Re: [PATCH v4] mm: memcg/slab: Stop reparented obj_cgroups from charging root

[Richard Palethorpe]

Hello Roman,

Roman Gushchin <guro@xxxxxx> writes:

> On Thu, Oct 22, 2020 at 04:59:56PM -0700, Shakeel Butt wrote:
>> On Thu, Oct 22, 2020 at 10:25 AM Roman Gushchin <guro@xxxxxx> wrote:
>> >
>> [snip]
>> > >
>> > > Since bf4f059954dc ("mm: memcg/slab: obj_cgroup API") is in 5.9, I
>> > > think we can take this patch for 5.9 and 5.10 but keep Roman's cleanup
>> > > for 5.11.
>> > >
>> > > What does everyone think?
>> >
>> > I think we should use the link to the root approach both for stable backports
>> > and for 5.11+, to keep them in sync. The cleanup (always charging the root cgroup)
>> > is not directly related to this problem, and we can keep it for 5.11+ only.
>> >
>> > Thanks!
>> 
>> Roman, can you send the signed-off patch for the root linking for
>> use_hierarchy=0?
>
> Sure, here we are.
>
> Thanks!
>
> --
>
> From 19d66695f0ef1bf1ef7c51073ab91d67daa91362 Mon Sep 17 00:00:00 2001
> From: Roman Gushchin <guro@xxxxxx>
> Date: Thu, 22 Oct 2020 17:12:32 -0700
> Subject: [PATCH] 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

I think it is worth mentioning that reparenting will always be to root
to avoid any confusion about what may happen with deeper, broken,
hierarchies.

> 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.
>
> The patch 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!
>
> Reported-by: ltp@xxxxxxxxxxxxxx
> Debugged-by: Richard Palethorpe <rpalethorpe@xxxxxxxx>

Much appreciated, thanks!

-- 
Thank you,
Richard.