mm/list_lru.c: fix list_lru_count_node() to be race free

list_lru_count_node() iterates over all memcgs to get the total number of
entries on the node but it can race with memcg_drain_all_list_lrus(),
which migrates the entries from a dead cgroup to another.  This can return
incorrect number of entries from list_lru_count_node().

Fix this by keeping track of entries per node and simply return it in
list_lru_count_node().

Change-Id: I19e3b527804e95be75f48ee363c8207c0e7ee2ff
Link: http://lkml.kernel.org/r/1498707555-30525-1-git-send-email-stummala@codeaurora.org


Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Alexander Polakov <apolyakov@beget.ru>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Patch-mainline: linux-mm @ 29/06/17, 09:09:15
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>