The move_lock is a per-memcg lock, but the VM accounting code that needs
to acquire it comes from the page and follows page->mem_cgroup under RCU
protection.  That means that the page becomes unlocked not when we drop
the move_lock, but when we update page->mem_cgroup.  And that assignment
doesn't imply any memory ordering.  If that pointer write gets reordered
against the reads of the page state - page_mapped, PageDirty etc.  the
state may change while we rely on it being stable and we can end up
corrupting the counters.

Place an SMP memory barrier to make sure we're done with all page state by
the time the new page->mem_cgroup becomes visible.

Also replace the open-coded move_lock with a lock_page_memcg() to make it
more obvious what we're serializing against.
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Link: http://lkml.kernel.org/r/20200508183105.225460-3-hannes@cmpxchg.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>