[PATCH 4.9 02/50] mm/huge_memory.c: reorder operations in __split_huge_page_tail()

Greg Kroah-Hartman
2018-12-04T11:12:46Z

commit 605ca5ede7643a01f4c4a15913f9714ac297f8a6 upstream.

THP split makes non-atomic change of tail page flags. This is almost ok
because tail pages are locked and isolated but this breaks recent
changes in page locking: non-atomic operation could clear bit
PG_waiters.

As a result concurrent sequence get_page_unless_zero() -> lock_page()
might block forever. Especially if this page was truncated later.

Fix is trivial: clone flags before unfreezing page reference counter.

This race exists since commit 62906027091f ("mm: add PageWaiters
indicating tasks are waiting for a page bit") while unsave unfreeze
itself was added in commit 8df651c7059e ("thp: cleanup
split_huge_page()").

clear_compound_head() also must be called before unfreezing page
reference because after successful get_page_unless_zero() might follow
put_page() which needs correct compound_head().

And replace page_ref_inc()/page_ref_add() with page_ref_unfreeze() which
is made especially for that and has semantic of smp_store_release().

Link: http://lkml.kernel.org/r/151844393341.210639.13162088407980624477.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/huge_memory.c | 36 +++++++++++++++---------------------
1 file changed, 15 insertions(+), 21 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 583ad61cc2f1..c14aec110e90 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1876,26 +1876,13 @@ static void __split_huge_page_tail(struct page *head, int tail,
struct page *page_tail = head + tail;

VM_BUG_ON_PAGE(atomic_read(&page_tail->_mapcount) != -1, page_tail);
- VM_BUG_ON_PAGE(page_ref_count(page_tail) != 0, page_tail);

/*
- * tail_page->_refcount is zero and not changing from under us. But
- * get_page_unless_zero() may be running from under us on the
- * tail_page. If we used atomic_set() below instead of atomic_inc() or
- * atomic_add(), we would then run atomic_set() concurrently with
- * get_page_unless_zero(), and atomic_set() is implemented in C not
- * using locked ops. spin_unlock on x86 sometime uses locked ops
- * because of PPro errata 66, 92, so unless somebody can guarantee
- * atomic_set() here would be safe on all archs (and not only on x86),
- * it's safer to use atomic_inc()/atomic_add().
+ * Clone page flags before unfreezing refcount.
+ *
+ * After successful get_page_unless_zero() might follow flags change,
+ * for exmaple lock_page() which set PG_waiters.
*/
- if (PageAnon(head)) {
- page_ref_inc(page_tail);
- } else {
- /* Additional pin to radix tree */
- page_ref_add(page_tail, 2);
- }
-
page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
page_tail->flags |= (head->flags &
((1L << PG_referenced) |
@@ -1907,14 +1894,21 @@ static void __split_huge_page_tail(struct page *head, int tail,
(1L << PG_unevictable) |
(1L << PG_dirty)));

- /*
- * After clearing PageTail the gup refcount can be released.
- * Page flags also must be visible before we make the page non-compound.
- */
+ /* Page flags must be visible before we make the page non-compound. */
smp_wmb();

+ /*
+ * Clear PageTail before unfreezing page refcount.
+ *
+ * After successful get_page_unless_zero() might follow put_page()
+ * which needs correct compound_head().
+ */
clear_compound_head(page_tail);

+ /* Finally unfreeze refcount. Additional reference from page cache. */
+ page_ref_unfreeze(page_tail, 1 + (!PageAnon(head) ||
+ PageSwapCache(head)));
+
if (page_is_young(head))
set_page_young(page_tail);
if (page_is_idle(head))
--
2.17.1