From: Roman Gushchin guro@fb.com Subject: mm: memcg: link page counters to root if use_hierarchy is false

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

[ 9.841552] ------------[ cut here ]------------ [ 9.841788] 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) [ 9.841982] Modules linked in: [ 9.842072] CPU: 0 PID: 12 Comm: kworker/0:1 Not tainted 5.9.0-rc7-22-default #77 [ 9.842266] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-48-gd9c812d-rebuilt.opensuse.org 04/01/2014 [ 9.842571] Workqueue: events drain_local_stock [ 9.842750] RIP: 0010:page_counter_uncharge (mm/page_counter.c:57 mm/page_counter.c:50 mm/page_counter.c:156) [ 9.842894] Code: 0f c1 45 00 4c 29 e0 48 89 ef 48 89 c3 48 89 c6 e8 2a fe ff ff 48 85 db 78 10 48 8b 6d 28 48 85 ed 75 d8 5b 5d 41 5c 41 5d c3 <0f> 0b eb ec 90 e8 4b f9 88 2a 48 8b 17 48 39 d6 72 41 41 54 49 89 [ 9.843438] RSP: 0018:ffffb1c18006be28 EFLAGS: 00010086 [ 9.843585] RAX: ffffffffffffffff RBX: ffffffffffffffff RCX: ffff94803bc2cae0 [ 9.843806] RDX: 0000000000000001 RSI: ffffffffffffffff RDI: ffff948007d2b248 [ 9.844026] RBP: ffff948007d2b248 R08: ffff948007c58eb0 R09: ffff948007da05ac [ 9.844248] R10: 0000000000000018 R11: 0000000000000018 R12: 0000000000000001 [ 9.844477] R13: ffffffffffffffff R14: 0000000000000000 R15: ffff94803bc2cac0 [ 9.844696] FS: 0000000000000000(0000) GS:ffff94803bc00000(0000) knlGS:0000000000000000 [ 9.844915] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 9.845096] CR2: 00007f0579ee0384 CR3: 000000002cc0a000 CR4: 00000000000006f0 [ 9.845319] Call Trace: [ 9.845429] __memcg_kmem_uncharge (mm/memcontrol.c:3022) [ 9.845582] drain_obj_stock (./include/linux/rcupdate.h:689 mm/memcontrol.c:3114) [ 9.845684] drain_local_stock (mm/memcontrol.c:2255) [ 9.845789] 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) [ 9.845898] worker_thread (./include/linux/list.h:282 kernel/workqueue.c:2416) [ 9.846034] ? process_one_work (kernel/workqueue.c:2358) [ 9.846162] kthread (kernel/kthread.c:292) [ 9.846271] ? __kthread_bind_mask (kernel/kthread.c:245) [ 9.846420] ret_from_fork (arch/x86/entry/entry_64.S:300) [ 9.846531] ---[ end trace 8b5647c1eba9d18a ]---

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!

Link: https://lkml.kernel.org/r/20201026231326.3212225-1-guro@fb.com Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") Signed-off-by: Roman Gushchin guro@fb.com Debugged-by: Richard Palethorpe rpalethorpe@suse.com Reported-by: ltp@lists.linux.it Reviewed-by: Shakeel Butt shakeelb@google.com Acked-by: Johannes Weiner hannes@cmpxchg.org Reviewed-by: Michal Koutný mkoutny@suse.com Cc: Michal Hocko mhocko@kernel.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memcontrol.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-)

--- a/mm/memcontrol.c~mm-memcg-link-page-counters-to-root-if-use_hierarchy-is-false +++ a/mm/memcontrol.c @@ -5345,17 +5345,22 @@ mem_cgroup_css_alloc(struct cgroup_subsy 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->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); 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->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->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 _