance and no
overhead.  So it's worth it.  Ideally direct-io should stop calling
get_page() on pages returned by get_user_pages().  The spinlock in
get_page() is already optimized away for no-THP builds but doing
get_page() on tail pages returned by GUP is generally a rare operation
and usually only run in I/O paths.

This new refcounting on page_tail->_mapcount in addition to avoiding new
RCU critical sections will also allow the working set estimation code to
work without any further complexity associated to the tail page
refcounting with THP.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: <stable@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 39 insertions, 38 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3b3e3b8bb706..f81b7b41930c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -356,36 +356,39 @@ static inline struct page *compound_head(struct page *page)
 	return page;
 }
 
+/*
+ * The atomic page->_mapcount, starts from -1: so that transitions
+ * both from it and to it can be tracked, using atomic_inc_and_test
+ * and atomic_add_negative(-1).
+ */
+static inline void reset_page_mapcount(struct page *page)
+{
+	atomic_set(&(page)->_mapcount, -1);
+}
+
+static inline int page_mapcount(struct page *page)
+{
+	return atomic_read(&(page)->_mapcount) + 1;
+}
+
 static inline int page_count(struct page *page)
 {
 	return atomic_read(&compound_head(page)->_count);
 }
 
+extern bool __get_page_tail(struct page *page);
+
 static inline void get_page(struct page *page)
 {
+	if (unlikely(PageTail(page)))
+		if (likely(__get_page_tail(page)))
+			return;
 	/*
 	 * Getting a normal page or the head of a compound page
-	 * requires to already have an elevated page->_count. Only if
-	 * we're getting a tail page, the elevated page->_count is
-	 * required only in the head page, so for tail pages the
-	 * bugcheck only verifies that the page->_count isn't
-	 * negative.
+	 * requires to already have an elevated page->_count.
 	 */
-	VM_BUG_ON(atomic_read(&page->_count) < !PageTail(page));
+	VM_BUG_ON(atomic_read(&page->_count) <= 0);
 	atomic_inc(&page->_count);
-	/*
-	 * Getting a tail page will elevate both the head and tail
-	 * page->_count(s).
-	 */
-	if (unlikely(PageTail(page))) {
-		/*
-		 * This is safe only because
-		 * __split_huge_page_refcount can't run under
-		 * get_page().
-		 */
-		VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
-		atomic_inc(&page->first_page->_count);
-	}
 }
 
 static inline struct page *virt_to_head_page(const void *x)
@@ -804,21 +807,6 @@ static inline pgoff_t page_index(struct page *page)
 }
 
 /*
- * The atomic page->_mapcount, like _count, starts from -1:
- * so that transitions both from it and to it can be tracked,
- * using atomic_inc_and_test and atomic_add_negative(-1).
- */
-static inline void reset_page_mapcount(struct page *page)
-{
-	atomic_set(&(page)->_mapcount, -1);
-}
-
-static inline int page_mapcount(struct page *page)
-{
-	return atomic_read(&(page)->_mapcount) + 1;
-}
-
-/*
  * Return true if this page is mapped into pagetables.
  */
 static inline int page_mapped(struct page *page)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3e01a19a91e8..5b42f1b34eb7 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -62,10 +62,23 @@ struct page {
 			struct {
 
 				union {
-					atomic_t _mapcount;	/* Count of ptes mapped in mms,
-							 * to show when page is mapped
-							 * & limit reverse map searches.
-							 */
+					/*
+					 * Count of ptes mapped in
+					 * mms, to show when page is
+					 * mapped & limit reverse map
+					 * searches.
+					 *
+					 * Used also for tail pages
+					 * refcounting instead of
+					 * _count. Tail pages cannot
+					 * be mapped and keeping the
+					 * tail page _count zero at
+					 * all times guarantees
+					 * get_page_unless_zero() will
+					 * never succeed on tail
+					 * pages.
+					 */
+					atomic_t _mapcount;
 
 					struct {
 						unsigned inuse:16;