Re: [PATCH v3] mm: memcg/slab: Stop reparented obj_cgroups from charging root

[Shakeel Butt <shakeelb@google.com>]

On Mon, Oct 19, 2020 at 2:59 AM Richard Palethorpe <rpalethorpe@suse.com> wrote:
>
> SLAB objects which outlive their descendant memcg are moved to their
> parent memcg where they may be uncharged. However if they are moved to
> the root memcg and use_hierarchy=0, uncharging will result in negative
> page counter values. This is because when use_hierarchy=0, the root
> memcg's page counters are disconnected from its children.
>
> To prevent this, we check whether we are about to uncharge the root
> memcg and whether use_hierarchy=0. If this is the case then we skip
> uncharging.
>
> Note that on the default hierarchy (CGroupV2 now) root always has
> use_hierarchy=1. So this only effects CGroupV1. Also it is possible to
> have a deeper hierarchy where descendants also have use_hierarchy=0;
> this is not considered valid by the kernel, but it is still allowed
> and in such cases reparenting may still result in negative page
> counter values.
>
> The warning can be, unreliably, reproduced with the LTP test
> madvise06 if the entire patch series
> https://lore.kernel.org/linux-mm/20200623174037.3951353-1-guro@fb.com/
> is present. Although the listed commit in 'fixes' appears to introduce
> the bug, I can not reproduce it with just that commit and bisecting
> runs into other bugs.
>
> [   12.029417] WARNING: CPU: 2 PID: 21 at mm/page_counter.c:57 page_counter_uncharge (mm/page_counter.c:57 mm/page_counter.c:50 mm/page_counter.c:156)
> [   12.029539] Modules linked in:
> [   12.029611] CPU: 2 PID: 21 Comm: ksoftirqd/2 Not tainted 5.9.0-rc7-22-default #76
> [   12.029729] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-48-gd9c812d-rebuilt.opensuse.org 04/01/2014
> [   12.029908] RIP: 0010:page_counter_uncharge (mm/page_counter.c:57 mm/page_counter.c:50 mm/page_counter.c:156)
> [ 12.029991] 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 db 47 36 27 48 8b 17 48 39 d6 72 41 41 54 49 89
> [   12.030258] RSP: 0018:ffffa5d8000efd08 EFLAGS: 00010086
> [   12.030344] RAX: ffffffffffffffff RBX: ffffffffffffffff RCX: 0000000000000009
> [   12.030455] RDX: 000000000000000b RSI: ffffffffffffffff RDI: ffff8ef8c7d2b248
> [   12.030561] RBP: ffff8ef8c7d2b248 R08: ffff8ef8c78b19c8 R09: 0000000000000001
> [   12.030672] R10: 0000000000000000 R11: ffff8ef8c780e0d0 R12: 0000000000000001
> [   12.030784] R13: ffffffffffffffff R14: ffff8ef9478b19c8 R15: 0000000000000000
> [   12.030895] FS:  0000000000000000(0000) GS:ffff8ef8fbc80000(0000) knlGS:0000000000000000
> [   12.031017] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   12.031104] CR2: 00007f72c0af93ec CR3: 000000005c40a000 CR4: 00000000000006e0
> [   12.031209] Call Trace:
> [   12.031267] __memcg_kmem_uncharge (mm/memcontrol.c:3022)
> [   12.031470] drain_obj_stock (./include/linux/rcupdate.h:689 mm/memcontrol.c:3114)
> [   12.031594] refill_obj_stock (mm/memcontrol.c:3166)
> [   12.031733] ? rcu_do_batch (kernel/rcu/tree.c:2438)
> [   12.032075] memcg_slab_free_hook (./include/linux/mm.h:1294 ./include/linux/mm.h:1441 mm/slab.h:368 mm/slab.h:348)
> [   12.032339] kmem_cache_free (mm/slub.c:3107 mm/slub.c:3143 mm/slub.c:3158)
> [   12.032464] rcu_do_batch (kernel/rcu/tree.c:2438)
> [   12.032567] rcu_core (kernel/rcu/tree_plugin.h:2122 kernel/rcu/tree_plugin.h:2157 kernel/rcu/tree.c:2661)
> [   12.032664] __do_softirq (./arch/x86/include/asm/jump_label.h:25 ./include/linux/jump_label.h:200 ./include/trace/events/irq.h:142 kernel/softirq.c:299)
> [   12.032766] run_ksoftirqd (./arch/x86/include/asm/irqflags.h:54 ./arch/x86/include/asm/irqflags.h:94 kernel/softirq.c:653 kernel/softirq.c:644)
> [   12.032852] smpboot_thread_fn (kernel/smpboot.c:165)
> [   12.032940] ? smpboot_register_percpu_thread (kernel/smpboot.c:108)
> [   12.033059] kthread (kernel/kthread.c:292)
> [   12.033148] ? __kthread_bind_mask (kernel/kthread.c:245)
> [   12.033269] ret_from_fork (arch/x86/entry/entry_64.S:300)
> [   12.033357] ---[ end trace 961dbfc01c109d1f ]---
>
> [    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 ]---
>
> Reported-by: ltp@lists.linux.it
> Signed-off-by: Richard Palethorpe <rpalethorpe@suse.com>
> Acked-by: Roman Gushchin <guro@fb.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
> ---
>
> V3: Handle common case where use_hierarchy=1 and update description.
>
>  mm/memcontrol.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6877c765b8d0..34b8c4a66853 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -291,7 +291,7 @@ static void obj_cgroup_release(struct percpu_ref *ref)
>
>         spin_lock_irqsave(&css_set_lock, flags);
>         memcg = obj_cgroup_memcg(objcg);
> -       if (nr_pages)
> +       if (nr_pages && (!mem_cgroup_is_root(memcg) || memcg->use_hierarchy))

If we have non-root memcg with use_hierarchy as 0 and this objcg was
reparented then this __memcg_kmem_uncharge() can potentially underflow
the page counter and give the same warning.

We never set root_mem_cgroup->objcg, so, no need to check for root
here. I think checking just memcg->use_hierarchy should be sufficient.

>                 __memcg_kmem_uncharge(memcg, nr_pages);
>         list_del(&objcg->list);
>         mem_cgroup_put(memcg);
> @@ -3100,6 +3100,7 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
>  static void drain_obj_stock(struct memcg_stock_pcp *stock)
>  {
>         struct obj_cgroup *old = stock->cached_objcg;
> +       struct mem_cgroup *memcg;
>
>         if (!old)
>                 return;
> @@ -3110,7 +3111,9 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
>
>                 if (nr_pages) {
>                         rcu_read_lock();
> -                       __memcg_kmem_uncharge(obj_cgroup_memcg(old), nr_pages);
> +                       memcg = obj_cgroup_memcg(old);
> +                       if (!mem_cgroup_is_root(memcg) || memcg->use_hierarchy)
> +                               __memcg_kmem_uncharge(memcg, nr_pages);
>                         rcu_read_unlock();
>                 }
>
> --
> 2.28.0
>