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Date: Thu, 9 Jul 2020 09:19:41 +0200
From: Paolo Valente <paolo.valente@...aro.org>
To: Dmitry Monakhov <dmonakhov@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>, linux-block@...r.kernel.org,
axboe@...nel.dk
Subject: Re: [PATCH] bfq: fix blkio cgroup leakage
> Il giorno 8 lug 2020, alle ore 19:48, Dmitry Monakhov <dmonakhov@...il.com> ha scritto:
>
> Paolo Valente <paolo.valente@...aro.org> writes:
>
>> Hi,
>> sorry for the delay. The commit you propose to drop fix the issues
>> reported in [1].
>>
>> 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.
>>
>> 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)"
>
> 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?
>
> #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.
>
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.
Paolo
> #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
>
> 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.
>
> Please correct me if I'm wrong.
>>
>> I'll check it ASAP, unless you do it before me.
>>
>> Thanks,
>> Paolo
>>
>> [1] https://lkml.org/lkml/2020/1/31/94
>>
>>> Il giorno 2 lug 2020, alle ore 12:57, Dmitry Monakhov <dmonakhov@...il.com> ha scritto:
>>>
>>> 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) <==== : Note1
>>> ->bfq_del_bfqq_busy
>>> ->bfq_deactivate_entity+0x53/0xc0 [bfq]
>>> ->__bfq_deactivate_entity+0x1b8/0x210 [bfq]
>>> -> bfq_forget_entity(is_in_service = true)
>>> entity->on_st_or_in_serv = false <=== :Note2
>>> if (is_in_service)
>>> return; ==> 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 == 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.
>>>
>>>
>>> This patch drop commit db37a34c563b ("block, bfq: get a ref to a group when adding it to a service tree")
>>> and add corresponding comment.
>>>
>>> ##TESTCASE_BEGIN:
>>> #!/bin/bash
>>>
>>> max_iters=${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
>>>
>>> # Prepare blkdev
>>> grep blkio /proc/cgroups
>>> truncate -s 1M img
>>> losetup /dev/loop0 img
>>> echo bfq > /sys/block/loop0/queue/scheduler
>>>
>>> grep blkio /proc/cgroups
>>> for ((i=0;i<max_iters;i++))
>>> do
>>> mkdir -p /sys/fs/cgroup/blkio/a
>>> echo 0 > /sys/fs/cgroup/blkio/a/cgroup.procs
>>> dd if=/dev/loop0 bs=4k count=1 of=/dev/null iflag=direct 2> /dev/null
>>> echo 0 > /sys/fs/cgroup/blkio/cgroup.procs
>>> rmdir /sys/fs/cgroup/blkio/a
>>> grep blkio /proc/cgroups
>>> done
>>> ##TESTCASE_END:
>>>
>>> Signed-off-by: Dmitry Monakhov <dmonakhov@...il.com>
>>> ---
>>> block/bfq-cgroup.c | 2 +-
>>> block/bfq-iosched.h | 1 -
>>> block/bfq-wf2q.c | 15 +++++----------
>>> 3 files changed, 6 insertions(+), 12 deletions(-)
>>>
>>> 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);
>>> }
>>>
>>> -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);
>>>
>>> #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 = bfq_entity_to_bfqq(entity);
>>>
>>> + /* 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));
>>> + }
>>> }
>>>
>>> /**
>>> @@ -649,14 +650,8 @@ static void bfq_forget_entity(struct bfq_service_tree *st,
>>>
>>> entity->on_st_or_in_serv = false;
>>> st->wsum -= 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));
>>> }
>>>
>>> /**
>>> --
>>> 2.7.4
>>>
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