[PATCH 5.9 003/255] mm: memcg: link page counters to root if use_hierarchy is false

[Greg Kroah-Hartman]

From: Roman Gushchin <guro@xxxxxx>

[ Upstream commit 8de15e920dc85d1705ab9c202c95d56845bc2d48 ]

Richard reported a warning which can be reproduced by running the LTP
madvise6 test (cgroup v1 in the non-hierarchical mode should be used):

  WARNING: CPU: 0 PID: 12 at mm/page_counter.c:57 page_counter_uncharge (mm/page_counter.c:57 mm/page_counter.c:50 mm/page_counter.c:156)
  Modules linked in:
  CPU: 0 PID: 12 Comm: kworker/0:1 Not tainted 5.9.0-rc7-22-default #77
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-48-gd9c812d-rebuilt.opensuse.org 04/01/2014
  Workqueue: events drain_local_stock
  RIP: 0010:page_counter_uncharge (mm/page_counter.c:57 mm/page_counter.c:50 mm/page_counter.c:156)
  Call Trace:
    __memcg_kmem_uncharge (mm/memcontrol.c:3022)
    drain_obj_stock (./include/linux/rcupdate.h:689 mm/memcontrol.c:3114)
    drain_local_stock (mm/memcontrol.c:2255)
    process_one_work (./arch/x86/include/asm/jump_label.h:25 ./include/linux/jump_label.h:200 ./include/trace/events/workqueue.h:108 kernel/workqueue.c:2274)
    worker_thread (./include/linux/list.h:282 kernel/workqueue.c:2416)
    kthread (kernel/kthread.c:292)
    ret_from_fork (arch/x86/entry/entry_64.S:300)

The problem occurs because in the non-hierarchical mode non-root page
counters are not linked to root page counters, so the charge is not
propagated to the root memory cgroup.

After the removal of the original memory cgroup and reparenting of the
object cgroup, the root cgroup might be uncharged by draining a objcg
stock, for example.  It leads to an eventual underflow of the charge and
triggers a warning.

Fix it by linking all page counters to corresponding root page counters
in the non-hierarchical mode.

Please note, that in the non-hierarchical mode all objcgs are always
reparented to the root memory cgroup, even if the hierarchy has more
than 1 level.  This patch doesn't change it.

The patch also doesn't affect how the hierarchical mode is working,
which is the only sane and truly supported mode now.

Thanks to Richard for reporting, debugging and providing an alternative
version of the fix!

Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
Reported-by: <ltp@xxxxxxxxxxxxxx>
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20201026231326.3212225-1-guro@xxxxxx
Debugged-by: Richard Palethorpe <rpalethorpe@xxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 mm/memcontrol.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9eefdb9cc2303..de51787831728 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5298,7 +5298,13 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 		memcg->swappiness = mem_cgroup_swappiness(parent);
 		memcg->oom_kill_disable = parent->oom_kill_disable;
 	}
-	if (parent && parent->use_hierarchy) {
+	if (!parent) {
+		page_counter_init(&memcg->memory, NULL);
+		page_counter_init(&memcg->swap, NULL);
+		page_counter_init(&memcg->memsw, NULL);
+		page_counter_init(&memcg->kmem, NULL);
+		page_counter_init(&memcg->tcpmem, NULL);
+	} else if (parent->use_hierarchy) {
 		memcg->use_hierarchy = true;
 		page_counter_init(&memcg->memory, &parent->memory);
 		page_counter_init(&memcg->swap, &parent->swap);
@@ -5306,11 +5312,11 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 		page_counter_init(&memcg->kmem, &parent->kmem);
 		page_counter_init(&memcg->tcpmem, &parent->tcpmem);
 	} else {
-		page_counter_init(&memcg->memory, NULL);
-		page_counter_init(&memcg->swap, NULL);
-		page_counter_init(&memcg->memsw, NULL);
-		page_counter_init(&memcg->kmem, NULL);
-		page_counter_init(&memcg->tcpmem, NULL);
+		page_counter_init(&memcg->memory, &root_mem_cgroup->memory);
+		page_counter_init(&memcg->swap, &root_mem_cgroup->swap);
+		page_counter_init(&memcg->memsw, &root_mem_cgroup->memsw);
+		page_counter_init(&memcg->kmem, &root_mem_cgroup->kmem);
+		page_counter_init(&memcg->tcpmem, &root_mem_cgroup->tcpmem);
 		/*
 		 * Deeper hierachy with use_hierarchy == false doesn't make
 		 * much sense so let cgroup subsystem know about this
-- 
2.27.0