[RFC 5/5] Revert "mm: memcg: fix race condition between memcg teardown and swapin"

[Michal Hocko <mhocko@suse.cz>]

This reverts commit 96f1c58d853497a757463e0b57fed140d6858f3a
because it is no longer needed after "memcg: make sure that memcg is not
offline when charging" which makes sure that no charges will be accepted
after mem_cgroup_reparent_charges has started.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
---
 mm/memcontrol.c | 36 ------------------------------------
 1 file changed, 36 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2904b2a6805a..591ced342036 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6395,42 +6395,6 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 static void mem_cgroup_css_free(struct cgroup_subsys_state *css)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
-	/*
-	 * XXX: css_offline() would be where we should reparent all
-	 * memory to prepare the cgroup for destruction.  However,
-	 * memcg does not do css_tryget() and res_counter charging
-	 * under the same RCU lock region, which means that charging
-	 * could race with offlining.  Offlining only happens to
-	 * cgroups with no tasks in them but charges can show up
-	 * without any tasks from the swapin path when the target
-	 * memcg is looked up from the swapout record and not from the
-	 * current task as it usually is.  A race like this can leak
-	 * charges and put pages with stale cgroup pointers into
-	 * circulation:
-	 *
-	 * #0                        #1
-	 *                           lookup_swap_cgroup_id()
-	 *                           rcu_read_lock()
-	 *                           mem_cgroup_lookup()
-	 *                           css_tryget()
-	 *                           rcu_read_unlock()
-	 * disable css_tryget()
-	 * call_rcu()
-	 *   offline_css()
-	 *     reparent_charges()
-	 *                           res_counter_charge()
-	 *                           css_put()
-	 *                             css_free()
-	 *                           pc->mem_cgroup = dead memcg
-	 *                           add page to lru
-	 *
-	 * The bulk of the charges are still moved in offline_css() to
-	 * avoid pinning a lot of pages in case a long-term reference
-	 * like a swapout record is deferring the css_free() to long
-	 * after offlining.  But this makes sure we catch any charges
-	 * made after offlining:
-	 */
-	mem_cgroup_reparent_charges(memcg);
 
 	memcg_destroy_kmem(memcg);
 	__mem_cgroup_free(memcg);
-- 
1.8.4.4

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