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Subject: Re: [PATCH] bfq: fix blkio cgroup leakage
From:   Paolo Valente <paolo.valente@linaro.org>
In-Reply-To: <87h7uhqewn.fsf@dmws.yandex.net>
Date:   Thu, 9 Jul 2020 10:35:49 +0200
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-block@vger.kernel.org,
        axboe@kernel.dk
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Message-Id: <22087F19-BC93-447E-848A-109392E0622D@linaro.org>
References: <20200702105751.20482-1-dmonakhov@gmail.com>
 <429E50C6-83BA-4A3F-BE9C-06C7C762AF33@linaro.org>
 <87k0zdrj7s.fsf@dmws.yandex.net>
 <545F1ABF-B2B2-4523-9259-D3F93A9BB330@linaro.org>
 <87h7uhqewn.fsf@dmws.yandex.net>
To:     Dmitry Monakhov <dmonakhov@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
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> Il giorno 9 lug 2020, alle ore 10:19, Dmitry Monakhov =
<dmonakhov@gmail.com> ha scritto:
>=20
> Paolo Valente <paolo.valente@linaro.org> writes:
>=20
>>> Il giorno 8 lug 2020, alle ore 19:48, Dmitry Monakhov =
<dmonakhov@gmail.com> ha scritto:
>>>=20
>>> Paolo Valente <paolo.valente@linaro.org> writes:
>>>=20
>>>> Hi,
>>>> sorry for the delay.  The commit you propose to drop fix the issues
>>>> reported in [1].
>>>>=20
>>>> Such a commit does introduce the leak that you report (thank you =
for
>>>> spotting it).  Yet, according to the threads mentioned in [1],
>>>> dropping that commit would take us back to those issues.
>>>>=20
>>>> Maybe the solution is to fix the unbalance that you spotted?
>>> I'm not quite shure that do I understand which bug was addressed for =
commit db37a34c563b.
>>> AFAIU both bugs mentioned in original patchset was fixed by:
>>> 478de3380 ("block, bfq: deschedule empty bfq_queues not referred by =
any proces")
>>> f718b0932 ( block, bfq: do not plug I/O for bfq_queues with no proc =
refs)"
>>>=20
>>> So I review commit db37a34c563b as independent one.
>>> It introduces extra reference for bfq_groups via =
bfqg_and_blkg_get(),
>>> but do we actually need it here?
>>>=20
>>> #IF CONFIG_BFQ_GROUP_IOSCHED is enabled:
>>> bfqd->root_group is holded by bfqd from bfq_init_queue()
>>> other bfq_queue objects are owned by corresponding blkcg from =
bfq_pd_alloc()
>>> So bfq_queue can not disappear under us.
>>>=20
>>=20
>> You are right, but incomplete.  No extra ref is needed for an entity
>> that represents a bfq_queue.  And this consideration mistook me =
before
>> I realized that that commit was needed.  The problem is that an =
entity
>> may also represent a group of entities.  In that case no reference is
>> taken through any bfq_queue.  The commit you want to remove takes =
this
>> missing reference.
> Sorry, It looks like I've mistyped sentance above, I ment to say =
bfq_group.
> So here is my statement corrected:
> #IF CONFIG_BFQ_GROUP_IOSCHED is enabled:
> bfqd->root_group is holded by bfqd from bfq_init_queue()
> other *bfq_group* objects are owned by corresponding blkcg, reference =
get from bfq_pd_alloc()
> So *bfq_group* can not disappear under us.
>=20
> So no extra reference is required for entity represents bfq_group. =
Commit is not required.

No, the entity may remain alive and on some tree after bfq_pd_offline =
has been invoked.

Paolo

>>=20
>> Paolo
>>=20
>>> #IF CONFIG_BFQ_GROUP_IOSCHED is disabled:
>>> we have only one  bfqd->root_group object which allocated from =
bfq_create_group_hierarch()
>>> and bfqg_and_blkg_get() bfqg_and_blkg_put() are noop
>>>=20
>>> Resume: in both cases extra reference is not required, so I continue =
to
>>> insist that we should revert  commit db37a34c563b because it tries =
to
>>> solve a non existing issue, but introduce the real one.
>>>=20
>>> Please correct me if I'm wrong.
>>>>=20
>>>> I'll check it ASAP, unless you do it before me.
>>>>=20
>>>> Thanks,
>>>> Paolo
>>>>=20
>>>> [1] https://lkml.org/lkml/2020/1/31/94
>>>>=20
>>>>> Il giorno 2 lug 2020, alle ore 12:57, Dmitry Monakhov =
<dmonakhov@gmail.com> ha scritto:
>>>>>=20
>>>>> commit db37a34c563b ("block, bfq: get a ref to a group when adding =
it to a service tree")
>>>>> introduce leak forbfq_group and blkcg_gq objects because of =
get/put
>>>>> imbalance. See trace balow:
>>>>> -> blkg_alloc
>>>>> -> bfq_pq_alloc
>>>>>   -> bfqg_get (+1)
>>>>> ->bfq_activate_bfqq
>>>>> ->bfq_activate_requeue_entity
>>>>>  -> __bfq_activate_entity
>>>>>     ->bfq_get_entity
>>> ->>         ->bfqg_and_blkg_get (+1)  <=3D=3D=3D=3D : Note1
>>>>> ->bfq_del_bfqq_busy
>>>>> ->bfq_deactivate_entity+0x53/0xc0 [bfq]
>>>>>  ->__bfq_deactivate_entity+0x1b8/0x210 [bfq]
>>>>>    -> bfq_forget_entity(is_in_service =3D true)
>>>>> 	 entity->on_st_or_in_serv =3D false   <=3D=3D=3D :Note2
>>>>> 	 if (is_in_service)
>>>>> 	     return;  =3D=3D> do not touch reference
>>>>> -> blkcg_css_offline
>>>>> -> blkcg_destroy_blkgs
>>>>> -> blkg_destroy
>>>>> -> bfq_pd_offline
>>>>>  -> __bfq_deactivate_entity
>>>>>       if (!entity->on_st_or_in_serv) /* true, because (Note2)
>>>>> 		return false;
>>>>> -> bfq_pd_free
>>>>>  -> bfqg_put() (-1, byt bfqg->ref =3D=3D 2) because of (Note2)
>>>>> So bfq_group and blkcg_gq  will leak forever, see test-case below.
>>>>> If fact bfq_group objects reference counting are quite different
>>>>> from bfq_queue. bfq_groups object are referenced by blkcg_gq via
>>>>> blkg_policy_data pointer, so  neither nor blkg_get() neither =
bfqg_get
>>>>> required here.
>>>>>=20
>>>>>=20
>>>>> This patch drop commit db37a34c563b ("block, bfq: get a ref to a =
group when adding it to a service tree")
>>>>> and add corresponding comment.
>>>>>=20
>>>>> ##TESTCASE_BEGIN:
>>>>> #!/bin/bash
>>>>>=20
>>>>> max_iters=3D${1:-100}
>>>>> #prep cgroup mounts
>>>>> mount -t tmpfs cgroup_root /sys/fs/cgroup
>>>>> mkdir /sys/fs/cgroup/blkio
>>>>> mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
>>>>>=20
>>>>> # Prepare blkdev
>>>>> grep blkio /proc/cgroups
>>>>> truncate -s 1M img
>>>>> losetup /dev/loop0 img
>>>>> echo bfq > /sys/block/loop0/queue/scheduler
>>>>>=20
>>>>> grep blkio /proc/cgroups
>>>>> for ((i=3D0;i<max_iters;i++))
>>>>> do
>>>>>  mkdir -p /sys/fs/cgroup/blkio/a
>>>>>  echo 0 > /sys/fs/cgroup/blkio/a/cgroup.procs
>>>>>  dd if=3D/dev/loop0 bs=3D4k count=3D1 of=3D/dev/null iflag=3Ddirect =
2> /dev/null
>>>>>  echo 0 > /sys/fs/cgroup/blkio/cgroup.procs
>>>>>  rmdir /sys/fs/cgroup/blkio/a
>>>>>  grep blkio /proc/cgroups
>>>>> done
>>>>> ##TESTCASE_END:
>>>>>=20
>>>>> Signed-off-by: Dmitry Monakhov <dmonakhov@gmail.com>
>>>>> ---
>>>>> block/bfq-cgroup.c  |  2 +-
>>>>> block/bfq-iosched.h |  1 -
>>>>> block/bfq-wf2q.c    | 15 +++++----------
>>>>> 3 files changed, 6 insertions(+), 12 deletions(-)
>>>>>=20
>>>>> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
>>>>> index 68882b9..b791e20 100644
>>>>> --- a/block/bfq-cgroup.c
>>>>> +++ b/block/bfq-cgroup.c
>>>>> @@ -332,7 +332,7 @@ static void bfqg_put(struct bfq_group *bfqg)
>>>>> 		kfree(bfqg);
>>>>> }
>>>>>=20
>>>>> -void bfqg_and_blkg_get(struct bfq_group *bfqg)
>>>>> +static void bfqg_and_blkg_get(struct bfq_group *bfqg)
>>>>> {
>>>>> 	/* see comments in bfq_bic_update_cgroup for why refcounting =
bfqg */
>>>>> 	bfqg_get(bfqg);
>>>>> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
>>>>> index cd224aa..7038952 100644
>>>>> --- a/block/bfq-iosched.h
>>>>> +++ b/block/bfq-iosched.h
>>>>> @@ -986,7 +986,6 @@ struct bfq_group *bfq_find_set_group(struct =
bfq_data *bfqd,
>>>>> struct blkcg_gq *bfqg_to_blkg(struct bfq_group *bfqg);
>>>>> struct bfq_group *bfqq_group(struct bfq_queue *bfqq);
>>>>> struct bfq_group *bfq_create_group_hierarchy(struct bfq_data =
*bfqd, int node);
>>>>> -void bfqg_and_blkg_get(struct bfq_group *bfqg);
>>>>> void bfqg_and_blkg_put(struct bfq_group *bfqg);
>>>>>=20
>>>>> #ifdef CONFIG_BFQ_GROUP_IOSCHED
>>>>> diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
>>>>> index 34ad095..6a363bb 100644
>>>>> --- a/block/bfq-wf2q.c
>>>>> +++ b/block/bfq-wf2q.c
>>>>> @@ -529,13 +529,14 @@ static void bfq_get_entity(struct bfq_entity =
*entity)
>>>>> {
>>>>> 	struct bfq_queue *bfqq =3D bfq_entity_to_bfqq(entity);
>>>>>=20
>>>>> +	/* Grab reference only for bfq_queue's objects, bfq_group ones
>>>>> +	 * are owned by blkcg_gq
>>>>> +	 */
>>>>> 	if (bfqq) {
>>>>> 		bfqq->ref++;
>>>>> 		bfq_log_bfqq(bfqq->bfqd, bfqq, "get_entity: %p %d",
>>>>> 			     bfqq, bfqq->ref);
>>>>> -	} else
>>>>> -		bfqg_and_blkg_get(container_of(entity, struct bfq_group,
>>>>> -					       entity));
>>>>> +	}
>>>>> }
>>>>>=20
>>>>> /**
>>>>> @@ -649,14 +650,8 @@ static void bfq_forget_entity(struct =
bfq_service_tree *st,
>>>>>=20
>>>>> 	entity->on_st_or_in_serv =3D false;
>>>>> 	st->wsum -=3D entity->weight;
>>>>> -	if (is_in_service)
>>>>> -		return;
>>>>> -
>>>>> -	if (bfqq)
>>>>> +	if (bfqq && !is_in_service)
>>>>> 		bfq_put_queue(bfqq);
>>>>> -	else
>>>>> -		bfqg_and_blkg_put(container_of(entity, struct bfq_group,
>>>>> -					       entity));
>>>>> }
>>>>>=20
>>>>> /**
>>>>> --=20
>>>>> 2.7.4